Network Working Group H. Berkowitz
Request for Comments: 2072 PSC International
Category: Informational January 1997
Router Renumbering Guide
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
IP addresses currently used by organizations are likely to undergo
changes in the near to moderate term. Change can become necessary
for a variety of reasons, including enterprise reorganization,
physical moves of equipment, new strategic relationships, changes in
Internet Service Providers (ISP), new applications, and the needs of
global Internet connectivity. Good IP address management may in
general simplify continuing system administration; a good renumbering
plan is also a good numbering plan. Most actions taken to ease
future renumbering will ease routine network administration.
Routers are the components that interconnect parts of the IP address
space identified by unique prefixes. Obviously, they will be
impacted by renumbering. Other interconnection devices, sUCh as
bridges, layer 2 switches (i.e., specialized bridges), and ATM
switches may be affected by renumbering. The interactions of these
lower-layer interconnection devices with routers must be considered
as part of a renumbering effort.
Routers interact with numerous network infrastructure servers,
including DNS and SNMP. These interactions, not just the pure
addressing and routing structure, must be considered as part of
router renumbering.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Motivations for Renumbering . . . . . . . . . . . . . . . . 3
4. Numbering and Renumbering. . . . . . . . . . . . . . . . . . 9
5. Moving toward a Renumbering-Friendly Enterprise. . . . . . . 13
6. Potential Pitfalls in Router Renumbering. . . . . . . . . 20
7. Tools and Methods for Renumbering . . . . . . . . . . . . 25
8. Router Identifiers . . . . . . . . . . . . . . . . . . . . . 29
9. Filtering and Access Control . . . . . . . . . . . . . . . . 35
10. Interior Routing . . . . . . . . . . . . . . . . . . . . . . 37
11. Exterior Routing . . . . . . . . . . . . . . . . . . . . . . 39
12. Network Management . . . . . . . . . . . . . . . . . . . . . 41
13. IP and Protocol Encapsulation . . . . . . . . . . . . . . . 43
14. Security Considerations. . . . . . . . . . . . . . . . . . . 44
15. Planning and Implementing the Renumbering . . . . . . . . . 44
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 46
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
18. Author"s Address . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction
Organizations can decide to renumber part or all of their IP address
space for a variety of reasons. Overall motivations for renumbering
are discussed in [RFC2071]. This document deals with the router-
related ASPects of a renumbering effort, once the decision to
renumber has been made.
A renumbering effort must be well-planned if it is to be successful.
This document deals with planning and implementation guidelines for
the interconnection devices of an enterprise. Of these devices,
routers have the clearest association with the IP numbering plan.
Planning begins with understanding the problem to be solved. Such
understanding includes both the motivation for renumbering and the
technical issues involved in renumbering.
1. Begin with a short and clear statement of the reason to
renumber. Section 3 of this document discusses common
reasons.
2. Understand the principles of numbering in the present and
planned environments. Section 4 reviews numbering and
suggests a method for describing the scope of renumbering.
3. Before the actual renumbering, it can be useful to evolve
the current environment and current numbering to a more
"renumbering-friendly" system. Section 5 discusses ways to
introduce renumbering friendliness into current systems.
4. Be aware of potential pitfalls. These are discussed in
Section 6.
5. Identify potential requirements for tools, discussed in
Section 7.
6. Evaluate the specific router mechanisms that will be affected
by renumbering. See Sections 8 through 13.
7. Set up a specific transition plan framework. Guidelines
for such planning are in Section 15.
When trying to understand the interactions of renumbering on routers,
remember there different aspects to the problem, depending on the
scope of the renumbering involved. Remember that even an
enterprise-wide renumbering probably will not affect all IP addresses
visible within the enterprise, since some addresses (e.g., Internet
service providers, external business partners) are outside the
address space under the control of the enterprise.
2. Disclaimer
The main part of this document is intended to be vendor-independent.
Not all features discussed, of course, have been implemented on all
routers. This document should not be used as a general comparison
of the richness of features of different implementations.
References here are only to those features affected by renumbering.
Some illustrative examples may be used that cite vendor-specific
characteristics. These examples do not necessarily reflect the
current status of products.
3. Motivations for Renumbering
Reasons to renumber can be technological, organizational, or both.
Technological reasons fall into several broad categories discussed
below. Just as there can be both technological and organizational
motivations for renumbering [RFC2071], there can be multiple
technological reasons.
There may not be a clear line between organizational and technical
reasons for renumbering. While networks have a charm and beauty all
their own, the organizational reasons should be defined first in
order to justify the budget for the technical renumbering. There
also may be pure technnical reasons to renumber, such as changes in
technology (e.g., from bridging to routing).
While this document is titled "Router Renumbering Guide," it
recognizes that renumbering may be required due to the initial
installation of routers in a bridged legacy network. Organizations
may have had an adequate bridging solution that did not scale with
growth. Some organizations could not able to move to routers until
router forwarding performance improved [Carpenter] to be comparable
to bridges.
Other considerations include compliance with routing outside the
organization. Routing issues here are primarily those of the global
Internet, but may also involve bilateral private links to other
enterprises.
Certain new transmission technologies have tended to redefine the
