RFC2519 - A Framework for Inter-Domain Route Aggregation

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　　Network Working Group E. Chen
Request for Comments: 2519 Cisco
Category: Informational J. Stewart
Juniper
February 1999

A Framework for Inter-Domain Route Aggregation

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

This document presents a framework for inter-domain route aggregation
and shows an example router configuration which 'implements' this
framework. This framework is flexible and scales well as it
emphasizes the philosophy of aggregation by the source, both within
routing domains as well as towards upstream providers, and it also
strongly encourages the use of the 'no-export' BGP community to
balance the provider-subscriber need for more granular routing
information with the Internet's need for scalable inter-domain
routing.

1. Introduction

The need for route aggregation has long been recognized. Route
aggregation is good as it reduces the size, and slows the growth, of
the Internet routing table. Thus, the amount of resources (e.g., CPU
and memory) required to process routing information is reduced and
route calculation is sped up. Another benefit of route aggregation
is that route flaps are limited in number, frequency and scope, which
saves resources and makes the global Internet routing system more
stable.

Since CIDR (Classless Inter-Domain Routing) [2] was introduced,
significant progress has been made on route aggregation, particularly
in the following two areas:

- Formulation and implementation of IP address allocation policies
by the top registries that conform to the CIDR principles [1].

This policy work is the cornerstone which makes efficient route
aggregation technically possible.

- Route aggregation by large (especially "Tier 1") providers. To
date, the largest reductions in the size of the routing table
have resulted from efficient aggregation by large providers.

However, the ability of various levels of the global routing system
to implement efficient aggregation schemes varies widely. As a
result, the size and growth rate of the Internet routing table, as
well as the associated route computation required, remain major
issues today. To support Internet growth, it is important to
maximize the efficiency of aggregation at all levels in the routing
system.

Because of the current size of the routing system and its dynamic
nature, the first step towards this goal is to establish a clearly
defined framework in which scaleable inter-domain route aggregation
can be realized. The framework described in this document is based
on the predominant and current experience in the Internet. It
emphasizes the philosophy of aggregation by the source, both within
routing domains as well as towards upstream providers. The framework
also strongly encourages the use of the "no-export" BGP community to
balance the providersubscriber need for more granular routing
information with the Internet's need for scalable inter-domain
routing. The advantages of this framework include the following:

- Route aggregation is done in a distributed fashion, with
emphasis on aggregation by the party or parties injecting the
aggregatable routing information into the global mesh.

- The flexibility of a routing domain to be able to inject more
granular routing information to an adjacent domain to control
the resulting traffic patterns, without having an impact on the
global routing system.

In addition to describing the philosophy, we illustrate it by
presenting sample configurations. IPv4 prefixes, BGP4 and ASs
are used in examples, though the principles are applicable to
inter-domain route aggregation in general.

Address allocation policies and technologies to renumber entire
networks, while very relevant to the realization of successful
and sustained inter-domain routing, are not the focus of this
document. The references section contains pointers to relevant
documents [8, 9, 11, 12].

2. Route Aggregation Framework

The framework of inter-domain route aggregation we are proposing can
be summarized as follows:

- Aggregation from the originating AS

That is, in its outbound route announcements, each AS aggregates
the BGP routes originated by itself, by dedicated AS and by
private-ASs [10]. ("Routes originated by an AS" refers to
routes which have that AS first in the AS path attribute. For
example, routes statically configured and injected into BGP fall
into this category.)

This framework does not depend on "proxy aggregation" which
refers to route aggregation done by an AS other than the
originating AS. This preserves the capability of a multi-homed
site to control the granularity of routing information injected
into the global routing system. Since proxy aggregation involves
coordination among multiple organizations, the complexity of
doing proxy aggregation increases with the number of parties
involved in the coordination. The complexity, in turn, impacts
the practicality of proxy aggregation.

An AS shall always originate via a stable mechanism (e.g.,
static route configuration) the BGP routes for the large
aggregates from which it allocates addresses to customers. This
ensures that it is safe for its customers to use BGP "no-
export".

