RFC1998 - An Application of the BGP Community Attribute in Multi-home Routing

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　　Network Working Group E. Chen
Request for Comments: 1998 MCI
Category: Informational T. Bates
cisco Systems
August 1996

An Application of the BGP Community Attribute
in Multi-home Routing

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

This document presents an application of the BGP community attribute
[2] in simplifying the implementation and configuration of routing
policies in the multi-provider Internet. It shows how the community
based configuration can be used to replace the AS-based customization
of the BGP "LOCAL_PREF" attribute, a common method used today. Not
only does the technique presented simplifies configuration and
management at the provider level, it also represents a paradigm shift
in that it gives the potential for the customer to control its own
routing policy with respect to its service provider, as well as
providing the ability for policy configuration to be done at a prefix
based granularity rather than the more common AS based granularity.

1. Introduction

In the multi-provider Internet, it is common for a service subscriber
(i.e., customer) to have more than one service provider, or to have
arrangements for redundant connectivity to the global connected
Internet. As discussed in [3], routing strategies in these cases
usually require coordination between the service subscriber and its
providers, which typically leads to customization of router
configurations (e.g., BGP "LOCAL_PREF") not only by the subscriber,
but also by its providers. Due to the large number of customers a
provider serves, customization of router configurations at the
provider level may present management and scalability problems.

This document presents an application of the BGP community attribute
in simplifying the implementation of routing strategies in the
multi-provider Internet. More specifically, the technique presented
uses a community-based, rather than the common AS-based,

configuration of the BGP "LOCAL_PREF". It essentially removes the
need for customized configuration of the BGP "LOCAL_PREF" attribute
at the provider level while maintaining the same level of routing
functionality and flexibility.

It also represents a paradigm shift in that it gives the potential
for the customer to control its own routing policy with respect to
its service provider, as well as providing the ability for policy
configuration to be done at a prefix based granularity rather than
the more common AS based granularity in use today.

2. AS-based Configuration and its Drawbacks

As discussed in [3], in today's multi-provider Internet, customized
configuration of the BGP "LOCAL_PREF" attribute is often required to
implement common routing strategies such as load-sharing or backup.
There are two main reasons:

o Lack of available implementations and deployment of routing
software that supports the "Destination Preference Attribute"
(DPA) as specified in [4].

DPA allows one to specify a globally transitive preference so
that return traffic favors certain path. As discussed in [3],
the attribute will be very useful in influencing route selection
for routes with identical "LOCAL_PREF" and equal AS-path length.

o In the multi-provider Internet, it is common for a provider
to assign higher BGP "LOCAL_PREF" values for routes from its
customers than from other service providers. This practice
provides some degree of protection for its customer routes,
and it facilitates implementation of certain routing
strategies. It, however, also complicates other routing
implementations such as backup arrangement, thus, requiring
customized "LOCAL_PREF" configuration.

Figure 1 shows a typical case of a backup arrangement in the multi-
provider Internet. In Figure 1, AS1 and AS2 are both providers, and
AS3 and AS4 are customers of AS1 and AS2, respectively. AS3 has
entered a bilateral agreement with AS4 to provide backup to each
other. That is, AS3 would use its direct link to AS4 to reach only
AS4 in the normal circumstance, and for transit in the case of a
failure between AS3 and AS1. To realize this routing agreement, AS3
requests that its provider AS1 adjust its BGP "LOCAL_PREF"
configuration so that AS1 reaches AS4 via AS2.

+------+ +------+
| AS1 |------| AS2 |
+------+ +------+
| |
+------+ +------+
| AS3 |------| AS4 |
+------+ +------+

Figure 1: Typical Backup Scenario

Primarily due to scalability and management concerns, most providers
only perform "LOCAL_PREF" customization based on ASs, not on IP
prefixes. If IP prefix-based "LOCAL_PREF" configuration is needed, a
technique known as as the BGP AS-path manipulation can be used.
However, it is currently only available in certain vendor's products.

