Concept Guide

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-dened 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 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 9. 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 dicult 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
Border Gateway Protocol (BGP)
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