basic notion of an IP subnet. The numbering plan needs to work with
these new ideas. Legacy bridged networks and leading-edge workgroup
switched networks may very well need changes in the subnetting
structure. Renumbering needs may also develop with the introduction
of new WAN technologies, especially nonbroadcast multiaccess (NBMA)
services such as frame relay. Other WAN technologies, dialup media
using modems or ISDN, also may have new routing and numbering
requirements. Switched virtual circuit services such as ATM, X.25,
or switched frame relay also interact with routing and addressing.
3.1 Internet Global Routing
Many discussions of renumbering emphasize interactions among
organizations" numbering plans and those of the global Internet
[RFC1900]. There can be equally strong motivations for renumbering
in organizations that never connect to the global Internet.
According to RFC1900, "Unless and until viable alternatives are
developed, extended deployment of Classless Inter-Domain Routing
(CIDR) is vital to keep the Internet routing system alive and to
maintain continuous uninterrupted growth of the Internet....To
contain the growth of routing information, whenever such an
organization changes to a new service provider, the organization"s
addresses will have to change.
Occasionally, service providers themselves may have to change to a
new and larger block of address space. In either of these cases, to
contain the growth of routing information, the organizations
concerned would need to renumber.... If the organization does not
renumber, then some of the potential consequences may include (a)
limited (less than Internet-wide) IP connectivity, or (b) extra cost
to offset the overhead associated with the organization"s routing
information that Internet Service Providers have to maintain, or
both."
3.2 Bridge Limitations; Internal Use of LAN Switching
Introducing workgroup switches may introduce suBTle renumbering
needs. Fundamentally, workgroup switches are specialized, high-
performance bridges, which make their main forwarding decisions
based on Layer 2 (MAC) address information. Even so, they rarely
are independent of Layer 3 (IP) address structure. Pure Layer 2
switching has a "flat" address space that will need to be renumbered
into a hierarchical, subnetted space consistent with routing.
Traditional bridged networks share many of the problems of workgroup
switches, but have additional performance problems when bridged
connectivity extends across slow WAN links.
Introducting single switches or stacks of switches may not have
significant impact on addressing, as long as it is remembered that
each system of switches is a single broadcast domain. Each broadcast
domain should map to a single IP subnet.
Virtual LANs (VLAN) further extend the complexity of the role of
workgroup switches. It is generally true that moving an end station
from one switch port to another within the same "color" VLAN will not
cause major changes in addressing. Many discussions of this
technology do not make it clear that moving the same end station
between different colors will move the end station into another IP
subnet, requiring a significant address change.
Switches are commonly managed by SNMP applications. These network
management applications communicate with managed devices using IP.
Even if the switch does not do IP forwarding, it will itself need IP
addresses if it is to be managed. Also, if the clients and servers
in the workgroup are managed by SNMP, they will need IP addresses.
The workgroup, therefore, will need to appear as one or more IP
subnets.
Increasingly, internetworking products are not purely Layer 2 or
Layer 3 devices. A workgroup switch product often includes a router
function, so the numbering plan must support both flat Layer 2 and
hierarchical Layer 3 addresses.
3.3 Internal Use of NBMA Cloud Services
"Cloud" services such as frame relay often are more economical than
traditional services. At first glance, when converting existing
enterprise networks to NBMA, it might appear that the existing subnet
structure should be preserved, but this is often not the case.
Many organizations often began by treating the "cloud" as a single
subnet, but eXPerience has shown it is often better to treat the
individual virtual circuits as separate subnets. When the individual
point-to-point VCs become separate subnets, efficient address
utilization requires the use of /30 prefixes for these subnets. This
typically means the addressing and routing plan must support multiple
prefix lengths, establishing one or more prefix lengths for LAN media
with more than two hosts, and subdividing one or more of these
shorter prefixes into longer /30 prefixes that minimize address loss.
There are alternative ways to configure routing over NBMA, using
special mechanisms to exploit or simulate point-to-multipoint VCs.
These often have a significant performance impact on the router, and
may be less reliable because a single point of failure is created.
Mechanics of these alternatives are discussed later in this section,
but the motivations for such alternatives tend to include:
1. A desire not to use VLSM. This is often founded in fear
rather than technology.
2. Router implementation issues that limit the number of subnets
or interfaces a given router can support.
3. An inherently point-to-multipoint application (e.g., remote
hosts to a data center). In such cases, some of the
limitations are due to the dynamic routing protocol in use.
In such "star" applications, static routing may actually be
preferable from performance and flexibility standpoints,
since it does not produce routing traffic and is unaffected
by split horizon.
To understand how use of NBMA services affects the addressing
structure and routers, it is worth reviewing what would appear to be
very basic concepts of IP subnets. The traditional view is that a
single subnet is associated with a single physical medium. All hosts
physically connected to this medium are assumed to be able to reach
all other hosts on the same medium, using data link level services.
These services are medium specific: hosts connected to a LAN medium
can broadcast to one another, while hosts connected to a point-to-
point line simply need to transmit to the other end.
When one host desires to transmit to another, it first determines if
