Rpl - If Prefix Is-Best-Path/Is-Best-Multipath - Cisco ASR 9000 Series Configuration Manual

Attach Points
The following example shows how to generate a default route if any of the routes that match 10.0.0.0/8 ge 8
le 25 are present in the RIB:
route-policy ospf-originate
if rib-has-route in (10.0.0.0/8 ge 8 le 25) then
pass
endif
end-policy
router ospf 1
default-information originate policy ospf-originate
.
.
.

RPL - if prefix is-best-path/is-best-multipath

Border Gateway Protocol (BGP) routers receive multiple paths to the same destination. As a standard, by
default the BGP best path algorithm decides the best path to install in IP routing table. This is used for traffic
forwarding.
BGP assigns the first valid path as the current best path. It then compares the best path with the next in the
list. This process continues, until BGP reaches the end of the list of valid paths. This contains all rules used
to determine the best path. When there are multiple paths for a given address prefix, BGP:
• Selects one of the paths as the best path as per the best-path selection rules.
• Installs the best path in its forwarding table. Each BGP speaker advertises only the best-path to its peers.
Note The advertisement rule of sending only the best path does not convey the full routing state of a destination,
present on a BGP speaker to its peers.
After the BGP speaker receives a path from one of its peers; the path is used by the peer for forwarding packets.
All other peers receive the same path from this peer. This leads to a consistent routing in a BGP network. To
improve the link bandwidth utilization, most BGP implementations choose additional paths satisfy certain
conditions, as multi-path, and install them in the forwarding table. Incoming packets for such are load-balanced
across the best-path and the multi-path(s). You can install the paths in the forwarding table that are not
advertised to the peers. The RR route reflector finds out the best-path and multi-path. This way the route
reflector uses different communities for best-path and multi-path. This feature allows BGP to signal the local
decision done by RR or Border Router. With this new feature, selected by RR using community-string (if
is-best-path then community 100:100). The controller checks which best path is sent to all R's. Border Gateway
Protocol routers receive multiple paths to the same destination. While carrying out best path computation
there will be one best path, sometimes equal and few non-equal paths. Thus, the requirement for a best-path
and is-equal-best-path.
The BGP best path algorithm decides the best path in the IP routing table and used for forwarding traffic. This
enhancement within the RPL allows creating policy to take decisions. Adding community-string for local
selection of best path. With introduction of BGP Additional Path (Add Path), BGP now signals more than the
best Path. BGP can signal the best path and the entire path equivalent to the best path. This is in accordance
to the BGP multi-path rules and all backup paths.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
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Implementing Routing Policy
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