BGP: Manual Summaries and AS_PATH Attribute

BGP can manually summarize advertised routes using the aggregate-address command differs from using auto-summary command.  It can summarize based on any routes in the BGP table, creating a summary of any prefix length.  It does not always suppress the advertisement of the component subnets, although it can be configured to do so.

The aggregate route must include the AS_PATH PA, this consists of up to four components or segments:

  • AS_SEQ
  • AS_STE
  • AS_CONFED_SEQ
  • AS_CONFED_SET

AS_SEQ lists all ASNs in order through which the route has been advertised.

Aggregate-address configuration requires the AS_SEQ to be null.  When component subnets of the summary route have different AS_SEQ values the router cannot create an accurate representation of AS_SEQ so it uses a null AS_SEQ.  This however opens the door to routing loops, bypassing thr native rule of ignoring prefixes whose AS_SEQ includes the local ASN of the router.

AS_PATH_SET segment solves this issue when the summary has a null AS_SEQ.  The AS_SET segment holds the unordered list of all the ASNs in all the component subnets AS_SEQ segments.

This list summarizes the actions generated by aggregate-address:

  • Does not create a summary if BGP table does not have any routes for the NLRI inside the summary
  • If all component subnets are withdrawn from the aggregating routers BGP RIB it also withdraws the aggregate route.
  • It sets the NEXT_ HOP address of the summary to 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 AS_SEQ to be exactly like the AS_SEQ of the component subnets
  • If the AS_SEQ of the component subnet 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 the if it is omitted, or advertises a subset using the suppress-map option configured.
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