Bgp Best-External Path - Cisco NCS 6000 Series Configuration Manual

Implementing BGP
• When provisioning NSR for line cards installed on a single rack, provision the active and standby
• When provisioning NSR for line cards installed on different racks, use one of the following three options:

BGP Best-External Path

The Border Gateway Protocol (BGP) best–external path functionality supports advertisement of the
best–external path to the iBGP and Route Reflector peers when a locally selected bestpath is from an internal
peer.
BGP selects one best path and one backup path to every destination. By default, selects one best path .
Additionally, BGP selects another bestpath from among the remaining external paths for a prefix. Only a
single path is chosen as the best–external path and is sent to other PEs as the backup path.
BGP calculates the best–external path only when the best path is an iBGP path. If the best path is an eBGP
path, then best–external path calculation is not required.
The procedure to determine the best–external path is as follows:
1. Determine the best path from the entire set of paths available for a prefix.
2. Eliminate the current best path.
3. Eliminate all the internal paths for the prefix.
4. From the remaining paths, eliminate all the paths that have the same next hop as that of the current best
path.
5. Rerun the best path algorithm on the remaining set of paths to determine the best–external path.
BGP considers the external and confederations BGP paths for a prefix to calculate the best–external path.
BGP advertises the best path and the best–external path as follows:
applications on the distributed route processor (DRP) of that rack. If a rack failure occurs, sessions are
dropped, because all line cards go down.
• Provision the active and standby applications on a distributed route processor (DRP) redundant pair,
where there is a separate route processor in each rack. This configuration uses up two
revenue-producing line-card slots on each rack, but is the most secure configuration.
• Provision the active and standby applications on a distributed route processor (DRP) pair that spans
two racks. In this configuration, the active/standby role of the line cards is not dependent on the
active/standby role of the DRPs. This is called flexible process redundancy and provides for rack
loss and efficient use of LC slots. Use of distributed BGP is not required with this solution.
Note Sessions on line cards in a lost rack are not protected with any of the above
options, because there is no line-card redundancy. These options ensure only that
sessions on other racks are not affected by a lost rack. However, lost sessions
from a lost rack may cause some traffic loss on other racks, because destinations
learned through those lost sessions may no longer have alternate routes. Also,
rack loss may cause the CPUs on route processors of active racks to slow as they
attempt to define new paths for some routes.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
BGP Best-External Path
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