Users Guide

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 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.
Four-Byte AS Numbers
You can use the 4-Byte (32-bit) format when configuring autonomous system numbers (ASNs).
The 4-Byte support is advertised as a new BGP capability (4-BYTE-AS) in the OPEN message. If a 4-Byte BGP speaker has sent and
received this capability from another speaker, all the messages will be 4-octet. The behavior of a 4-Byte BGP speaker is different with the
peer depending on whether the peer is a 4-Byte or 2-Byte BGP speaker.
Where the 2-Byte format is 1-65535, the 4-Byte format is 1-4294967295. Enter AS numbers using the traditional format. If the ASN is
greater than 65535, the dot format is shown when using the show ip bgp commands. For example, an ASN entered as 3183856184
appears in the show commands as 48581.51768; an ASN of 65123 is shown as 65123. To calculate the comparable dot format for an ASN
from a traditional format, use ASN/65536. ASN%65536.
Traditional
Format
DOT Format
65001 0.65501
154 Border Gateway Protocol IPv4 (BGPv4)