the destination is local or remote. A local destination is on the
same subnet and assumed to be reachable through data link services.
A remote destination is on a different subnet, and it is assumed that
router intervention is needed to reach it.
The first NBMA problem comes up when a single subnet is implemented
over an NBMA service. Frame Relay provides single virtual circuits
between hosts that have connectivity. It is quite common to design
Frame Relay services as partial meshes, where not all hosts have VCs
to all others. When the set of hosts in a partial mesh is in a
single IP subnet, partial mesh violates the local model of full
connectivity. Even when there is full meshing, a pessimistic but
reasonable operational model must consider that individual VCs do
fail, and full connectivity may be lost transiently.
There are several ways to deal with this violation, each with their
own limitations. If a specific "central" host has connectivity to N
all other hosts, that central host can replicate all frames it
receives from one host onto outgoing VCs connecting it with the (N-1)
other hosts in the subnet. Such replication usually causes an
appreciable CPU load in the replicating router. The replicating
router also is a single point of failure for the subnet. This method
does not scale well when extended to fuller meshes within the subnet.
In a routing protocol, such as OSPF, that has a concept of designated
routers, explicit configuration usually is needed. Other problems in
using a meshed subnet is that all VCs may not have the same
performance, but the router cannot prefer individual paths within the
subnet.
One of the simplest methods is not to attempt to emulate a broadcast
medium, but simply to treat each VC as a separate subnet. This will
cause a need for renumbering. Efficient use of the address space
dictates a /30 prefix be used for the per-VC subnets. Such a prefix
often needs VLSM support in the routers.
3.4 Expansion of Dialup Services
Dialup services, especially public Internet access providers, are
undergoing explosive growth. This success represents a particular
drain on the available address space, especially with a commonly used
practice of assigning unique addresses to each customer.
In this practice, individual users announce their address to the
access server using PPP"s IP configuration option [RFC1332]. The
server may validate the proposed address against some user
identifier, or simply make the address active in a subnet to which
the access server (or set of bridged access servers) belongs.
These access server functions may be part of the software of a
"router" and thus are within the scope of this Guide.
The preferred technique [Hubbard] is to allocate dynamic addresses to
the user from a pool of addresses available to the access server.
Various mechanisms are used actually to do this assignment, and are
discussed in Section 5.5.
3.5 Internal Use of Switched Virtual Circuit Services
Services such as ATM virtual circuits, switched frame relay, etc.,
present challenges not considered in the original IP design. The
basic IP decision in forwarding a packet is whether the destination
is local or remote, in relation to the source host"s subnet. Address
resolution mechanisms are used to find the medium address of the
destination in the case of local destinations, or to find the medium
address of the router in the case of remote routers.
In these new services, there are cases where it is far more effective
to "cut-through" a new virtual circuit to the destination. If the
destination is on a different subnet than the source, the cut-through
typically is to the egress router that serves the destination subnet.
The advantage of cut-through in such a case is that it avoids the
latency of multiple router hops, and reduces load on "backbone"
routers. The cut-through decision is usually made by an entry router
that is aware of both the routed and switched environments.
This entry router communicates with a address resolution server using
the Next Hop Resolution Protocol (NHRP) [Cansever] [Katz]. This
server maps the destination network address to either a next-hop
router (where cut-through is not appropriate) or to an egress router
reached over the switched service. Obviously, the data base in such
a server may be affected by renumbering. Clients may have a hard-
coded address of the server, which again may need to change.
While the NHRP work is in progress at the time of this writing,
commercial implementations based on drafts of the protocol standard
are in use.
4. Numbering and Renumbering
What is the role of any numbering plan? To understand the general
problem, it can be worthwhile to review the basic principles of
routers. While most readers will have a good intuitive sense of
this, the principles have refined in the current usage of IP.
A router receives an inbound IP datagram on one of its interfaces,
and examines some number of bits of the destination address. The
sequence of bits examined by the router always begin at the left of
the address (i.e., the most significant bit). We call this sequence
a "prefix."
Routing decisions are made on totalPrefix bits, which start at the
leftmost (i.e., most significant) bit position of the IP address.
Those totalPrefix bits may be completely under the control of the
enterprise (e.g., if they are in the private address space), or the
enterprise may control the loWorderPrefix bits while the
highOrderPrefix bits are assigned by an outside organization.
The router looks up the prefix in its routing table (formally called
a Forwarding Information Base). If the prefix is in the routing
table, the router then selects an outgoing interface that will take
the routed packet to the next hop IP address in the end-to-end route.
If the prefix cannot be found in the routing table, the router
returns an ICMP Destination Unreachable message to the source address
in the received datagram.
Assuming the prefix is found in the routing table, the router then
transmits the datagram through the indicated outgoing interface. If
multicast routing is in effect, the datagram may be copied and sent
out multiple outgoing interfaces.
4.1 Categorizing the topology
From the router renumbering perspective, renumbering impact is apt to
be greatest in highly connected parts of "backbones," and least in
"stub" parts of the routing domain that have a single route to the
backbone.
Global Internet
^


