Administrator Guide
Network Next Hop Metric LocPrf Weight Path
*> 7.0.0.0/29 10.114.8.33 0 0 18508 ?
*> 7.0.0.0/30 10.114.8.33 0 0 18508 ?
*>a 9.0.0.0/8 192.0.0.0 32768 18508 701 {7018 2686 3786} ?
Configuring BGP Confederations
Another way to organize routers within an AS and reduce the mesh for IBGP peers is to configure BGP
confederations.
As with route reflectors, BGP confederations are recommended only for IBGP peering involving many IBGP
peering sessions per router. Basically, when you configure BGP confederations, you break the AS into smaller
sub-AS, and to those outside your network, the confederations appear as one AS. Within the confederation
sub-AS, the IBGP neighbors are fully meshed and the MED, NEXT_HOP, and LOCAL_PREF attributes are
maintained between confederations.
To configure BGP confederations, use the following commands.
• Specifies the confederation ID.
CONFIG-ROUTER-BGP mode
bgp 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 mode
bgp 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 ASN support.
To view the configuration, use the show config command in CONFIGURATION ROUTER BGP mode.
Enabling Route Flap Dampening
When EBGP routes become unavailable, they “flap” and the router issues both WITHDRAWN and UPDATE
notices.
A flap is when a route:
• is withdrawn
• is readvertised after being withdrawn
• has an attribute change
The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP process.
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. In Dell, that
penalty value is 1024. As time passes and the route does not flap, the penalty value 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
Border Gateway Protocol IPv4 (BGPv4) 243