Cisco CCNP / BSCI Certification: The Local Preference BGP AttributePage: 1 | 2 | 3 | 4 | 5 | 6 | 7 or Go To our Certification Articles Section
When you're preparing to pass the BSCI exam and earn your
CCNP certification, one of the biggest challenges is learning
BGP. BGP is totally different from any protocol you learned
to earn your CCNA certification, and one of the differences
is that BGP uses path attributes to favor one path over another
when multiple paths to or from a destination exist.
Notice I said "to or from". In earlier free BGP
tutorials, I discussed the BGP attributes "weight"
and "local preference". These attributes are used
to favor one path to a destination over another; for example,
if BGP AS 100 has two paths to a destination in AS 200, these
two attributes can be set in AS 100 to favor one path over
another. But what if AS 100 wants to inform the routers in
AS 200 as to which path it should use to reach a given destination
in AS 100?
That's where the BGP attribute "Multi-Exit Discriminator",
or MED, comes in. The MED value can be set in AS 100 to tell
AS 200 which path it should use to reach a given network in
AS 100.
As with many BGP attributes, the MED can be set with a route-map.
What you need to watch is that there is no "set med"
value in route maps. To change the MED of a path, you need
to change the metric of that path. Let's say that there are
two entry paths for AS 200 to use to reach destinations in
AS 100. You want AS 200 to use the 100.1.1.0/24 path over
the 100.2.2.0/24 path. First, identify the two paths with
two separate ACLs.
R1(config)#access-list 22 permit 100.1.1.0 0.0.0.255
R1(config)#access-list 23 permit 100.2.2.0 0.0.0.255
Next, write a route-map that assigns a lower metric to the
more-desirable path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 22
R1(config-route-map)#set metric 100
R1(config-route-map)#route-map PREFER_PATH permit 20
R1(config-route-map)#match ip address 23
R1(config-route-map)#set metric 250
Finally, apply the route-map to the neighbor or neighbors.
R1(config-route-map)#router bgp 100
R1(config-router)#neighbor 22.2.2.2 route-map PREFER_PATH
out
The key points to keep in mind is that while many BGP attributes
prefer a higher value, the MED is basically an external metric
- and a lower metric is preferred, just as with the protocols
you've already studied to earn your CCNA certification.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNP and CCNA tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages. For a FREE copy of his latest e-books, "How To Pass The CCNA" and "How To Pass The CCNP", just visit the website! You can also get FREE CCNA and CCNP exam questions every day! Pass the CCNP exam with The Bryant Advantage!
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