Best Path Change Suppression; Bgp Update Generation And Update Groups - Cisco NCS 5500 Series Configuration Manual

Implementing BGP
3 A set of paths is formed containing the best path selected from each group in Step 2. The overall best path
is selected from this set of paths, by iterating through them as in Step 2.

Best Path Change Suppression

The third part of the implementation is to determine whether the best-path change can be suppressed or
not—whether the new best path should be used, or continue using the existing best path. The existing best
path can continue to be used if the new one is identical to the point at which the best-path selection algorithm
becomes arbitrary (if the router-id is the same). Continuing to use the existing best path can avoid churn in
the network.
Note This suppression behavior does not comply with the IETF Networking Working Group
draft-ietf-idr-bgp4-24.txt document, but is specified in the IETF Networking Working Group
draft-ietf-idr-avoid-transition-00.txt document.
The suppression behavior can be turned off by configuring the bgp bestpath compare-routerid command.
If this command is configured, the new best path is always preferred to the existing one.
Otherwise, the following steps are used to determine whether the best-path change can be suppressed:
1 If the existing best path is no longer valid, the change cannot be suppressed.
2 If either the existing or new best paths were received from internal (or confederation) peers or were locally
generated (for example, by redistribution), then the change cannot be suppressed. That is, suppression is
possible only if both paths were received from external peers.
3 If the paths were received from the same peer (the paths would have the same router-id), the change cannot
be suppressed. The router ID is calculated using rules in
4 If the paths have different weights, local preferences, origins, or IGP metrics to their next hops, then the
change cannot be suppressed. Note that all these values are calculated using the rules in
of Paths, on page
5 If the paths have different-length AS paths and the bgp bestpath as-path ignore command is not configured,
then the change cannot be suppressed. Again, the AS path length is calculated using the rules in
Pairs of Paths, on page
6 If the MED of the paths can be compared and the MEDs are different, then the change cannot be suppressed.
The decision as to whether the MEDs can be compared is exactly the same as the rules in
of Paths, on page
7 If all path parameters in Step 1 through Step 6 do not apply, the change can be suppressed.

BGP Update Generation and Update Groups

The BGP Update Groups feature separates BGP update generation from neighbor configuration. The BGP
Update Groups feature introduces an algorithm that dynamically calculates BGP update group membership
based on outbound routing policies. This feature does not require any configuration by the network operator.
Update group-based message generation occurs automatically and independently.
130.
130.
130, as is the calculation of the MED value.
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
Information about Implementing BGP
Comparing Pairs of Paths, on page 130.
Comparing Pairs
Comparing
Comparing Pairs
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