Dell Force10 S4810P Configuration Manual page 231

Configure 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 a large number of 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.
Use the following commands in the CONFIGURATION ROUTER BGP mode to configure BGP
confederations.
Command Syntax
bgp confederation identifier
as-number
bgp confederation peers
as-number [... as-number]
Use the
show config
configuration.
Enable 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When EBGP routes become unavailable, they "flap" and the router issues both WITHDRAWN and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• is withdrawn
• is readvertised after being withdrawn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The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP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
FTOS, 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
Command Mode
CONFIG-ROUTER-BGP
CONFIG-ROUTER-BGP
All Confederation routers must be either 4-Byte or 2-Byte. You cannot have a mix of
router ASN support,
command in the CONFIGURATION ROUTER BGP mode to view the
Purpose
Specifies the confederation ID.
AS-number: 0-65535 (2-Byte) or 1-4294967295
(4-Byte)
Specifies which confederation sub-AS are peers.
AS-number: 0-65535 (2-Byte) or 1-4294967295
(4-Byte)
Border Gateway Protocol | 231