Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 189

Implement BGP The following sections describe how BGP is implemented on the switch. Additional Path (Add-Path) Support The add-path feature reduces convergence times by advertising multiple paths to its peers for the same address prefix without replacing existing paths with new ones. By default, a BGP speaker advertises only the best path to its peers for a given address prefix. If the best path becomes unavailable, the BGP speaker withdraws its path from its local RIB and recalculates a new best path. This situation requires both IGP and BGP convergence and can be a lengthy process. BGP add-path also helps switchover to the next new best path when the current best path is unavailable. Advertise IGP Cost as MED for Redistributed Routes When using multipath connectivity to an external AS, you can advertise the MED value selectively to each peer for redistributed routes. For some peers you can set the internal/IGP cost as the MED while setting others to a constant pre-defined metric as MED value. Use the set metric-type internal command in a route-map to advertise the IGP cost as the MED to outbound EBGP peers when redistributing routes. The configured set metric value overwrites the default IGP cost. By using the redistribute command with the route-map command, you can specify whether a peer advertises the standard MED or uses the IGP cost as the MED. When configuring this functionality: • If the redistribute command does not have metric configured and the BGP peer outbound route-map does have metric-type internal configured, BGP advertises the IGP cost as MED. • If the redistribute command has metric configured (route-map set metric or redistribute route-type metric) and the BGP peer outbound route-map has metric-type internal configured, BGP advertises the metric configured in the redistribute command as MED. • If BGP peer outbound route-map has metric configured, all other metrics are overwritten by this configuration. NOTE: When redistributing static, connected, or OSPF routes, there is no metric option. Simply assign the appropriate route-map to the redistributed route. The following table lists some examples of these rules. Table 8. Redistributed Route Rules Command Settings BGP Local Routing Information Base MED Advertised to Peer MED Advertised to Peer WITH route-map WITHOUT route-map metric-type internal metric-type internal redistribute isis (IGP cost MED: IGP cost 20 = 20) MED = 20 MED = 0 redistribute isis routemap set metric 50 MED: IGP cost 50 MED: 50 MED: 50 MED: 50 MED: 50 redistribute isis metric 100 MED: IGP cost 100 MED: 100 MED: 100 Border Gateway Protocol IPv4 (BGPv4) 189