Manual Summaries and the AS_PATH Path Attribute

The aggregate route must include the AS_PATH PA, just like it is required for every other NLRI
in the BGP table. However, to fully understand what this command does, you need to take a closer
look at the AS_PATH PA.
The AS_PATH PA consists of up to four different components, called segments, as follows:
■ AS_SEQ (short for AS Sequence)
■ AS_SET
■ AS_CONFED_SEQ (short for AS Confederation Sequence)
■ AS_CONFED_SET

The most commonly used segment is called AS_SEQ. AS_SEQ is the idea of AS_PATH , However, the aggregate-address command can create a summary route for which the AS_SEQ
must be null. When the component subnets of the summary route have differing AS_SEQ values,
the router simply can’t create an accurate representation of AS_SEQ, so it uses a null AS_SEQ.
However, this action introduces the possibility of creating routing loops, because the contents of
AS_PATH, specifically AS_SEQ, are used so that when a router receives an update, it can ignore
prefixes for which its own ASN is listed.



The following list summarizes the actions taken by the aggregate-address command when it
creates a summary route:
■ It does not create the summary if the BGP table does not currently have any routes for NLRI
inside the summary.
■ If all the component subnets are withdrawn from the aggregating router’s BGP table, it also
then withdraws the aggregate. (In other words, the router tells its neighbors that the aggregate
route is no longer valid.)
■ It sets the NEXT_HOP address of the summary, as listed in the local BGP table, as 0.0.0.0.
■ It sets the NEXT_HOP address of the summary route, as advertised to neighbors, to the
router’s update source IP address for each neighbor, respectively.
■ If the component subnets inside the summary all have the same AS_SEQ, it sets the new
summary route’s AS_SEQ to be exactly like the AS_SEQ of the component subnets.
■ If the AS_SEQ of the component subnets differs in any way, it sets the AS_SEQ of the new
summary route to null.
■ When the as-set option has been configured, the router creates an AS_SET segment for the
aggregate route, but only if the summary route’s AS_SEQ is null.
■ As usual, if the summary is advertised to an eBGP peer, the router prepends its own ASN to
the AS_SEQ before sending the Update.
■ It suppresses the advertisement of all component subnets if the summary-only keyword is
used; advertises all of them if the summary-only keyword is omitted; or advertises a subset
if the suppress-map option is configured.