Users Guide

Implement BGP with the Dell Networking OS
The following sections describe how to implement BGP on the 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 prex without replacing
existing paths with new ones. By default, a BGP speaker advertises only the best path to its peers for a given address prex. 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 reduces the time taken for BGP convergence by advertising multiple paths to its peers for the same address prex 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-dened metric as MED value.
The Dell Networking OS version 8.3.1.0 and later support conguring 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 congured
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 conguring this functionality:
• If the redistribute command does not have metric congured and the BGP peer outbound route-map does have metric-
type internal congured, BGP advertises the IGP cost as MED.
• If the redistribute command has metric congured (route-map set metric or redistribute route-type
metric) and the BGP peer outbound route-map has metric-type internal congured, BGP advertises the metric congured
in the
redistribute command as MED.
• If BGP peer outbound route-map has metric congured, all other metrics are overwritten by this conguration.
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 6. 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
184 Border Gateway Protocol IPv4 (BGPv4)