Back1-------------------Back2

+-----------+ +----------+

Reg1.1------Reg1.2 Reg2.1-----Reg2.2


Branch Branch Branch Branch
1.1.1 to 1.2.1 to 2.1.1 to 2.2.1 to
1.1.N 1.2.N 2.1.N 2.2.N
In this drawing, assume Back1 and Back2 exchange full routes; Back1
is also the exterior router. Regional routers (Reg) exchange full
routes with one another and aggregate addresses to the backbone
routers. Branch routers default to regional routers.
From a pure topological standpoint, the higher in the hierarchy, the
greater are apt to be the effects of renumbering. This is a first
approximation to scoping the task, assuming addresses have been
assigned systematically. Systematic address space is rarely the case
in legacy networks.
4.2 Categorizing the address space
An inventory of present and planned address space is a prerequisite
to successful renumbering. Begin by identifying the prefixes in or
planned into your network, and whether they have been assigned in a
systematic and hierarchical manner.
+--Unaffected by renumbering [A]


+--Existing prefixes to be renumbered


+----To be directly renumbered on "flag day"


+----Initially to be renumbered to temporary address


+--Existing prefixes to be retired


+--Planned new prefixes


+---totalPrefix change, no length change


+---highOrderPart change only, no length change


+---lowOrderPart change only, no length change


+---highOrderPart change only, high length change


+---lowOrderPart change only, low length change


+---totalPrefix change only, changes in high and low


+---highOrderPart change only, no length change
Ideally, a given prefix should either be "unchanged," "old," or
"new." Renumbering will be easiest when each "old" prefix can be
mapped to a single "new" prefix.
Unfortunately, the ideal often will not be attainable. It may be
necessary to run parts of the new and old address spaces in parallel.
Renumbering applies first to prefixes and then to host numbers to the
right of the prefix. To understand the scope of renumbering, it is
essential to:
1. Identify the prefixes (and possibly host fields) potentially
affected by the renumbering operation.
2. Identify the authority that controls the values of the prefix,
or part of the prefix, affected by renumbering.
In a given enterprise, prefixes may be present that will be under the
complete or partial control of the enterprise, as well as totally
outside the control of the enterprise. Let us review the principles
of control over address space.
More commonly, the most significant bits of the prefix are assigned
to the enterprise by an address registry (e.g., InterNIC, RIPE, or
APNIC) or by an Internet Service Provider (ISP). This assignment of
a value in the most significant bit positions historically has been
called a "network number," when the assigned high-order part is 8,
16, or 24 bits long. More recent usage does not limit the assigned
part to a byte boundary. The preferred term for the assigned part is
a "CIDR block" of a certain number of bits [RFC1518].
The enterprise then extends the prefix to the right, creating
"subnets." It is critical to realize that routers make routing
decisions based on the total prefix of interest, regardless of who
controls which bits. In other words, the router really doesn"t know
or care about subnet boundaries.
The way to think about subnetting is that it creates a longer prefix.
Even before CIDR, we collapsed multiple subnets into a single network
number advertisement sent to external routers. In a more general
way, we now think of extending the prefix to the right as subnetting
and collapsing it to the left as supernetting, aggregating, or
summarizing. Depending on the usage of subnetting or aggregation,
different prefix lengths are significant at different router
interfaces.
4.3 Renumbering Scope
Prefixes may be taken from the private address space [RFC1918] that
is not routable on the global Internet. Since these addresses are
not routable on the global Internet, changing parts of private
address space prefixes is an enterprise-local decision.
If a prefix is totally outside the control of the enterprise, it is
external, and will be minimally affected by routing. Potential
interactions of external prefixes with enterprise renumbering
include:
1) Inadvertent alteration or deletion of external addresses
as part of router reconfiguration.
2) Loss of connectivity to application servers inside the
enterprise, because the external client no longer knows
the internal address of the server.
3) DNS/BGP
4) Security
Prefixes partially under the control of the enterprise may change.
The scope of this will vary depending on whether only the externally
controlled part of the prefix changes, or if part of the internally
controlled part is to be renumbered. If the length of either the
high-order or low-order parts change, the process becomes more
complex.
High-order-part-only renumbering is most common when an organization
changes ISPs, and needs to renumber into the new provider"s space.
The old prefix may have been assigned to the enterprise but will no
longer be used for global routing, or the old prefix may have been
assigned to the previous provider. Note that administrative
procedures may be necessary to return the previous prefix, although
this usually will be done by the previous provider. There often will
need to be a period of coexistence between the old and new prefixes.
Low-order-part-only renumbering can occur when an enterprise modifies
its internal routing structure, and the changes only affect the
internal subnet structure of the enterprise network. This is typical
of efforts involved in increasing the number of available subnets
(e.g., for more point-to-point media) or increasing the number of
hosts on a medium (e.g., in greater use of workgroup switches).
Both the high-order and low-order parts may change. This might
happen when the enterprise changes to a new ISP, who assigns address
space from a CIDR block rather than a classful network previously
used. With a different high-order prefix length, the enterprise
might be forced to change its subnet structure.
5. Moving toward a Renumbering-Friendly Enterprise
Renumbering affects both the configuration of specific router
"boxes," and the overall system of routers in a routing domain. The
emphasis of this section is on making the current enterprise more
renumbering-friendly, before any prefixes are actually changed.
Renumbering will have the least impact when the minimum number of
reconfiguration options are needed. When planning renumbering on
routers, consider that many existing configurations may contain
hard-coded IP addresses that may not be necessary, even if
renumbering were not to occur. Part of a router renumbering effort
should include, wherever possible, replacing router mechanisms based
on hard-coded addresses with more flexible mechanisms.
Renumbering will also generally be easier if the configuration
changes can be made offline on appropriate servers, and then
downloaded to the router if the router implementation permits.
5.1 Default Routes
A well-known method for reducing the amount of reference by one
router to other routers is to use a default route to a higher-level,
better-connected router. This assumes a hierarchical network design,
which is generally desirable in the interest of scaling.
Default routes are most appropriate for stub routers inside a routing
domain, and for boundary routers that connect the domain to a single
ISP.
5.2 Route Summarization and CIDR
When routes need to be advertised, summarize as much as is practical.
Summarization is most effective when address prefixes have been
assigned in a consistent and contiguous manner, which is often not
the case in legacy networks. Nevertheless, there is less to change
when we can refer to blocks of prefixes.
Not all routing mechanisms support general summarization. Interior