- Using BGP community "no-export" toward upstream providers

That is, in its route announcements toward its upstream
provider, an AS tags the BGP community "no-export" to routes it
originates that do not need to be propagated beyond its upstream
provider (e.g., prefixes allocated by the upstream provider).

This framework is illustrated in Figure 1. A "Tier 1" provider does
not use "no-export" in its announcement as it does not have an
upstream provider. However, it shall aggregate the routes it
originates in its outbound announcements towards both peer providers
and customers. An AS with an upstream provider shall aggregate the
routes it originates and use "no-export" toward its upstream provider
for routes that do not need to be propagated beyond its provider's
AS. This recursion shall apply to all levels of the routing
hierarchy.

Tier 1
+-- Provider <--+
| |
o aggregates routes | | o announces customer routes
it originates | | o aggregates routes it originates
| ^ o uses "no-export" if appropriate
|
+---> Tier 2 <--+
Provider |
V |
| |
o aggregates routes | | o announces customer routes
it originates | | o aggregates routes it originates
| | o uses "no-export" if appropriate
| |
| ^
-> Customer AS

Figure 1

This framework scales well as aggregation is done at all levels of
the routing system. It is flexible because the originating AS
controls whether routes of finer granularity are injected to, and/or
propagated by, its upstream provider. It facilitates multi-homing
without compromising route aggregation.

This framework is detailed in the following sections.

3. Aggregation from the Originating AS

It has been well recognized that address allocation and address
renumbering are keys to containing the growth of the Internet routing
table [1, 2, 8, 9, 11, 12].

Although the strategies discussed in this document do not assume a
perfect address allocation, it is strongly urged that an AS receive
allocation from its upstream service providers' address block.

3.1 Intra-Domain Aggregation

To reduce the number of routes that need to be injected into an AS,
there are a couple of principles that shall be followed:

- Carry in its BGP table the large route block allocated from its
upstream provider or an address registry (e.g., InterNIC, RIPE,
APNIC). This can be done by either static configuration of the
large block or by aggregating more specific BGP routes. The
former is recommended as it does not depend on other routes.

- Allocate sub-blocks to the access routers where further
allocation is done. That is, the address allocation shall be
done such that only a few, less specific routes (instead of many
more, specific ones) need to be known to the other routers
within the AS.

For example, a prefix of /17 can be further allocated to
different access routers as /20s which can then be allocated to
customers connected to different interfaces on that router (as
shown in Figure 2). Then in general only the /20 needs to be
injected into the whole AS. Exceptions need to be made for
multi-homed static routes.

access router
+------------+
| x.x.x.x/20 |
+------------+
| | |
| | |
/24 /22 /25

Figure 2

It is noted that rehoming of customers without renumbering even
within the same AS may lead to injection of more specific routes.
However, in general the more-specifics do not need to be advertised
outside of that AS. Such routes can either be tagged with the BGP
community "no-export" or filtered out by a prefix-based filter to
prevent them from being advertised out.

3.2 Inter-Domain Aggregation

There are at least two types of routes that need to be advertised by
an AS: routes originated by the AS and routes originated by its BGP
customers. An AS may need to advertise full routes to certain BGP
customers, in which case the routing announcements include routes
originated by non-customer ASs. Clearly an AS can, and should,
safely aggregate the routes originated by itself and by its BGP
customers multi-homed only to it (using, e.g., the dedicated-AS and

by the private-AS mechanism [10]) in its outbound announcement. But
it is far more dangerous to aggregate routes originated by customer
ASs due to multi-homing.

However, there are several cases in which a route originated by a BGP
customer (other than using the dedicated AS or private AS) does not
need to be advertised out by its upstream providers. For example,

- The route is a more-specific of the upstream provider's block.
However, the customer is either singly homed; or its connection
to this particular upstream provider is used for backup only.

- The more-specifics of a larger block are announced by the
customer in order to balance traffic over the multiple links to
the upstream provider.

Our approach to suppress such routes is to give control to the ASs
that originate the more-specifics (as seen by its upstream providers)
and let them tag the BGP community "no-export" to the appropriate
routes.

