Juniper JUNOSE 11.2.X BGP AND MPLS, Руководство по настройке

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Suppose, however that router Dublin was not configured to set the MED for route
192.168.33.0/24 in its outbound route map 10. Would router London receive a MED of 50
passed along by router Paris through router Dublin? No, because the MED attribute is
nontransitive. Router Dublin does not transmit any MED that it receives. A MED is only of
value to a direct peer.
bgp always-compare-med
•
Use to enable the comparison of the MED for paths from neighbors in different ASs.
•
Unless you specify the bgp always-compare-med command, the router compares
MED attributes only for paths from external neighbors that are in the same AS.
•
The BGP path decision algorithm selects a lower MED value over a higher one.
•
Unlike local preferences, the MED attribute is exchanged between ASs, but does not
leave the AS.
•
The value is used for decision making within the AS only.
•
When BGP propagates a route received from outside the AS to another AS, it removes
the MED.
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Example
host1(config-router)# bgp always-compare-med
•
Changes apply automatically whenever BGP subsequently runs the best-path decision
process for a destination prefix; that is, whenever a best route is picked for a given
prefix.
To force BGP to run the decision process on routes already received, you must use the
clear ip bgp command to perform an inbound soft clear or hard clear of the current
BGP session.
•
Use the no version to disable the feature.
•
See bgp always-compare-med.
set metric
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Use to set the metric value—for BGP, the MED—for a route.
•
Sets an absolute metric. You cannot use both an absolute metric and a relative metric
within the same route map sequence. Setting either metric overrides any previously
configured value.
•
Example
host1(config)# route-map nyc1 permit 10
host1(config-route-map)# set metric 10
•
Use the no version to delete the set clause from a route map.
•
See set metric.
Missing MED Values
By default, a route that arrives with no MED value is treated as if it had a MED of 0, the
most preferred value. You can use the
bgp bestpath missing-as-worst command to
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Chapter 1: Configuring BGP Routing