routing mechanisms that do include RIPv2, OSPF, EIGRP, IS-IS, and
systems of static routes. RIPv1 and IGRP do support classful
summarization (i.e., at Class A/B/C network boundaries only).
If existing addresses have been assigned hierarchically, it may be
possible to renumber below the level of summarization, while hiding
the summarization to the rest of the network. In other words, if all
the address bits being renumbered are to the right of the summarized
prefix length, the change can be transparent to the overall routing
system.
Even when effective summarization is possible to hide the details of
routing, DNS, filters, and other services may be affected by any
renumbering.
5.3 Server References in Routers
Routers commonly communicate with an assortment of network management
and other infrastructural servers. Examples of these servers are
given in the "Network Management" section below. DNS itself,
however, may be an important exception.
Wherever possible, servers should be referenced by DNS name rather
than by IP address. If a specific router implementation only
supports explicit address references, this should be documented as
part of the renumbering plan.
Routers may also need to forward end host broadcasts to other
infrastructure services (e.g., DNS, DHCP/BOOTP). Configurations that
do this are likely to contain hard-coded IP addresses of the
destination hosts or their subnets, which will need to be changed as
part of renumbering.
5.4 DNS and Router Renumbering
The Domain Name Service is a powerful tool in any renumbering effort,
and can help routers as well as end hosts. If traceroute displays
DNS names rather than IP addresses, certain debugging options can be
transparent through the address transition.
Be aware that dynamically learned names and addresses may be cached
in router tables. For a router to learn changes in address to name
correspondence, it may be necessary to restart the router or
explicitly clear the cache.
Alternatively, router configuration files may contain hard-coded
address/name correspondences that will not be affected by a change in
the DNS server.
Different DNS databases are affected by renumbering. For example,
the enterprise usually controls its own "forward" data base, but the
reverse mapping data base may be maintained by its ISP. This can
require coordination when changing providers.
Commonly, router renumbering goes through a transition period.
During this transition, old and new addresses may coexist in the
routing system. Coexistence over a significant period of time is
especially likely for DNS references to addresses that are known in
the global Internet [deGroot]. Various DNS servers throughout the
world may cache addresses for periods of days.
If, for example, a given router interface may have a coexisting new
and old address, it can be appropriate to introduce the new address
as an additional A record for the new address.
DNS RR statements can end with a semicolon, indicating the rest of
the line is a comment. This can be used as the basis of tools to
renumber DNS names for router addresses, by putting a comment (e.g.,
";newaddr") at the end of the A statements for the new addresses. At
an appropriate time, a script could generate a new zone file in which
the new addresses become the primary definitions on A records, and
the old addresses could become appropriately commented A records. At
a later time, these commented entries could be removed.
Care should be taken to assure that PTR reverse mapping entries are
defined for new addresses, because some router vendor tools depend on
reverse mapping.
5.5 Dynamic Addressing
Renumbering is easiest when addresses need to be changed in the least
possible number of places. Dynamic address assignment is especially
attractive for end hosts, and routers may play a key role in this
process. Routers may act as servers and actually assign addresses,
or may be responsible for forwarding end host address assignment
requests to address assignment servers.
The most common use of dynamic address assignment is to provide IP
addresses to end systems. Dynamic address assignment, however, is
also used to assign IP addresses to router interfaces. An address
assignment server may assign an IP address to a router either in the
usual DHCP way, based on a MAC address in the router, or simply based
on the physical connectivity of the new router. In other words, any
router connected on a specific interface of the configuring router
would be assigned the same IP address.
5.5.1 Router Roles in LAN-based DHCP Address Assignment
End hosts attached to LANs often obtain address assignments from
BOOTP or DHCP servers. If the server is not on the same medium as
the end hosts, routers may need to play a role in establishing
connectivity between the end host and the address server.
If the client is not on the same medium as the address assignment
server, routers either must act as address assignment services, or
forward limited broadcasts to the location of appropriate servers.
If the router acts as an address assignment server, its database of
addresses that it can assign may change during renumbering. If the
router forwards to a DHCP or BOOTP server, it must know the address
of that server. That server address can itself change as a result of
renumbering.
While the usual perception of DHCP is that it assigns addresses from
a pool, such that assignments to a given host at a given time is
random within the pool, DHCP can also return a constant IP address
for a specific MAC address. This may be much easier to manage and
troubleshoot, especially during renumbering.
Clearly, if the DHCP server identifies end hosts based on their MAC
address, consideration must be given to making that address unique,
and changing the DHCP database if either the MAC address or the IP
address changes. One way to reduce such reconfiguration is to use
Locally-Administered Addresses (LAA) on end hosts, rather than
globally unique addresses stored in read-only memory (ROM). Using
LAAs solves the problem of MAC addresses changing when a network
interface card changes, but LAAs have their own management problems
of configuration into end systems and maintaining uniqueness within
the enterprise.
5.5.2 Router Roles in Dialup Address Assignment
There are several possible ways in which an dialup end host interacts
with address assignment. Routers/access servers can play critical
roles in this process, either to provide connectivity between client
and server, or directly to assign addresses.
Different vendors handle address assignment in different ways.
Methods include:
1. The access server forwards the request to a DHCP server, using
a unique 48-bit identifier associated with the client. Note
that this usually should not be the MAC address of the access
server, since the same MAC address would then be associated
with different hosts.
2. The server forwards the request to an authentication server,
which in turn gets a dynamic address either from:
a. internal pools
b. a DHCP server to which it forwards
3. The server selects an address from locally configured pools
and provides it to the dialing host without the intervention
of other services.
When the router contains assignable addresses, these may need to
change as part of renumbering. Alternatively, hard-coded references
to address assignment or authentication servers may need to change.
5.5.3 Router Autoonfiguration
This initial address assignment may provide an address only for a
single connection (i.e., between the new and configuring routers).
The newly assigned address may then be used to "bootstrap" a full
configuration into the new router.
Dynamic address assignment to routers is probably most common at
outlying "stub" or "edge" routers that connect via WAN links to a
central location with a configuration server. Such edge devices may
be shipped to a remote site, plugged in to a WAN line, and use
