Information Technology Reference
In-Depth Information
and the oldest path is listed at the end. In the output of
show ip bgp
, the newest path is listed
at the top, and the oldest path is listed at the bottom. To select the best path for a given
destination, BGP generally uses a sequential comparison method. It assigns the first path
(the newest path) as the current best path. It then compares the current best path to the next
path in the list until it reaches the end of the list of valid paths. For example, for three paths
received sequentially—1, 2, and 3—BGP first compares paths 3 (received last) and 2. The
resulting best path is then compared to path 1 (received first). The best path of the second
comparison becomes the final best path for the destination.
A path is not a valid candidate in the best-path selection process if it meets any of the
following conditions:
•
The path's next hop is unreachable
•
The path is not synchronized, and synchronization is enabled
•
The path is denied by inbound BGP policies, and inbound soft reset is configured
•
The route is dampened
Path selection in Cisco IOS software currently has 13 steps (www.cisco.com/warp/
customer/459/25.shtml). Each step is evaluated sequentially until a preference is found:
WEIGHT is the first parameter considered. The path with the highest WEIGHT is pre-
ferred. WEIGHT is a Cisco-proprietary parameter and is local to the router on which
it is configured. By default, paths originated locally have an equal WEIGHT of 32768,
and all other paths have a WEIGHT of 0.
1
The path with the highest LOCAL_PREF is preferred. The default LOCAL_PREF is
100 in Cisco IOS software.
2
The routes are evaluated based on the origination, with preference to the path that was
sourced locally on the router. Here is the complete order with decreasing preference:
default-originate
(configured per neighbor),
default-information originate
(config-
ured per address family),
network
,
redistribute
,
aggregate-address
.
3
The length of AS_PATH is evaluated, with preference to the path with the shortest
AS_PATH list. However, the configuration of
bgp bestpath as-path ignore
(a hidden
command) bypasses this step.
4
Keep the following in mind when evaluating the path length:
•
An AS_SET is counted as 1, no matter how many autonomous systems are in
the set.
•
The AS_CONFED_SEQUENCE is not included in the AS_PATH length.
The route's ORIGIN is evaluated here, with preference to the path with the lowest
ORIGIN type. IGP is lower than EGP, and EGP is lower than INCOMPLETE.
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