Dell Force10 Z9000 FTOS Configuration Guide for Z9000 System - Page 217

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] Command Mode Purpose CONFIG-ROUTER-BGP Specifies the confederation ID. AS-number: 0-65535 (2-Byte) or 1-4294967295 (4-Byte) CONFIG-ROUTER-BGP Specifies which confederation sub-AS are peers. AS-number: 0-65535 (2-Byte) or 1-4294967295 (4-Byte) All Confederation routers must be either 4-Byte or 2-Byte. You cannot have a mix of router ASN support, Use the show config command in the CONFIGURATION ROUTER BGP mode to view the 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 Enable route flap dampening When EBGP routes become unavailable, they "flap" and the router issues both WITHDRAWN and UPDATE notices. A flap is when a route • is withdrawn • is readvertised after being withdrawn • has an attribute change The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP process.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 Border Gateway Protocol | 217