Cisco CCNP / BSCI Exam Tutorial: The BGP Attribute MED - 0 views

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  #1 Walter Sunesen on 19 Sep 13

   

  Your BSCI exam and CCNP accreditation success rely on learning BGP, and a big element of that's knowing how and when to utilize the many BGP attributes. And for those of you with an eye on the CCIE, trust in me - you have got to know BGP characteristics such as the right back of one's hand. One BGP credit could be the Multi-Exit Discriminator, or MED.
  
  The MED credit is sent from a router or routers in a single AS to another AS to show what course the remote AS must use to send information to the local AS.
  
  That seems only a little confusing written down, therefore let's walk via an example. R1 is in AS 1, and R2, R3, and R4 come in AS 234. To study more, please consider checking out: company web site. R4 is promoting 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 us see which of the two routes R1 is using.
  
  R1#show ip address bgp 4.4.4.4
  
  BGP routing table entry for 4.4.4.4/32, version 8
  
  Paths: (2 available, most useful number 2, table Default-IP-Routing-Table)
  
  Flag: 0x208
  
  Advertised to low peer-group peers:
  
  172.12.123.3
  
  234
  
  172.12.123.3 from 172.12.123.3 (3.3.3.3)
  
  Source IGP, localpref 100, appropriate, outer
  
  234
  
  172.12.123.2 from 172.12.123.2 (2.2.2.2)
  
  Beginning IGP, localpref 100, valid, additional, best
  
  R1 is using 172.12.123.2 since the next-hop to enter AS 234. If all prices are left at their default, we could have 100 routes being advertised from AS 234 to AS 1 and the next-hop would remain the same.
  
  We are able to configure R3 and R2 to deliver various MED values to R1, and the router sending the lowest MED is the preferred next-hop. (The MED is a metric, and the best metric is definitely preferred.) We'll manage the MED attribute on both R2 and R3, sending a of 200 from R2 and 100 from R3.
  
  R2( config )#route-map SET_MED_200 enable 10
  
  R2( config-route-map )#set metric 200
  
  R2( config-route-map )#router bgp 234
  
  R2( config-router )#neighbor 172.12.123.1 route-map SET_MED_200 out
  
  R3( config )#route-map SET_MED_100 let 10
  
  R3( config-route-map )#set metric 100
  
  R3( config-route-map )#router bgp 234
  
  R3( config-router )#neighbor 172.12.123.1 route-map SET_MED_100 out
  
  After removing the BGP dining table on R1, R1 will still see both next-hop addresses and will still contemplate both to be valid, but the route through R3 will be selected due to its lower metric.
  
  Just keep in mind that the MED is truly a full, and lower measurements are far more appealing in route selection. That may set you one step closer to moving the BSCI and earning your CCNP Certification!.

   

  <div class="cArrow"> </div><div class="cContentInner">Your BSCI exam and CCNP accreditation success rely on learning BGP, and a big element of that's knowing how and when to utilize the many BGP attributes. And for those of you with an eye on the CCIE, trust in me - you have got to know BGP characteristics such as the right back of one's hand. One BGP credit could be the Multi-Exit Discriminator, or MED. <br /> <br /> The MED credit is sent from a router or routers in a single AS to another AS to show what course the remote AS must use to send information to the local AS. <br /> <br /> That seems only a little confusing written down, therefore let's walk via an example. R1 is in AS 1, and R2, R3, and R4 come in AS 234. To study more, please consider checking out: <a rel="nofollow" href="http://www.brakeconnect.com/product-category/banjo-bolts">company web site</a>. R4 is promoting 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 us see which of the two routes R1 is using. <br /> <br /> R1#show ip address bgp 4.4.4.4 <br /> <br /> BGP routing table entry for 4.4.4.4/32, version 8 <br /> <br /> Paths: (2 available, most useful number 2, table Default-IP-Routing-Table) <br /> <br /> Flag: 0x208 <br /> <br /> Advertised to low peer-group peers: <br /> <br /> 172.12.123.3 <br /> <br /> 234 <br /> <br /> 172.12.123.3 from 172.12.123.3 (3.3.3.3) <br /> <br /> Source IGP, localpref 100, appropriate, outer <br /> <br /> 234 <br /> <br /> 172.12.123.2 from 172.12.123.2 (2.2.2.2) <br /> <br /> Beginning IGP, localpref 100, valid, additional, best <br /> <br /> R1 is using 172.12.123.2 since the next-hop to enter AS 234. If all prices are left at their default, we could have 100 routes being advertised from AS 234 to AS 1 and the next-hop would remain the same. <br /> <br /> We are able to configure R3 and R2 to deliver various MED values to R1, and the router sending the lowest MED is the preferred next-hop. (The MED is a metric, and the best metric is definitely preferred.) We'll manage the MED attribute on both R2 and R3, sending a of 200 from R2 and 100 from R3. <br /> <br /> R2( config )#route-map SET_MED_200 enable 10 <br /> <br /> R2( config-route-map )#set metric 200 <br /> <br /> R2( config-route-map )#router bgp 234 <br /> <br /> R2( config-router )#neighbor 172.12.123.1 route-map SET_MED_200 out <br /> <br /> R3( config )#route-map SET_MED_100 let 10 <br /> <br /> R3( config-route-map )#set metric 100 <br /> <br /> R3( config-route-map )#router bgp 234 <br /> <br /> R3( config-router )#neighbor 172.12.123.1 route-map SET_MED_100 out <br /> <br /> After removing the BGP dining table on R1, R1 will still see both next-hop addresses and will still contemplate both to be valid, but the route through R3 will be selected due to its lower metric. <br /> <br /> Just keep in mind that the MED is truly a full, and lower measurements are far more appealing in route selection. That may set you one step closer to moving the BSCI and earning your CCNP Certification!.</div>

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