Showing posts with label policy. Show all posts
Showing posts with label policy. Show all posts

Monday, March 14, 2016

BGP MED routing decision - Always-compare-med

MED (Multi-Exit Discriminator) is used to influence routing decision. By default, a router which receives MED attribute from neighbors, only compares it if they are coming from the same AS. As you can see in the diagram below:
  • R3 send to R4 a MED of 50 for subnet 172.16.1.0/24
  • R2 send to R5 a MED of 100 for subnet 172.16.1.0/24
  • The lowest MED is the preferred path

In a first step, the configuration below has been applied:
R1:
interface Loopback1
ip address 172.16.1.1 255.255.255.0
router bgp 50
bgp log-neighbor-changes
network 172.16.1.0 mask 255.255.255.0
neighbor 10.1.1.2 remote-as 100
neighbor 10.1.3.3 remote-as 200
R2:
router bgp 100
bgp log-neighbor-changes
neighbor 10.1.1.1 remote-as 50
neighbor 10.1.2.3 remote-as 200
neighbor 10.1.4.4 remote-as 300
neighbor 10.1.4.4 route-map MED out
!
access-list 1 permit 172.16.1.0 0.0.0.255
!
route-map MED permit 10
match ip address 1
set metric 100
!
route-map MED permit 20
R3:
router bgp 200
bgp log-neighbor-changes
neighbor 10.1.2.2 remote-as 100
neighbor 10.1.3.1 remote-as 50
neighbor 10.1.5.4 remote-as 300
neighbor 10.1.5.4 route-map MED out
!
access-list 1 permit 172.16.1.0 0.0.0.255
!
route-map MED permit 10
match ip address 1
set metric 50
!
route-map MED permit 20
R4:
router bgp 300
bgp log-neighbor-changes
neighbor 10.1.4.2 remote-as 100
neighbor 10.1.5.3 remote-as 200

MED is seen in R4 but R4 set R2 has next-hop. R4 doesn't compare MED because information is coming from 2 different ASs. R2 is used due to the fact that he has the lowest Router-id 10.1.4.2.
R4#sho ip bgp 172.16.1.0
BGP routing table entry for 172.16.1.0/24, version 2
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     3
  Refresh Epoch 1
  100 50
    10.1.4.2 from 10.1.4.2 (10.1.4.2)
      Origin IGP, metric 100, localpref 100, valid, external, best
  Refresh Epoch 1
  200 50
    10.1.5.3 from 10.1.5.3 (10.1.5.3)
      Origin IGP, metric 50, localpref 100, valid, external

Now, we add the command 'bgp always-compare-med' on R4:
R4:
router bgp 300
bgp always-compare-med

As you can, R4 has now choosen R3 for the next-hop. MED is used:
R4#sho ip bgp 172.16.1.0
BGP routing table entry for 172.16.1.0/24, version 2
Paths: (2 available, best #1, table default)
  Advertised to update-groups:
     4
  Refresh Epoch 1
  200 50
    10.1.5.3 from 10.1.5.3 (10.1.5.3)
      Origin IGP, metric 50, localpref 100, valid, external, best
  Refresh Epoch 1
  100 50
    10.1.4.2 from 10.1.4.2 (10.1.4.2)
      Origin IGP, metric 100, localpref 100, valid, external