Dell S4048T, Configuration Manual

Page: 219 / 1277

Implement BGP with Dell Networking
OS
The following sections describe how to implement BGP on Dell Networking OS.
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.
Border Gateway Protocol IPv4 (BGPv4) 219