Reference Guide

204 | Border Gateway Protocol IPv4 (BGPv4)
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BGP add-path in FTOS reduces the time taken for BGP convergence by advertising multiple paths to its
peers for the same address prefix without new paths implicitly replacing the existing paths. An iBGP
speaker that receives multiple paths from its peers should calculate the best path in its own. BGP add-path
helps switchover to next new best path based on IGP convergence time when best path becomes
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.
FTOS 8.3.1.0 and later support 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 in conjunction with the route-map command, you can specify whether
a peer advertises the standard MED or uses the IGP cost as the MED.
Note the following when configuring this functionality:
• If the
redistribute command does not have any metric configured and BGP Peer out-bound 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 out-bound route-map has metric-type internal configured, BGP advertises the
metric configured in the redistribute command as MED.
• If BGP peer out-bound route-map has
metric configured, then all other metrics are overwritten by this.
Table 12-1, "Example MED advertisement," in Border Gateway Protocol IPv4 (BGPv4)gives some
examples of these rules.
Note: When redistributing static, connected or OSPF routes, there is no metric option. Simply assign the
appropriate route-map to the redistributed route.
Table 12-1. Example MED advertisement
Command Settings
BGP Local Routing
Information Base MED Advertised to Peer
WITH route-map
metric-type internal
WITHOUT route-map
metric-type internal
redistribute isis
(IGP cost = 20)
MED: IGP cost 20 MED = 20 MED = 0
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: 100