MED - Multi Exit Discriminator - Metric
- MED can be used to manipulate the Incoming traffic entering into your AS.
- The route with lower MED value is preferred.
- The default value is 0
- MED is exchanged between Autonomous Systems.
- It is propagated to all the routers within the neighbor AS
- But can't go beyond the neighbor AS - Hence it is Optional Non-Transitive.
R1#sh int des | i up
Et2/0 up up to R2
Et2/1 up up to R3
R1#sh ip int brief | i up
Ethernet2/0 12.1.1.1 YES manual up up
Ethernet2/1 13.1.1.1 YES manual up up
R2#sh int des | i up
Et2/0 up up to R1
Et2/1 up up to R3
R2#
R2#sh ip int brief | i up
Ethernet2/0 12.1.1.2 YES manual up up
Ethernet2/1 23.1.1.2 YES manual up up
R2#
R3#sh int des | i up
Et2/0 up up to R1
Et2/1 up up to R2
R3#
R3#sh ip int brief | i up
Ethernet2/0 13.1.1.3 YES manual up up
Ethernet2/1 23.1.1.3 YES manual up up
R3#
! Lets configure an additional loopback on R1
R1(config)#int loopback 100
R1(config-if)#ip address 100.1.1.1 255.255.255.0
R1(config-if)#exit
R1(config)#exit
R1#
*Jan 13 08:01:29.523: %SYS-5-CONFIG_I: Configured from console by console
! BGP Configuration
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router bgp 100
R1(config-router)#neighbor 12.1.1.2 remote-as 100
R1(config-router)#neighbor 13.1.1.3 remote-as 200
R1(config-router)#network 100.1.1.0 mask 255.255.255.0
R1(config-router)#exit
R1(config)#exit
R1#wr
R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#router bgp 100
R2(config-router)#neighbor 12.1.1.1 remote-as 100
R2(config-router)#
*Jan 13 08:05:02.079: %BGP-5-ADJCHANGE: neighbor 12.1.1.1 Up
R2(config-router)#neighbor 23.1.1.3 remote-as 200
R2(config-router)#exit
R2(config)#exit
R2#wr
Building configuration...
[OK]
R3#config t
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router bgp 200
R3(config-router)#neighbor 13.1.1.1 remote-as 100
R3(config-router)#
*Jan 13 08:06:21.163: %BGP-5-ADJCHANGE: neighbor 13.1.1.1 Up
R3(config-router)#neighbor 23.1.1.2 remote-as 100
R3(config-router)#
*Jan 13 08:06:37.443: %BGP-5-ADJCHANGE: neighbor 23.1.1.2 Up
R3(config-router)#exit
R3(config)#exit
R3#wr
Building configuration...
*Jan 13 08:06:43.707: %SYS-5-CONFIG_I: Configured from console by console[OK]
!!Verify the status of the BGP
R1#sh ip bgp summar
BGP router identifier 100.1.1.1, local AS number 100
BGP table version is 2, main routing table version 2
1 network entries using 144 bytes of memory
1 path entries using 80 bytes of memory
1/1 BGP path/bestpath attribute entries using 136 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 360 total bytes of memory
BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
12.1.1.2 4 100 7 7 2 0 0 00:02:39 0
13.1.1.3 4 200 5 6 2 0 0 00:01:20 0
R2#sh ip bgp summary
BGP router identifier 23.1.1.2, local AS number 100
BGP table version is 2, main routing table version 2
1 network entries using 144 bytes of memory
1 path entries using 80 bytes of memory
1/1 BGP path/bestpath attribute entries using 136 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 360 total bytes of memory
BGP activity 1/0 prefixes, 1/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
12.1.1.1 4 100 8 7 2 0 0 00:02:59 1
23.1.1.3 4 200 8 6 2 0 0 00:01:24 0
R3#sh ip bgp summary
BGP router identifier 23.1.1.3, local AS number 200
BGP table version is 2, main routing table version 2
1 network entries using 144 bytes of memory
2 path entries using 160 bytes of memory
2/1 BGP path/bestpath attribute entries using 272 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 600 total bytes of memory
BGP activity 1/0 prefixes, 2/0 paths, scan interval 60 secs
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
13.1.1.1 4 100 7 6 2 0 0 00:01:57 1
23.1.1.2 4 100 6 8 2 0 0 00:01:41 1
!!If you check on R3 for the network 100.1.1.0/24 the perferred path is through R1
R3#sh ip bgp
BGP table version is 2, local router ID is 23.1.1.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 100.1.1.0/24 23.1.1.2 0 100 i
*> 13.1.1.1 0 0 100 i
R3#
R3#sh ip bgp 100.1.1.0
BGP routing table entry for 100.1.1.0/24, version 2
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 1
100
23.1.1.2 from 23.1.1.2 (23.1.1.2)
Origin IGP, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Refresh Epoch 1
100
13.1.1.1 from 13.1.1.1 (100.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Now using can change the incoming the traffic
Lets make few changes on R1
R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#route-map TEST
R1(config-route-map)#set metric ?
+/-<metric> Add or subtract metric
<0-4294967295> Metric value or Bandwidth in Kbits per second
R1(config-route-map)#set metric 100
R1(config)#router bgp 100
R1(config-router)#neighbor 13.1.1.3 route-map TEST out
R1(config-router)#exit
R1(config)#exit
R1#
R1#clear ip bgp * soft out
R3#sh ip bgp
BGP table version is 3, local router ID is 23.1.1.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 100.1.1.0/24 23.1.1.2 0 100 i
* 13.1.1.1 100 0 100 i
Thanks for Reading
amartechstuff
