Principles of Route Selection
On the NE80E/40E, when there are multiple active routes to the same destination, BGP selects
routes according to the following principles:
NOTE
BGP selects routes by comparing the route attributes in the following sequence. When the optimal route
is selected based on the first route attribute, the subsequent attributes will be ignored.
1.
Prefers the route with the highest PreVal.
PrefVal is a Huawei-specific attribute, valid only on the Huawei device where it is
configured.
2.
Prefers the route with the highest Local_Pref.
If a route carries no Local_Pref, 100 or the value set using the
default local-preference
command is considered as its default Local_Pref value.
3.
Prefers a locally originated route. A locally originated route takes precedence over a route
learned from a peer.
Locally originated routes include routes imported using the
network
command or the
import-route
command, manually summarized routes, and automatically summarized
routes.
a.
A summarized route is preferred, and takes precedence over a non-summarized route.
b.
A route obtained using the
aggregate
command is preferred over a route obtained
using the
summary automatic
command.
c.
A route imported using the
network
command is preferred over a route imported using
the
import-route
command.
4.
Prefers a route that carries the Accumulated Interior Gateway Protocol Metric (AIGP)
attribute.
l
The priority of a route that carries the AIGP attribute is higher than the priority of a
route that does not carry the AIGP attribute.
l
If two routes both carry the AIGP attribute, the route with a smaller AIGP attribute value
plus IGP metric of the iterated next hop is preferred over the other route.
5.
Prefers the route with the shortest AS_Path.
l
The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the
AS_Path length.
l
An AS_SET counts as 1, regardless of how many ASs are in the set.
l
After the
bestroute as-path-ignore
command is run, the AS_Path attributes of routes
are not compared in the route selection process.
6.
Prefers the route with the highest Origin type. IGP is higher than EGP, and EGP is higher
than Incomplete.
7.
Prefers the route with the lowest Multi—Exit Discriminator (MED).
l
The MEDs of routes only from the same AS (but not a confederation sub-AS) are
compared. MEDs of two routes are compared only when the first AS number in the
AS_SEQUENCE (excluding AS_CONFED_SEQUENCE) is the same for the two
routes.
HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
8 BGP Configuration
Issue 02 (2014-09-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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