Application of the BGP Community in Multi-home Routing

Stan Barber's Notes

BGP LOCAL_PREF Configuration in Multihome Routing

There are many multi-home customers. Common routing strategies are for load sharing or backup.

Customized LOCAL_PREF is often required since there is a lack of the Destination Preference (DPA) today.

Many providers assign higher values of LOCAL_PREF for routes from customers than from ISPs.

AS-based configuration (AS LIST) is widely used.

AS-Based Configuration & Its Drawbacks

Inefficient Process: Customer requests, provider implements, repeat the process for any change

Scalability issue for providers (more customers, more operational costs)

Prefix-based configuration is not possible without AS-path manipulation.

Hard to maintain.

How can BGP Community help?

Overview of Community

Optional, Transitive, and Variable Length

Four octet values :: Each value specifies a community

Encoding: AS:number (e.g. 3561:70)

A Community share a common policy.

Community-based Configuration

A provider defines a set of community values to mapped to asscoaites LOCAL_PREF values

A customer tapes proper community to routes requiring special LOCAL_PREF configuration by its provider. The provider apples a uniform configuration that automatically sets up the correct LOCAL_PREF based on the community values a received

This is effecient and scalable.

Examples

Community list is defined:

        ip community-list 70 permit 0x0DE90046 (3561:70)
        ip community-list 80 permit 0x0DE90050 (3561:80)
        ip community-list 90 permit 0x0DE9005A (3561:90)

Customer Backup Routes -- LOCAL_PREF 90 (3561:90)

ISP Routes -- LOCAL_PREF 80 (3561:80)

Customer-provided backups -- LOCAL_PREF 70 (3561:70)

Default -- 100

Sean Doran comments that folks could send bogus information into MCI and affect MCI's routes. Enke says that today, MCI overrides information from other ISPs before it is injected into MCI's routing network.

Enke notes that both Bay Networks and Cisco have implemented these community mechanisms.

Route-Map for BGP Community

	route-map set-customer-local-pref permit 10
	match community 70
	set local-preference 70
	route-map set-customer-local-pref premit 20
	match community 80
	set local-preference 80
etc.

Summary

Community-based configuration cab be used to replace the AS-based configuation It represents a paradigm shift in that it gives full control and distributes the configuration work to customers.

The current draft is available as internet draft entitled draft-chen-community-usage-00.txt.

Questions

Sean Doran comments that he thinks this is a good idea. He is concerned that without continuing to use route and as based filtering, there is still a potential for abuse. He'd really like to see DPA happen.

How many of MCI customers using this? 15% require special configuration and all have been migrated to this.

What about the case where a customer is connected to two different ISP customers of MCI? The DPA would be the best solution for this. That is not currently available. MEDs and Communities should be used instead.