proprietary methods to acquire part or all of their address
configuration.
When such autoconfiguration is used on edge routers, it may be
necessary to force a restart of the edge router after renumbering.
Restarting may be the only way to force the autoconfigured router to
learn its new address. Other out-of-band methods may be available to
change the edge router addresses.
5.6 Network Address Translation
Network address translation (NAT) is a valuable technique for
renumbering, or even for avoiding the need to renumber significant
parts of an enterprise [RFC1631]. It is not always transparent to
network layer protocols, upper layer protocols, and network
management tools, and must not be regarded as a panacea.
In the classic definition of NAT, certain parts of the routing system
are designated as stub domains, and connect to the global domain only
through NAT functions. The NAT contains a translation mechanism that
maps a stub address to a global address. This mechanism may map
statically or dynamically.
A more general NAT mechanism often is implemented in firewall bastion
hosts, which isolate "inside" and "outside" addresses through
transport- or application-level authenticated gateways. Mappings
from a "local" or "inside" address to a global address often is not
one-to-one, because an inside address is dynamically mapped to a TCP
or UDP port on an outside interface address.
In general, if there are multiple NATs, their translation mechanisms
should be synchronized. There are specialized cases when a given
stub address appears in more than one stub domain, and ambiguity
develops if one wishes to map, say, from 10.1.0.1/16 in stub domain A
to 10.1.0.1/16 in stub domain B. In this case, both 10.1.0.1
addresses identify different hosts. Special mechanisms would have
to exist to map the domain A local address into one global address,
and to map the domain B local address into a different global
address.
NAT can have a valuable role in renumbering. Its intelligent use can
greatly minimize the amount of renumbering that needs to be done.
NAT, however, is not completely transparent.
Specifically, it can interfere with the proper operation of any
protocol that carries an IP address as data, since NAT does not
understand passenger fields and is unaware numbers need to change.
Examples of protocols affected are:
--TCP and UDP checksums that are in part based on the
IP header. This includes end-to-end encryption schemes
that include the TCP/UDP checksum
--ICMP messages containing IP addresses
--DNS queries that return addresses or send addresses
--FTP interactions that use an ASCII-encoded IP address
as part of the PORT command
It may be possible to avoid conflict if only certain hosts use
affected protocols. Such hosts could be assigned only global
addresses, if the network topology and routing plan permit.
Early NAT experiments suggested that it needs a sparse end-to-end
traffic mapping database for reasonable performance. This may or may
not be an issue in more hardware-based NAT implementations.
Another concern with NAT is that unique host addresses are hidden
outside the local stub domains. This may in fact be desirable for
security, but may present network management problems. One
possibility would be to develop a NAT MIB that could be queried by
SNMP to find the specific local-to-global mappings in effect.
There are also issues for DNS, DHCP, and other address management
services. There presumably would need to be local servers within
stub domains, so address requests would be resolved uniquely in each
stub (or would return appropriate global addresses).
6. Potential Pitfalls in Router Renumbering
One way to categorize potential pitfalls is to look at those
associated with the overall numbering plan itself and routing
advertisement, and those associated with protocol behavior. In
general, the former case is static and the latter is dynamic.
6.1 Static
Problems can be implicit to the address/routing structure itself.
These can include failures of components to understand arbitrary
prefix addressing (i.e., classless routing), reachability due to
inappropriate default or aggregated routes, etc.
6.1.1 Classless Routing Considerations
Among the major reasons to renumber is to gain globally routable
address space. In the global Internet, routable address space is
based on arbitrary-length prefixes rather than traditional address
classes. Classless Inter-Domain Routing (CIDR) is the administrative
realization of prefix addressing in the global Internet. Inside
enterprises, arbitrary prefix length addressing often is called
Variable Length Subnet MaSKINg (VLSM) or "subnetting a subnet."
The general rules of prefix addressing must be followed in CIDR.
Among these are permitting use of the all-zeroes and all-one subnets
[RFC1812], and not assuming that a route to a "Class A/B/C network
number" implies routes to all subnets of that network. Assumptions
also should not be made that a prefix length is implied by the
structure of the high-order bits of the IP address (i.e., the "First
Octet Rule").
This ideal, unfortunately, is not understood by a significant number
of routers (and terminal access servers that participate in routing),
and an even more significant number of host IP implementations.
When planning renumbering, network designers must know if the new
address has been allocated using CIDR rules rather than traditional
classful addressing. If CIDR rules have been followed in address
assignment, then steps need to be taken to assure the router
understands them, or appropriate steps need to be taken to interface
the existing environment to the CIDR environment.
Current experience suggests that it is best, when renumbering, to
maintain future compatibility by moving to a CIDR-supportive routing
environment. While this is usually thought to mean introducing a
classless dynamic routing protocol, this need not mean that routing
become much more complex. In a RIPv1 environment, moving to RIPv2
may be a relatively simple change. Alternative simple methods
include establishing a default route from a non-CIDR-compliant
routing domain to a CIDR-compliant service provider, or making use of
static routes that are CIDR-compliant.
Some routers support classless routing without further
configuration, other routers support classless routing but require
specific configuration steps to enable it, while other routers only
understand classful routing. In general, most renumbering will
eventually require classless routing support. It is essential to
know if a given router can support classless routing. If it does
not, workarounds may be possible. Workarounds are likely to be
necessary.
6.1.1.1 Aggregation Problems
In experimenting with the CIDR use of a former Class A network
number, it was shown in RFC1879 that CIDR compliance rules must be
enabled explicitly in some routers, while other routers do not
understand CIDR rules.
RFC1897 demonstrated problems with some existing equipment,
especially "equipment that depends on use of a classful routing
protocol, such as RIPv1 are prone to misconfiguration. Tested
examples are current Ascend and Livingston gear, which continue to
use RIPv1 as the default/only routing protocol. RIPv1 use will
create an aggregate announcement.... The Ascend was told to announce
39.1.28/24, but since RIPv1 can"t do this, the Ascend instead sent
39/8." RIPv1, like all classful interior protocols, is obsolescent.
6.1.1.2 Discontiguous Networks
Another problem that can occur with routers or routing mechanisms
that do not understand arbitrary length prefix addressing is that of
discontiguous networks. This problem is easy to create
inadvertently when renumbering. In the example below, assume the
enterprise has been using 10.0.0.0/8 as its primary prefix, but has
introduced an ISP whose registered addresses were in 172.31.0.0/16.
Assume a RIPv1 system of three routers:
10.1.0.1/16 10.2.0.1/16


