1.5.e Convergence and scalability

1.5.e i Route reflector

A route reflector is a BGP router that is allowed to ignore the iBGP loop avoidance rule and as such, is allowed to advertise updates received from an iBGP peer to another iBGP peer under specific conditions. The deployment of a route reflector can eliminate the BGP full-mesh requirement and greatly simplifies the building of an iBGP network. A route reflector is essentially a route server, serving routes to route reflector clients and non-client peers. Any routes received from a route reflector clients, are effectively ‘reflected’ to both client and non-client neighbors.

1.5.e ii Aggregation, as-set

When you use a BGP aggregate-address command without any additional keywords, you will lose any information of route attributes such as AS-Path which can cause problems since this is a loop prevention mechanism.  All is not lost however, as we can use the aggregate-address command in conjunction with the following:

aggregate-address <network address> <mask> summary-only as-set

Aggregation (other options)

For more controlled summarization, use the command:

aggregate-address <summary address> <subnet mask> [args]

Arguments (args)

• summary-only
  • suppress more specific routes
• suppress-map
  • suppress subset of more specific routes
• as-set
  • un-order list of AS numbers and communities are associated with the
    aggregate; inherited from more specific routes.
• advertise-map
  • attributes of prefix or prefixes matched with advertised-map are inherited in
    aggregate address
  • Used in conjunction with as-set
    • Prevents a specific AS from being included in the AS_SET of the new
      aggregate
    • Circumvents loop prevention
• attribute-map | route-map
  • Used to modify attributes associated with aggregate address
  • Used in conjunction with as-set
    • Can set or remove attributes
    • E.g. remove “no-export” community, add MED