Reference Guide

Implement BGP with FTOS
The following sections describe how to implement BGP on FTOS.
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.
FTOS version 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 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
WITH route-map metric-
type internal
MED Advertised to Peer
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 MED: 50 MED: 50
redistribute isis metric 100 MED: IGP cost 100 MED: 100 MED: 100
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