The BGP community "no-export" is a well known BGP community [6, 7].
A route with this attribute is not propagated beyond an AS boundary.
So, if a route is tagged with this community in its announcement to
an upstream provider and is accepted by the upstream provider, the
route will not be announced beyond the upstream provider's AS. This
achieves the goal of suppressing the more-specifics in the upstream
provider's outbound announcement.

In this framework, the BGP community "no-export" shall be tagged to
routes that are to be advertized to, but not propagated by, its
upstream provider. They may include routes allocated out of its
upstream provider's block or the more specific routes announced to
its upstream provider for the purpose of load balancing. This
aggregation strategy can be implemented via prefix-based filtering as
shown in the example of Section 5.

For its own protection, a downstream AS shall announce only its own
routes and its customer routes to its upstream providers. Thus, the
outbound routing announcement and aggregation policy can be expressed
as follows:

For routes originated by itself/dedicated-AS/private-AS:
tag with "no-export" when appropriate, and advertise the
large block and suppress the more-specifics

For routes originated by customer ASs:
advertise to upstream ASs

For any other routes:
do not advertise to upstream ASs

This approach is flexible and scales well as it gives control to the
party with the special needs, distributes the workload and avoids the
coordination overhead required by proxy aggregation.

4. Aggregation by a Provider

A provider shall aggregate all the routes it originates, as
documented in Section 3. The only difference is that the provider
may be providing full routes to certain BGP customers where no
outbound filtering is presently in place. Experience has shown that
inconsistent route announcement (e.g., aggregate at the interconnects
but not toward certain customers) can cause serious routing problems
for the Internet as a whole because of longest-match routing. In
certain cases announcing the more-specifics to customers might
provide for more accurate IGP metrics and could be useful for better
load-balancing. However, the potential risk seems to outweigh the
benefit, especially given the increasing complexity of connectivity
that a customer may have. As a result, every effort shall be made to
ensure consistent route aggregation for all BGP peers. This means
deploying filters for the BGP peers which receive full routes.

In summary, the aggregation strategy for a provider shall be:

- In announcing customer routes:

For routes originated by itself/dedicated-AS/private-AS:
tag with "no-export" when appropriate, and advertise the
large block and suppress the more-specifics

For routes originated by other customer ASs:
advertise

For any other routes:
do not advertise

- In announcing full routes:

For routes originated by itself/dedicated-AS/private-AS:
tag with "no-export" when appropriate, and advertise the
large block and suppress the more-specifics

For any other routes:
advertise

5. An Example

Consider the example shown in Figure 3 where AS 1000 is a "Tier 1"
provider with two large aggregates 208.128.0.0/12 and 166.55.0.0/16,
and AS 2000 is a customer of AS 1000 with a "portable address"
160.75.0.0/16 and an address 208.128.0.0/19 allocated from AS 1000.
Assume that 208.128.0.0/19 does not need to be propagated beyond AS
1000.

+----------------+
| AS 1000 |
| 208.128.0.0/12 |
| 166.55.0.0/16 |
+----------------+
|
| BGP
|
|
+----------------+
| AS 2000 |
| 208.128.0.0/19 |
| 160.75.0.0/16 |
+----------------+

Figure 3

Then, based on the framework presented, AS 1000 would

- originate and advertise the BGP routes 208.128.0.0/12 and
166.55.0.0/16, and suppress more-specifics originated by
itself/private-ASs/dedicated-ASs

- advertise the routes received from the customer AS 2000

and AS 2000 would

- originate BGP route 208.128.0.0/19 and 160.75.0.0/16

- advertise both 160.75.0.0/16 and 208.128.0.0/19 to its provider
AS 1000 and suppress the more specifics originated by
itself/private-AS/dedicated-AS, tagging the route 208.128.0.0/19
with "no-export"

- advertise both 160.75.0.0/16 and 208.128.0.0/19 to its BGP
customers (if any) and suppress the more-specifics originated by
itself/private-AS/dedicated-AS, plus any other routes the
customers may desire to receive

The sample configuration which implement these policies (in Cisco
syntax) is given in Appendix A.

6. Acknowledgments

The authors would like to thank Roy Alcala of MCI for a number of
interesting hallway discussions related to this work. The IETF's IDR
Working Group also provided many helpful comments and suggestions.