There are several drawbacks with the the practice of AS-based BGP
"LOCAL_PREF" configuration at the provider level:

o The implementation tends to less efficient due to the process
of coordination and configuration. More importantly, the
process needs to be repeated each time a change (e.g., adding
a new AS) occurs.

o The AS-based customization complicates router configuration
and increases complexity of network operation. It has become
a serious scalability issue for providers.

o It can not implement prefix-based configuration without the
AS-path manipulation (i.e., using fake AS).

o Keeping configuration up-to-date is some times problematic.

3. How the BGP Community Attribute Can Help

3.1 Overview of the Community Attribute

The BGP community path attribute is an optional transitive attribute
of variable length [1,2]. The attribute consists of a set of four
octet values, each of which specify a community. The community
attribute values are encoded using an AS number in the first two
octets, with the remaining two octets defined by the AS. As defined
in [2], a community is a group of destinations (i.e. prefixes) that
share some common attribute. Each destination can belong to multiple
communities. All prefixes with the community attribute belong to the
communities listed in the attribute.

The BGP community allows one to group a set of prefixes and perform
routing decisions based on the identity of the group.

The well-known communities NO_EXPORT (0xFFFFFF01) and NO_ADVERTISE
(0xFFFFFF02) are intuitive, and can be used for optimizing routing
and for improving route aggregation.

3.2 Community-based Configuration

With the BGP community attribute [2], a provider can now use
community-based, rather than AS-based, configuration of BGP
"LOCAL_PREF". The provider first needs to coordinate with its
customers a set of communities to be mapped to certain BGP
"LOCAL_PREF" values. The provider can then apply a uniform BGP
configuration to all its customers that would capture routes with the
community values, and set up the appropriate BGP "LOCAL_PREF" values
accordingly. A customer that requires customization in its provider
BGP "LOCAL_PREF" configuration can simply send the appropriate
community values in its routing announcements.

The major advantages of using this technique include:

o The customer has full control in the process, which makes a
lot of sense as the customer is in a position to have better
understanding about its own topology and routing policy
requirement.

o The effect of route-based customization in BGP "LOCAL_PREF"
configuration by providers can now be achieved, thus, removing
the need of AS-Path manipulation in certain cases.

o It addresses the scalability issue facing providers as it
distributes the configuration work to the customer that
requires customization.

4. A Real-World Implementation Example

MCI currently makes heavy use of the BGP "LOCAL_PREF" attribute value
as part of its routing policy configuration process. Different BGP
"LOCAL_PREF" values are assigned for routes from different sources.
Table 1 details these values:

+-------------------------+------------+
| Category | LOCAL_PREF |
+-------------------------+------------+
|Customer Routes | 100 |
|Customer backup Routes | 90 |
|Other ISP Routes | 80 |
|Customer-Provided backup | 70 |
+-------------------------+------------+

Table 1: Defined LOCAL_PREF Values

Note:

o The value '100' is the default value used within our network
configuration.

o In most cases, the MED attribute set by a customer is
sufficient for customer backup routes (e.g., T1 backs up T3).
However, in certain cases configuration of "LOCAL_PREF" will
still be necessary until the BGP DPA attribute is available.

To make use of the BGP community attribute, several community values
(MCI's AS number: 3561 = 0x0DE9) have been defined that can be used
by customers to tag routes so that the appropriate "LOCAL_PREF"
values are configured. Table 2 lists the appropriate community
attribute values (and the mappings of community to LOCAL_PREF):

+---------------------+------------+
| community | LOCAL_PREF |
+---------------------+------------+
|3561:70 (0x0DE90046) | 70 |
|3561:80 (0x0DE90050) | 80 |
|3561:90 (0x0DE9005A) | 90 |
+---------------------+------------+

Table 2: Community to LOCAL_PREF Mapping

A customer requiring MCI to configure BGP "LOCAL_PREF" values other
than the default can tag their routes with the defined communities.
The community values can be configured either based on an AS path
list or an IP address access list. A cisco systems software specific
configuration example is given in Appendix A to show how this can be
achieved.

