Setup Guide

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 10. 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
Ignore Router-ID in Best-Path Calculation
You can avoid unnecessary BGP best-path transitions between external paths under certain conditions. The bgp bestpath
router-id ignore command reduces network disruption caused by routing and forwarding plane changes and allows for
faster convergence.
AS Number Migration
With this feature you can transparently change the AS number of an entire BGP network and ensure that the routes are
propagated throughout the network while the migration is in progress.
When migrating one AS to another, perhaps combining ASs, an eBGP network may lose its routing to an iBGP if the ASN
changes. Migration can be difficult as all the iBGP and eBGP peers of the migrating network must be updated to maintain
network reachability. Essentially, Local-AS provides a capability to the BGP speaker to operate as if it belongs to "virtual" AS
network besides its physical AS network.
The following illustration shows a scenario where Router A, Router B, and Router C belong to AS 100, 200, and 300,
respectively. Router A acquired Router B; Router B has Router C as its customer. When Router B is migrating to Router
A, it must maintain the connection with Router C without immediately updating Router C’s configuration. Local-AS allows
this behavior to happen by allowing Router B to appear as if it still belongs to Router B’s old network (AS 200) as far as
communicating with Router C is concerned.
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Border Gateway Protocol (BGP)