BGP Route Summarisation

By reducing the number of routes presented, it can reduce the router resources required to operate the BGP process and reduce the overall size of the BGP table.

By summarising routes dynamic route flaps from downstream routers can be hidden from BGP advertisements, providing stability.

There are two main techniques for summarising routes:

Static

Create a static route with a destination next-hop of Null0, and then advertise that prefix out of BGP with the network statement.

The summary route is always advertised, although it will still be advertised even if the summarised networks are not available.

Dynamic

With a aggregation network prefix created, and routes that match the aggregate network prefix are entered into the BGP table, the summarised/aggregated network prefix is entered into the BGP table. The next-hop of this route is set to Null0 to prevent route loops.

Aggregate Routes

Dynamic aggregate route summarisation is accomplished with the command aggregate-address 192.168.0.0 255.255.0.0 with option keywords of summary-only and as-set

Without the summary-only suffix, the non-summarised routes and the summarised route will be be advertised into the BGP table. To advertise only the aggregated/summarised routes, use the suffix summary-only

Atomic Aggregate

Aggregated routes act like brand new BGP routes with a shorter prefix length. When being advertised as a summarised route, BGP does not advertise the AS_Path information from prior to the summarisation. The same is true for the MED and BGP communities.

The attribute ‘Atomic Aggregate’ indicates to other routes via a flag that a loss of path information has occurred.

To keep this AS_Path history, use the suffix as-set on the aggregate-address command