BGP - Route Summarization

4 important questions on BGP - Route Summarization

What are two techniques for BGP Route Summarization?

1. Static: Creating a Null0 route for the summary network prefix and advertise it using a Network statement. Downfall is that it will always be advertised, even if the networks are not available.

2. Dynamic: Configure an aggregate network prefix. When viable component routes that match the aggregate network prefix enter the BGP table, then the aggregate prefix is created. The originating router sets the next-hop to Null0 as a discard route for the aggregated prefix to prevent loops.

What BGP command is used for Dynamic Route Summarization?

In the BGP Address Family configuration: Aggregate-address network subnet-mask [Summary-only][as-set]

With the Summary-Only option the component network prefixes are prevented from being advertised.

What does the BGP attribute Atomic Aggregate indicate?

It indicates that a route is learned from a router where the route is summarized and path attributes like AS_Path, MED and BGP Communities have been lost.
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What is the AS_SET option in the BGP Summary command used for?

It is used to indicate that the AS_PATH attribute needs to be copied from the component routes to the aggregate route.
It is stored in AS_SET attribute which is displayed within brackets.

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