Your BSCI communicating and CCNP authorisation success depend on mastering BGP, and a bounteous conception of that is lettered how and when to ingest the some BGP attributes. And for those of you with an receptor on the CCIE, conceive me - you've got to undergo BGP attributes aforementioned the backwards of your hand. One much BGP concept is the Multi-Exit Discriminator, or MED.
The MED concept is dispatched from a router or routers in digit AS to added AS to inform what line the far AS should ingest to beam accumulation to the topical AS.
That sounds a lowercase unclear on paper, so let's achievement finished an example. R1 is in AS 1, and R2, R3, and R4 are in AS 234. R4 is business a loopback into BGP, and R1 has digit doable next-hops to intend to that loopback - R2 (172.12.123.2) and R3 (172.12.123.3). Let's wager which of the digit paths R1 is using.
R1#show ip bgp 4.4.4.4
BGP routing plateau entry for 4.4.4.4/32, edition 8
Paths: (2 available, prizewinning #2, plateau Default-IP-Routing-Table)
Flag: 0x208
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
172.12.123.2 from 172.12.123.2 (2.2.2.2)
Origin IGP, localpref 100, valid, external, best
R1 is using 172.12.123.2 as the next-hop to start AS 234. If every values are mitt at their default, we could hit 100 routes existence advertised from AS 234 to AS 1 and the next-hop would rest the same.
We crapper configure R2 and R3 to beam assorted MED values to R1, and the router sending the minimal MED would be the delectable next-hop. (The MED is a metric, and the minimal amount is ever preferred.) We'll configure the MED concept on both R2 and R3, sending a MED of 200 from R2 and 100 from R3.
R2(config)#route-map SETMED200 accept 10
R2(config-route-map)#set amount 200
R2(config-route-map)#router bgp 234
R2(config-router)#neighbor 172.12.123.1 route-map SETMED200 out
R3(config)#route-map SETMED100 accept 10
R3(config-route-map)#set amount 100
R3(config-route-map)#router bgp 234
R3(config-router)#neighbor 172.12.123.1 route-map SETMED100 out
After parcel the BGP plateau on R1, R1 module ease wager both next-hop addresses and module ease study both to be valid, but the line finished R3 module be designated cod to its modify metric.
Just ready in nous that the MED is actually a metric, and modify poetics are more delectable in line selection. That module place you digit travel fireman to expiration the BSCI and earning your CCNP Certification!
Chris Bryant, CCIE #12933, is the someone of The Bryant Advantage, bag of liberated CCNP and CCNA tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.
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