Transitive attributes

• Aggregator

- Specifies IP address and AS number of the router that performed route aggregation

- Used for route tagging

One of the nontransitive optional attributes is the multi-exit discriminator (MED) attribute, which also influences the BGP route selection process. Whenever there are several links between two adjacent autonomous systems, one AS can use the MED attribute to tell another AS to prefer one of the links for specific destinations.

Transitive optional attributes include the following:

■ Aggregator: Identifies the AS and the router within that AS that created a route summarization, or aggregate.

■ Community: A numerical value that can be attached to certain routes as they pass a specific point in the network. The community value can later be examined by other routers at different points in the network for filtering or route selection purposes. BGP configuration may cause routes with a specific community value to be treated differently than others.

© 2005, Cisco Systems, Inc. BGP Overview 1-25

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