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Multi-Exit-Discriminator (MED) attribute
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Local_Pref attribute
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Community attribute
Principles of Route Selection
On the S6700, when there are multiple active routes to the same destination, BGP selects routes
according to the following principles:
1.
Prefers the route with the highest PreVal.
PrefVal is a Huawei-specific parameter. It is valid only on the device where it is configured.
2.
Prefers the route with the highest Local_Pref.
A route without Local_Pref is considered to have had the value set by using the
default
local-preference
command or to have a value of 100 by default.
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 by using the
network
command or the
import-route
command, manually aggregated routes, and automatically summarized
routes.
a.
A summarized route is preferred. A summarized route takes precedence over a non-
summarized route.
b.
A route obtained by using the
aggregate
command is preferred over a route obtained
by using the
summary automatic
command.
c.
A route imported by using the
network
command is preferred over a route imported
by using the
import-route
command.
4.
Prefers the route with the shortest AS_Path.
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The AS_CONFED_SEQUENCE and AS_CONFED_SET are not included in the
AS_Path length.
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An AS_SET counts as 1, no matter how many ASs are in the set.
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After the
bestroute as-path-ignore
command is run, the AS_Path attributes of routes
are not compared in the route selection process.
5.
Prefers the route with the highest Origin type. IGP is higher than EGP, and EGP is higher
than Incomplete.
6.
Prefers the route with the lowest Multi Exit Discriminator (MED).
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The MEDs of only routes 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.
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A route without any MED is assigned a MED of 0, unless the
bestroute med-none-as-
maximum
command is run. If the
bestroute med-none-as-maximum
command is run,
the route is assigned the highest MED of 4294967295.
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After
compare-different-as-med
command is run, the MEDs in routes sent from peers
in different ASs are compared. Do not use this command unless it is confirmed that
different ASs use the same IGP and route selection mode. Otherwise, a loop may occur.
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If the
bestroute med-confederation
command is run, MEDs are compared for routes
that consist only of AS_CONFED_SEQUENCE. The first AS number in the
AS_CONFED_SEQUENCE must be the same for the routes.
S6700 Series Ethernet Switches
Configuration Guide - IP Routing
7 BGP Configuration
Issue 01 (2012-03-15)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
363