A uniform BGP configuration (see Appendix B, again cisco systems
software specific) is applied by MCI to peers with customers that
configure the appropriate "LOCAL_PREF" values based on the
communities received.

This technique has been tested and is in use with several customers,
and the response has been very positive. We are in the process of
migrating all other customized BGP "LOCAL_PREF" configurations to
this uniform community based configuration approach.

5. References

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

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

[3] Chen, E., and T. Bates, "Current Practice of Implementing
Symmetric Routing and Load Sharing in the Multi-Provider
Internet", Work in Progress.

[4] Chen, E., and T. Bates, "Destination Preference Attribute for
BGP", Work in Progress.

[5] Chen, E., and T. Bates, "Application of the BGP Destination
Preference Attribute in Implementing Symmetric Routing",
Work in Progress.

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

6. Security Considerations

Security issues are not discussed in this memo.

7. Acknowledgments

The authors would specifically like to thank Ravi Chandra, Tony Li
and Paul Traina of cisco systems for devising and implementing the
community attribute.

8. Authors' Addresses

Enke Chen
MCI
2100 Reston Parkway
Reston, VA 22091

Phone: +1 703 715 7087
EMail: enke@mci.net

Tony Bates
cisco Systems
170 West Tasman Drive
San Jose, CA 95134

Phone: +1 408 527 2470
EMail: tbates@cisco.com

Appendix

These appendices list cisco systems software specific configuration
examples for configuring communities, and for uniform route-map
definition that sets up the appropriate "LOCAL_PREF" values based on
the corresponding community values. These examples are given purely
to show a working example of how the desired effect discussed in this
document can be achieved. Please refer to [6] for more specific
information on cisco configuration and syntax.

Appendix A. Community Configuration

The community values can be configured either based upon an AS path
list or based an IP address access list. Here is an example that
includes both cases:

!!
router bgp xxxx
neighbor x.x.x.x remote-as 3561
neighbor x.x.x.x filter-list 20 out
neighbor x.x.x.x route-map config-community out
neighbor x.x.x.x send-community
!
!!# match all
ip as-path access-list 1 permit .*
!
!!# list of customer ASs
ip as-path access-list 20 permit ^$
ip as-path access-list 20 permit ^64700_
ip as-path access-list 20 deny .*
!
!!# AS path based matching, backup for another ISPs customer
ip as-path access-list 40 permit _64710_
ip as-path access-list 40 permit _64711_
ip as-path access-list 40 deny .*
!
!!# route-map
route-map config-community permit 10
match as-path 40
set community 0x0DE90046
route-map config-community permit 20
match as-path 1
!

Note: The community can also be configured based on IP prefixes
instead of AS numbers. For example,

!
access-list 101 permit ip 192.160.154.0 0.0.0.0 255.255.255.0 0.0.0.0
!
route-map config-community permit 10
match ip address 101
set community 0x0DE90046
route-map config-community permit 20
match as-path 1
!

Appendix B. Uniform Route-map Configuration

Here is the uniform route-map that can be used for all BGP
customers:

!!# routes primary via another ISP
ip community-list 70 permit 0x0DE90046
ip community-list 70 deny
!
!!# routes also homed to another ISP, but with DPA or
!!# AS-path length as the tie-breaker
ip community-list 80 permit 0x0DE90050
ip community-list 80 deny
!
!!# customer backup routes
ip community-list 90 permit 0x0DE9005A
ip community-list 90 deny
!
!!# the route-map applied to BGP customers
route-map set-customer-local-pref permit 10
match community 70
set local-preference 70
route-map set-customer-local-pref permit 20
match community 80
set local-preference 80
route-map set-customer-local-pref permit 30
match community 90
set local-preference 90
route-map set-customer-local-pref permit 40
match as-path 1
set local-preference 100
!

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