Configuring BGP ConfederationsAnother way to organize routers within an AS and reduce the mesh for IBGP peers is to configure BGPconfederations.As with route reflectors, BGP confederations are recommended only for IBGP peering involving manyIBGP peering sessions per router. Basically, when you configure BGP confederations, you break the ASinto smaller sub-AS, and to those outside your network, the confederations appear as one AS. Within theconfederation sub-AS, the IBGP neighbors are fully meshed and the MED, NEXT_HOP, and LOCAL_PREFattributes are maintained between confederations.To configure BGP confederations, use the following commands.• Specifies the confederation ID.CONFIG-ROUTER-BGP modebgp confederation identifier as-number– as-number: from 0 to 65535 (2 Byte) or from 1 to 4294967295 (4 Byte).• Specifies which confederation sub-AS are peers.CONFIG-ROUTER-BGP modebgp confederation peers as-number [... as-number]– as-number: from 0 to 65535 (2 Byte) or from 1 to 4294967295 (4 Byte).All Confederation routers must be either 4 Byte or 2 Byte. You cannot have a mix of router ASNsupport.To view the configuration, use the show config command in CONFIGURATION ROUTER BGP mode.Enabling Route Flap DampeningWhen EBGP routes become unavailable, they “flap” and the router issues both WITHDRAWN and UPDATEnotices.A flap is when a route:• is withdrawn• is readvertised after being withdrawn• has an attribute changeThe constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGPprocess. To minimize this instability, you may configure penalties (a numeric value) for routes that flap.When that penalty value reaches a configured limit, the route is not advertised, even if the route is up. InDell Networking OS, that penalty value is 1024. As time passes and the route does not flap, the penaltyvalue decrements or is decayed. However, if the route flaps again, it is assigned another penalty.The penalty value is cumulative and penalty is added under following cases:• Withdraw• Readvertise• Attribute changeWhen dampening is applied to a route, its path is described by one of the following terms:214 Border Gateway Protocol IPv4 (BGPv4)
