BGP Summary and Aggregated Routes

I spent this morning briefly covering how to summarize routes in BGP using CIDR Aggregated Routes. BGP specifically uses the Atomic Aggregate attribute which is considered one of the well-known discretionary attributes. BGP also uses the optional transitive attribute called an Aggregator which specifies the BGP ID and the AS that performed the aggregation in BGP updates. If you aren't careful when planning which routes to summarize your AS could easily claim routes that it really doesn't own which could upset other AS's in the BGP system! AS's doesn't really use aggregation as much as they could because some are multihomed to many ISP's and would rather make sure that all of the routes that own are being advertised without being summarized into one route.