Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 187

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 metrictype 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 routemap to the redistributed route. The following table lists some examples of these rules. Table 8. Redistributed Route Rules Command Settings BGP Local Routing Information MED Advertised to Peer Base WITH route-map metric-type internal redistribute isis (IGP cost = 20) MED: IGP cost 20 MED = 20 redistribute isis route-map set metric 50 MED: IGP cost 50 MED: 50 MED: 50 redistribute isis metric 100 MED: IGP cost 100 MED: 100 MED Advertised to Peer WITHOUT route-map metrictype internal MED = 0 MED: 50 MED: 50 MED: 100 Ignore Router-ID in Best-Path Calculation You can avoid unnecessary BGP best-path transitions between external paths under certain conditions. The bgp bestpath routerid ignore command reduces network disruption caused by routing and forwarding plane changes and allows for faster convergence. Four-Byte AS Numbers You can use the 4-Byte (32-bit) format when configuring autonomous system numbers (ASNs). The 4-Byte support is advertised as a new BGP capability (4-BYTE-AS) in the OPEN message. If a 4-Byte BGP speaker has sent and received this capability from another speaker, all the messages will be 4-octet. The behavior of a 4-Byte BGP speaker is different with the peer depending on whether the peer is a 4-Byte or 2-Byte BGP speaker. Where the 2-Byte format is 1-65535, the 4-Byte format is 1-4294967295. Enter AS numbers using the traditional format. If the ASN is greater than 65535, the dot format is shown when using the show ip bgp commands. For example, an ASN entered as 3183856184 appears in the show commands as 48581.51768; an ASN of 65123 is shown as 65123. To calculate the comparable dot format for an ASN from a traditional format, use ASN/65536. ASN%65536. Border Gateway Protocol version 4 (BGPv4) 187