BGP Auto Summary

Auto summarizes prefixes.
- Requires subnet of network to be present.
- No mask required on network/igp statement
- Classful
Disabled by Default.

Summarization can occur anywhere, unlike OSPF and IS-IS (link state)
Config
- Under router bgp ASN process, ‘aggregate-address <network><mask>’
config arguments
- summary-only
  - suppress more specific routes
- suppress-map
  - suppress subset of more specific routes
- as-set
  - list of AS numbers and communities associated with the aggregate; inherited from more specific routes.
- advertise-map
  - attributes of prefixes matched with advertised map are inherited in aggregate address
  - Used in conjunction with as-set
    - Prevents a specific AS from being included in AS_SET of the new aggregate.
    - Circumvents loop prevention
- attribute-map | route-map
  - Used to modify attributes with aggregate address.
  - Used with as-set
    - Can set or remote attributes
    - E.g. remove “no-export” community, add MED.

Topology:

R8, R10 = ASN 810.1
R5, R2 and R3 = ASN 100
R1 and R4 = ASN 200

R10 is advertising subnets 172.16.10.0/24, 172.16.20.0/24, and 172.16.30.0/24 into BGP, which is seen in the route table of both ASN 200 routers.

summary-only config:
To summarize the subnets from R10 into ASN 200, we’ll have to add two commands to R2 and R3, which are the routers providing paths to ASN 200 for these IP ranges.