DELL 9.7(0.0) User Manual
Implement BGP
The following sections describe how BGP is implemented on the Z9500 switch.
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.
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.
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
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.
assign the appropriate route-map to the redistributed route.
The following table lists some examples of these rules.
Table 6. Redistributed Route 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)
= 20)
MED: IGP cost 20
MED = 20
MED = 0
redistribute isis route-
map set metric 50
map set metric 50
MED: IGP cost 50
MED: 50 MED: 50
MED: 50 MED: 50
redistribute isis metric
100
100
MED: IGP cost 100
MED: 100
MED: 100
Border Gateway Protocol IPv4 (BGPv4)
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