Users Guide

*> 9.141.128.0/24 10.114.8.33 0 18508 701 7018 2686 ?
Dell#
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When EBGP routes become unavailable, they “flap” and the router issues both WITHDRAWN and UPDATE notices.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• is withdrawn
• is readvertised after being withdrawn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The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP process.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 Networking OS, 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
• Readvertise
• Attribute change
When dampening is applied to a route, its path is described by one of the following terms:
• history entry — an entry that stores information on a downed route
• dampened path — a path that is no longer advertised
• penalized path — a path that is assigned a penalty
Border Gateway Protocol IPv4 (BGPv4) 201