Implement Bgp With Dell Networking Os; Additional Path (Add-Path) Support; Advertise Igp Cost As Med For Redistributed Routes - Dell Z9500 Configuration Manual

NOTE: It is possible to configure BGP peers that exchange both unicast and multicast network layer reachability
information (NLRI), but you cannot connect multiprotocol BGP with BGP. Therefore, you cannot redistribute multiprotocol
BGP routes into BGP.

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 6. Redistributed Route Rules
Command Settings
redistribute isis (IGP cost =
20)
redistribute isis route-map set
metric 50
redistribute isis metric 100
BGP Local Routing
Information Base
MED: IGP cost 20
MED: IGP cost 50
MED: IGP cost 100
MED Advertised to Peer
WITH route-map metric-type
internal
MED = 20
MED: 50 MED: 50
MED: 100
Border Gateway Protocol IPv4 (BGPv4)
MED Advertised to Peer
WITHOUT route-map
metric-type internal
MED = 0
MED: 50 MED: 50
MED: 100
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