7. References

[1] Rekhter, Y. and T. Li, "An Architecture for IP Address Allocation
with CIDR", RFC1518, September 1993.

[2] Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.

[3] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC1771, March 1995.

[4] Rekhter, Y. and P., Gross, "Application of the Border Gateway
Protocol in the Internet", RFC1772, March 1995.

[5] Rekhter, Y., "Routing in a Multi-provider Internet", RFC1787,
April 1995.

[6] Chandra, R., Traina, P. and T. Li, "BGP Communities Attribute",
RFC1997, August 1996.

[7] Chen, E. and T. Bates, "An Application of the BGP Community
Attribute in Multi-home Routing", RFC1998, August 1996.

[8] Ferguson, P. and H. Berkowitz, "Network Renumbering Overview: Why
would I want it and what is it anyway?", RFC2071, January 1997.

[9] Berkowitz, H., "Router Renumbering Guide", RFC2072, January
1997.

[10] Stewart, J., Bates, T., Chandra, R., and Chen, E., "Using a
Dedicated AS for Sites Homed to a Single Provider", RFC2270,
January 1998.

[11] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
Behaviour Today", RFC2101, February 1997.

[12] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
1900, February 1996.

[13] Cisco systems, Cisco IOS Software Version 10.3 Router Products
Configuration Guide (Addendum), May 1995.

8. Authors' Addresses

Enke Chen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134-1706

Phone: +1 408 527 4652
EMail: enkechen@cisco.com

John W. Stewart, III
Juniper Networks, Inc.
385 Ravendale Drive
Mountain View, CA 94043

Phone: +1 650 526 8000
EMail: jstewart@juniper.net

A. Appendix A: Example Cisco Configuration

This appendix lists the Cisco configurations for AS 2000 of the
examples presented in Section 5. The configuration here uses the
AS-path for outbound filtering although it can also be based on BGP
community. Several route-maps are defined that can be used for
peering with the upstream provider, and for peering with customers
(announcing full routes or customer routes).

!!# inject aggregates
ip route 160.75.0.0 255.255.0.0 Null0 254
ip route 208.128.0.0 255.255.224.0 Null0 254
!
router bgp 2000
network 160.75.0.0 mask 255.255.0.0
network 208.128.0.0 mask 255.255.224.0
neighbor x.x.x.x remote-as 1000
neighbor x.x.x.x route-map export-routes-to-provider out
neighbor x.x.x.x send-community
!
!!# match all
ip as-path access-list 1 permit .*
!
!!# List of internal AS and private ASs that are safe to aggregate
ip as-path access-list 10 permit ^$
ip as-path access-list 10 permit ^64999_
ip as-path access-list 10 deny .*
!
!!# list of other customer ASs
ip as-path access-list 20 permit ^3000_

!!# List of prefixes to be tagged with "no-export"
access-list 101 permit ip 208.128.0.0 0.0.0.0 255.255.224.0 0.0.0.0
!!# Filter out the more specifics of large aggregates, and permit the rest
access-list 102 permit ip 160.75.0.0 0.0.0.0 255.255.0.0 0.0.0.0
access-list 102 deny ip 160.75.0.0 0.0.255.255 255.255.128.0 0.0.127.255
access-list 102 permit ip 208.128.0.0 0.0.0.0 255.255.224.0 0.0.0.0
access-list 102 deny ip 208.128.0.0 0.0.31.255 255.255.240.0 0.0.16.255
access-list 102 permit ip any any
!

!!# route-map with the upstream provider
route-map export-routes-to-provider permit 10
match ip address 101
set community no-export
route-map export-routes-to-provider permit 20
match as-path 10
match ip address 102

route-map export-routes-to-provider permit 30
match as-path 20
!
!!# route-map with BGP customers that desire only customer routes
route-map export-customer-routes permit 10
match as-path 10
match ip address 102
route-map export-customer-routes permit 20
match as-path 20
!
!!# route-map with BGP customers that desire full routes
route-map export-full-routes permit 10
match as-path 10
match ip address 102
route-map export-full-routes permit 20
match as-path 1
!

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