+-------------------------------------+
Router 1
+-------------------------------------+
172.31.1.1/24


172.31.1.2/24
+-------------------------------------+
Router 2 <------OUTSIDE
+-------------------------------------+
172.31.2.1/24


172.31.2.2/24
+-------------------------------------+
Router 3
+-------------------------------------+


10.3.0.1/16 10.4.0.1/16
Router 1 can reach its two locally connected subnets, 10.1.0.0/16 and
10.2.0.0/16. It will aggregate them into a single announcement of
10.0.0.0/8 when it advertises out the 172.31.1.1 interface.
In like manner, Router 3 can reach its two locally connected subnets,
0.3.0.0/16 and 10.4.0.0/16. It will aggregate them into a single
announcement of 10.0.0.0/8 when it advertises out the 172.31.2.2
interface.
When Router 2 receives a packet from its "outside" interface
destined, say, to 10.1.1.56/16, where does it send it? Router 2 has
received two advertisements of 10.0.0.0 on different interfaces,
without any detail of subnets inside 10.0.0.0. Router 2 has an
ambiguous routing table in terms of the next hop to a subnet of
10.0.0.0. We call this problem, when parts of the same classful
network are separated by different networks, discontigous subnets.
Two problems occur in this configuration. Router 2 does not know
where to send outside packets destined for a subnet of 10.0.0.0.
Connectivity, however, also will break between Routers 1 and 3,
because Router 2 does not know the next hop for any subnet of
10.0.0.0.
There are several workarounds to this problem. Obviously, one would
be to change to a routing mechanism that does advertise subnets. An
alternative would be to establish an IP-over-IP tunnel through Router
2, and give this a subnet in 10.0.0.0. This additional subnet would
be visible only in Routers 1 and 3. It would solve the connectivity
problem between Routers 1 and 3, but Router 2 would still not be able
to forward outside packets. This might be a perfectly acceptable
solution if Router 2 is simply being used to connect two parts of
10.0.0.0.
Another way to deal with the discontiguous network problem is to
assign secondary addresses in 10.0.0.0 to the R1-R2 and R2-R3
interfaces, which would allow the 10.0.0.0 subnets to be advertised
to R2. This would work as long as there is no problem in advertising
the 10.0.0.0 subnets into the R2 routing system. There would be a
problem, for example, if the 10.0.0.0 address were in the private
address space but the R2 primary addresses were registered, and R2
advertised the 10.0.0.0 addresses to the outside.
This problem can be handled if R2 has filtering mechanisms that can
selectively block 10.0.0.0 advertisements to the outside world. The
configuration, however, will become more and more complicated.
6.1.1.3 Router-Host Interactions
The situation may not be as bleak if hosts do not understand prefix
addressing but routers do. Methods exist for hiding a VLSM structure
from end hosts that do not understand it. These do involve protocol
mechanisms as workarounds, but the fundamental problem is hosts"
understanding of arbitrary prefix lengths.
A key mechanism is proxy ARP [Carpenter]. The basic mechanism of
using proxy ARP in prefix-based renumbering is to have the hosts
issue an ARP whenever they want to communicate with a destination.
If the destination is actually on the same subnet, it will respond
directly to the ARP. If the destination is not, the router will
respond as if it were the destination, and the originating host will
send the datagram to the router. Once the datagram is in the router,
the VLSM-aware router can forward it.
Many end hosts, however, will only issue an ARP if they conclude the
destination is on their own subnet. All other traffic is sent to a
hard-coded default router address. In such cases, workarounds may be
needed to force the host to ARP for all destinations.
One workaround involves a deliberate misconfiguration of hosts.
Hosts that only understand default routers also are apt only to
understand classful addressing. If the host is told its major (i.e.,
classful) network is not subnetted, even though the address plan
actually is subnetted, this will often persuade it to ARP to all
destinations.
This also works for hosts that do not understand subnetting at all.
An example will serve for both levels of host understanding. Assume
the enterprise uses 172.31.0.0/16 as its major address, which is in
the Class B format. This is actually subnetted with eight bits of
subnetting -- 172.31.0.0/24. Individual hosts unaware of VLSM,
however, either infer Class B from the address value, or are told
that the subnet mask in effect is 255.255.0.0.
Yet another approach that helps hosts find routers is to run passive
RIP on the hosts, so that they hear routing updates. They assume any
host that issues routing updates must be a router, so traffic for
non- local destinations can be forwarded there. While RIPv1 does not
support arbitrary prefixes, the router(s) issuing the routing updates
may have additional capabilities that let them correctly forward such
traffic. The priority, therefore, is to get the non-local routers to
a router that understands the overall routing structure, and passive
RIP may be a viable way to do this.
It may be appropriate to cut over on a site-by-site basis [Lear]. In
such an approach, some sites have VLSM-aware hosts but others do not.
As long as the routing structure supports VLSM, workarounds can be
applied where needed.
6.1.2 MAC Address Interactions
While it is uncommon now for a router to acquire any of its interface
addresses as a DHCP client, this may become more common. When a
router so acquires addresses, care must be taken that the MAC address
presented to the DHCP server is in fact unique.
Modern routers usually support protocol architectures besides IP.
Some of these architectures, notably DECnet, Banyan VINES, Xerox
Network Services, and IPX, may modify MAC addresses of routers such
that a given MAC address appears on more than one interface. While
this is normally not a problem, it could cause difficulties when the
MAC address is assumed to be unique.
Other situations occur when the same MAC address can appear on more
than one interface. There are high-availability IBM options which
establish primary and backup instances of the same MAC address on
different physical interfaces of 37x5 communications controllers.
Some end hosts running protocol stacks other than IP, notably DECnet,
may change their MAC address from the globally assigned built-in
address.
6.2 Dynamic
Dynamic protocol mechanisms that to some extent depend on IP
addresses may be affected by router renumbering. These include
mechanisms that assign or resolve addresses (e.g., DHCP, DNS),
mechanisms that use IP addresses for identification (e.g., SNMP),
security and authentication mechanisms, etc. The listed mechanisms
are apt to have configuration files that will be affected by
renumbering.
Another area of dynamic behavior that can be affected is caching.
Application servers, Directory functions such as DNS, etc., may cache
IP addresses. When the router is renumbered, these servers may point
to old addresses. Certain proxy server functions may reside on
routers, and the router may need to be restarted to reset the cache.
The endpoints of TCP tunnels terminating on routers may be internally
identified by address/port pairs at each end. If the address
changes, even if the port remains the same, the tunnel is likely to
need to be reestablished.
7. Tools and Methods for Renumbering
The function of a renumbering tool can be realized either as manual
procedures as well as software. This section deals with functionality
of hypothetical yet general renumbering tools rather than their
implementation.
General caveat: tools should know whether the environment supports
classless addressing. If it does not, newly generated addresses
