Dell PowerSwitch S4820T 9.50.1 Configuration Guide for the S4820T System - Page 192

Multiprotocol BGP Multiprotocol extensions for BGP (MBGP) is defined in IETF RFC 2858. MBGP allows different types of address families to be distributed in parallel. MBGP for IPv4 multicast is supported on the S4820T platform. MBGP allows information about the topology of the IP multicast-capable routers to be exchanged separately from the topology of normal IPv4 and IPv6 unicast routers. It allows a multicast routing topology different from the unicast routing topology. MBGP uses either an IPv4 address configured on the interface (which is used to establish the IPv6 session) or a stable IPv4 address that is available in the box as the next-hop address. As a result, while advertising an IPv6 network, exchange of IPv4 routes does not lead to martian next-hop message logs. NOTE: It is possible to configure BGP peers that exchange both unicast and multicast network layer reachability information (NLRI), but you cannot connect multiprotocol BGP with BGP. Therefore, you cannot redistribute multiprotocol BGP routes into BGP. Implement BGP with Dell Networking OS The following sections describe how to implement BGP on Dell Networking OS. Additional Path (Add-Path) Support BGP add-path is supported on the S4820T platform. 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. Dell Networking OS supports configuring 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 192 Border Gateway Protocol IPv4 (BGPv4)