Convergence and scalability

1.5.e i Route reflector

General information on “BGP Route reflector”:

• Alternative to iBGP full-mesh
• Every iBGP router only peers with the RR
• Configuration is done on the designated RR, no additional configuration on peers needed
• Configuration is done per address-family
• RR adds two new attributes to a route:
  • ORIGINATOR_ID: RID of the advertising RR client/non-client
  • CLUSTER_LIST: RID of the RR, but can also be set manually, manual configuration of the BGP cluster ID needed if a cluster has multiple RRs
• RR does the best path selection for all clients
• RR only reflects routes considered as best
• RR is not allowed the change any attribute of the reflected route (loop prevention mechanism)
• Multiple RRs can exist within a single Cluster for redundancy
• Two peer types:
  • Client (RR Client): “Sits” within a cluster, no full-mesh required (only peering with RR), RR reflects client-routes to all other clients and non-clients
  • Non-Client (reg. iBGP): “Sits” outside of a cluster, full-mesh required (peering with RR and other non-clients), RR reflects non-client-routes only to all other clients
• Routes get also reflected back to the originator who denies it because of the ORIGINATOR_ID attribute
• RR clients should be only peered with the respective RRs
• RR clusters should be interconnected by the RRs because RR clients don’t examine (= ignore) the CLUSTER_LIST attribute which could lead to potential loops
• All RRs within a cluster as well as all routers not belonging to a RR cluster must be fully meshed
• Disadvantage of route reflection is that the RR makes all decisions about the best path, which can result in that the exit/egress point of a RR client is not the best one from his perspective
• Important: When using multiple RRs in the same Cluster, all clients and non-clients must be connected to both or else routes could disappear (because CLUSTER_LIST behaves like AS_PATH, therefor blocking updates which have the same CLUSTER-ID).

// Graphic missing - Coming soon //

“BGP Route reflector” CLI configuration commands:

## Configuring a BGP cluster ID manually
Router(config-router)# bgp cluster-id <value>

## Configuring a BGP neighbor as RR client
Router(config-router)# neighbor <ip> route-reflector-client

1.5.e Confederations (Not on blueprint)

General information on “BGP Confederations”:

• Alternative to iBGP full-mesh
• Multiple routers within an AS are grouped into sub-AS
• Within these sub-AS, iBGP full-mesh is still required
• eBGP neighbors “see” the confederations AS as one single AS
• The NEXT_HOP, MED and LOCAL_ PREF attributes are preserved through the whole confederated AS

“BGP Confederations” configuration considerations:

• The BGP process ID is the ASN of the sub-AS
• The main AS, to which the sub-AS belongs to, is configured within the BGP process
• Only directly peered sub-AS routers need to be explicitly specified

// Graphic missing - Coming soon //

“BGP Confederations” CLI configuration commands:

## Configuring a BGP conederations sub-AS:
Router(config)# router bgp <sub-asn>

## Configuring a BGP confederations main-AS:
Router(config-router)# bgp confederation identifier <main-asn>

## Configuring the directly connected BGP confederations peers:
Router(config-router)# bgp confederation peers <sub-asn1, sub-asn2, ...>

1.5.e ii Aggregation, AS set

General information on “BGP Aggregation”:

• Aggregation = Route summarization within BGP
• To aggregate, subnets must exist in the BGP table
• Routers don’t need to “own” (originate) the routes to do the aggregation
• By default, the aggregated address and the individual prefixes get advertised

Configuration considerations:

• Suppress-Map:
  • Configured under the aggregate-address statement.
  • Should be used when only single prefixes need to be suppressed
• Unsuppress-Map:
  • Configured under the neighbor statement.
  • Should be used when only single prefixes need to be unsuppressed.

Configuration arguments:

• summary-only: Only advertises the summary address and not the individual prefixes
• suppress-map: Based on a route-map, advertises the summary address and suppresses individual prefixes
• as-set: The advertised summary route includes all the AS’es of

“BGP Aggregation” CLI configuration commands:

## Configuring BGP aggregation
Router(config-router-af)# aggregate-address <network> <mask> [arguments]

## Configuring a BGP aggregation unsuppress-map on a per-neighbor basis
Router(config-router-af)# neighbor <ip> unsuppress-map [NAME]