should be checked to see they do not fall into the all-zeroes or
all-ones subnet values.
7.1 Search Mechanisms
Tools will be needed to search configuration files and other
databases to identify addresses and names that will be affected by
reorganization. This search should be based on the address inventory
described above.
Especially when searching for names, common search tools using
regular expressions (e.g., grep) may be very useful. Some additional
search tools may be needed. One would convert dotted decimal search
arguments to binary and only then makes the comparison.
The comparison may need to be done under the constraint of a mask.
Such a comparator would also be relevant as the second phase that
looks for ipAddress and other relevant strings in MIBs.
7.2 Address Modification
The general mechanism for address modification is substitution of an
arbitrary number of bits in an address. In the simplest cases, there
is a one-to-one correspondence between old and new bit positions.
Especially when address modification is manual, it should be
remembered that the affected bits can be obscured by dotted decimal
notation. Work in, or at least consider, binary notation to assure
accuracy.
Several basic functions can be defined:
zerobits(position,length):
clear <length> bits starting at <position>
orbits(position,length,newbits)
OR the bit string <newbits> of <length> starting at <position>
In these examples, the index [j] is used to identify entries in the
address inventory database for existing addresses, while [k]
identifies new addresses.
Remember that these tools operate at a bit level, so the new address
will have to be converted back into dotted decimal, MIB ASN.1, or
other notation before it can be replaced into configuration files.
7.2.1 Prefix Change, No Change in Length
If the entire new prefix has the same number of bits as the old
external part, requiring no change to subnetting, the router part of
renumbering may be fairly simple. If the router configurations are
available in machine-readable form, as text files or parsable SNMP
data, it is a relatively simple task to define a tool to examine IP
addresses in the configuration, identify those beginning with the old
prefix, and substitute the new prefix bit-by-bit.
for each address[j],
zerobits(0,PrefixLength[j])
orbits(0,PrefixLength[j],NewPrefix[j])
Note that the host part is unaffected. Both subnet specifications
(e.g., for route advertisements) and specific host references will be
changed correctly in this simple case.
7.2.2 highOrderPart change
Matters are slightly more complex when the change applies only to the
externally-controlled part of the prefix, as might be the case when
an organization changes ISPs and renumbers into the ISP"s address
space, without changing the internal subnet structure.
The substitution process is much as the previous case, except only
the high-order bits change:
for each address,
zerobits(0,highOrderPartLength[j])
orbits(0,highOrderPartLength,newHighOrderPart[k])
7.2.3 lowOrderPart change
Organizations might renumber only the lowOrderPart (i.e., the subnet
field) of their address space. This might be done to clean up an
address space into contiguous blocks prior to introducing a routing
system that aggregates addresses, such as OSPF.
for each address[j],
zerobits(highOrderPartLength[j],lowOrderPartLength[j])
orbits(highOrderPartLength[j],
lowOrderPartLength[j],newLowOrderPart[k])
7.2.4 lowOrderPart change, Change in lowOrderPart length
When the length of the subnet field changes in all or part of the
address space, things become significantly more complex. A fairly
simple case arises when the host field is consistently too long, at
least in the affected subnets. This is common, for example, when
address space is being recovered in an existing Class B network with
8 bits of subnetting. Certain /24 bit prefixes are being extended to
/30 and reallocated to point-to-point real or virtual (e.g., DLCI)
media.
for each address [j]
if address is affected by renumbering
if newLowOrderPartLength[k] > oldLowOrderPartLength[j]
then
zerobits(highOrderPartLength[k],newLowOrderPartLength[k])
orbits(highOrderPartLength[k],newLowOrderPart[k])
end
7.2.5 highOrderPart change, Change in highOrderPart length
When the length of the high-order part changes, but it is desired to
keep the existing subnet structure, constraints apply. The situation
is fairly simple if the new high-order part is shorter than the
existing high order part.
If the new high-order part is longer than the old high order part,
constraints are more complex. The key is to see if any of the <k>
most significant bits of the oldHighOrderPart, which overlap the <k>
least significant bits of the newHighOrderPart, are nonzero. If no
bits are nonzero, it may be simply to overlay the new prefix bits.
7.3 Naming
It is worthwhile to distinguish that a router"s use of a DNS name
does not necessarily mean that name is defined in a name server.
Routers often contain static address to name mappings local to the
router, so both the DNS zone files and the router configurations will
need to be checked.
What we first want to do is generate a list of name/address mappings,
the mapping mechanism for each instance (e.g., static entry in
configuration file, RR in our zone"s DNS, RR in a zone file outside
ours), the definition statement (or equivalent if the routers are
configured with SNMP), and current IP address. We then want to
compare the addresses in this list to the list defined earlier of
prefixes affected by renumbering. The intersection of these lists
define where we need to make changes.
Note that the explicit definition statement, or at leasts its
variables, should be kept. In the real world, static IP address
mappings in hosts may not have been maintained as systematically as
are RR records in a DNS server. It is entirely possible that
different host mapping entries for the same name point to different
addresses.
7.3.1 DNS Tools
The DNS itself can both delay and and speed router renumbering.
Caches in DNS servers both inside and outside the organization may
have sufficient persistence that a "flag day" cutover is not
practical if worldwide connectivity is to be kept. DNS can help,
however, make a period of old and new address coexistence work.
If, for example, a given router interface may have a coexisting new
and old address, it can be appropriate to introduce the new address
as a CNAME alias for the new address.
DNS RR statements can end with a semicolon, indicating the rest of
the line is a comment. This can be used as the basis of tools to
renumber DNS names for router addresses, by putting a comment (e.g.,
";newaddr") at the end of the CNAME statements for the new addresses.
At an appropriate time, a script could generate a new zone file in
which the new addresses become the primary definitions on A records,
and the old addresses could become appropriately commented CNAME
records. At a later time, these commented CNAME entries could be
removed.
Care should be taken to assure that PTR reverse mapping entries are
defined for new addresses, because some router vendor tools depend on
reverse mapping.
7.3.2 Related name tools
Especially on UNIX and othe that do routing, there may be static name
definitions. Such definitions are probably harder to keep maintained
than entries in the DNS, simply because they are more widely
distributed.
Several tools are available to generate more maintainable entries. A
perl script called h2n converts host tables into zone data files that
can be added to the DNS server. It is available as
ftp://ftp.uu.net/published/oreilly/nutshell/dnsbind/dns.tar.Z.
Another tool, makezones, is part of the current BIND distribution,
and can also be obtained from
ftp://ftp.cus.cam.ac.uk/pub/software/programs/DNS/makezones
See the DNS Resources Directory at http://www.dns.net/dnsrd.
8. Router Identifiers
Configuration commands in this category assign I