Cisco CCNP / BSCI Exam Tutorial: The BGP Attribute MED
Posted in Computers on June 30th, 2009 by cahyoYour BSCI exam and CCNP certification success depend on mastering BGP, and a big part of that is knowing how and when to use the many BGP attributes. And for those of you with an eye on the CCIE, believe me – you’ve got to know BGP attributes like the back of your hand. One such BGP attribute is the Multi-Exit Discriminator, or MED.
The MED attribute is sent from a router or routers in one AS to another AS to indicate what path the remote AS should use to send data to the local AS.
That sounds a little confusing on paper, so let’s walk through an example. R1 is in AS 1, and R2, R3, and R4 are in AS 234. R4 is advertising a loopback into BGP, and R1 has two possible next-hops to get to that loopback – R2 (172.12.123.2) and R3 (172.12.123.3). Let’s see which of the two paths R1 is using.
BGP routing table entry for 4.4.4.4/32, version 8
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0×208
Advertised to non peer-group peers:
172.12.123.3
234
172.12.123.3 from 172.12.123.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external
234
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