Dell S4048-ON, Configuration Manual

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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.
Table 11. 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
Border Gateway Protocol IPv4 (BGPv4)
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