BGP MED Configuration Step by Step [Gns3 Lab]

Topology

Initial configuration without MED Manipulation

* R1 Configuration:

interface Ethernet0/0
 ip address 10.0.0.1 255.255.255.252
 half-duplex
!
interface Ethernet1/0
 ip address 10.0.0.5 255.255.255.252
 half-duplex
!
router bgp 266
 no synchronization
 bgp log-neighbor-changes
 network 10.0.0.0 mask 255.255.255.252
 network 10.0.0.4 mask 255.255.255.252
 neighbor 10.0.0.2 remote-as 277
 neighbor 10.0.0.6 remote-as 277
 no auto-summary

* R2 Configuration:

interface Ethernet0/0
 ip address 10.0.0.2 255.255.255.252
 half-duplex
!
interface Ethernet1/0
 ip address 10.0.0.9 255.255.255.252
 half-duplex


router bgp 277
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.1 remote-as 266
neighbor 10.0.0.10 remote-as 277
no auto-summary

* R3 Configuration:

interface Ethernet0/0
 ip address 10.0.0.6 255.255.255.252
 half-duplex
!
interface Ethernet1/0
 ip address 10.0.0.13 255.255.255.252
 half-duplex
!
router bgp 277
 no synchronization
 bgp log-neighbor-changes
 redistribute connected
 neighbor 10.0.0.5 remote-as 266
 neighbor 10.0.0.5 route-map MEDI out
 neighbor 10.0.0.14 remote-as 277
 no auto-summary

* R4 Configuration:

interface Loopback0
 ip address 4.4.4.2 255.255.255.255
!
interface Loopback1
 ip address 4.4.4.3 255.255.255.255
!
interface Ethernet0/0
 ip address 10.0.0.10 255.255.255.252
 half-duplex
!
interface Ethernet1/0
 ip address 10.0.0.14 255.255.255.252
 half-duplex
!
router bgp 277
 no synchronization
 bgp log-neighbor-changes
 network 4.4.4.2 mask 255.255.255.255
 network 4.4.4.3 mask 255.255.255.255
 neighbor 10.0.0.9 remote-as 277
 neighbor 10.0.0.13 remote-as 277
 no auto-summary

Verifications:

So, before modifying the MED value, we can see that the subnets 4.4.4.2 and 4.4.4.3 are preferred via 10.0.0.2 which is the Router R2.

R1#show ip bgp
BGP table version is 22, local router ID is 10.0.0.5
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  4.4.4.2/32       10.0.0.6                               0 277 i
*>                  10.0.0.2                               0 277 i
*  4.4.4.3/32       10.0.0.6                               0 277 i
*>                  10.0.0.2                               0 277 i
*  10.0.0.0/30      10.0.0.2                 0             0 277 ?
*>                  0.0.0.0                  0         32768 i
*  10.0.0.4/30      10.0.0.6                 0             0 277 ?
*>                  0.0.0.0                  0         32768 i
*> 10.0.0.8/30      10.0.0.2                 0             0 277 ?
*> 10.0.0.12/30     10.0.0.6                 0             0 277 ?



R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 2 subnets
B       4.4.4.2 [20/0] via 10.0.0.2, 00:00:45
B       4.4.4.3 [20/0] via 10.0.0.2, 00:00:45
     10.0.0.0/30 is subnetted, 4 subnets
B       10.0.0.8 [20/0] via 10.0.0.2, 00:00:45
B       10.0.0.12 [20/0] via 10.0.0.6, 00:00:25
C       10.0.0.0 is directly connected, Ethernet0/0
C       10.0.0.4 is directly connected, Ethernet1/0
R1#

Our target in the next part is to modify the MED value in order to:

make the Endpoint 4.4.4.2 prefered via Router R2, to do this we will configure R2 to advertise this subnet to R1 with a lower metric (MED value) than the metric advertised by R3.
make the Endpoint 4.4.4.3 prefered via Router R3, to do this we will configure R3 to advertise the subnet to R1 with a lower metric (MED value) than the metric advertised by R2.

MED Configuration

Let’s configure BGP and change the MED for each destination:

in Router R2: We will prefer route 4.4.4.2 by giving it a lower BGP metric (MED) than route 4.4.4.3:

we will set the MED value to 100 for 4.4.4.2 IP when advertising this route to R1.
we will set the MED value to 200 for 4.4.4.3 IP when advertising this route to R1.

in Router R2: We will do the exact opposite in order to prefer route 4.4.4.3 by giving it a lower BGP metric (MED) than route 4.4.4.2:

we will set the MED value to 100 for 4.4.4.3 IP when advertising this route to R1.
we will set the MED value to 200 for 4.4.4.2 IP when advertising this route to R1.

* R2 configuration:

We will prefer the 4.4.4.2 to pass through R2 by giving it a lower metric (100) and 4.4.4.3 a higher metric (200).

interface Ethernet0/0
 ip address 10.0.0.2 255.255.255.252
 half-duplex
!
interface Ethernet1/0
 ip address 10.0.0.9 255.255.255.252
 half-duplex


router bgp 277
no synchronization
bgp log-neighbor-changes
neighbor 10.0.0.1 remote-as 266
neighbor 10.0.0.1 route-map MEDI out
neighbor 10.0.0.10 remote-as 277
no auto-summary

access-list 1 permit 4.4.4.2
access-list 2 permit 4.4.4.3
!

route-map MEDI permit 10
match ip address 1
set metric 100
!

route-map MEDI permit 20
match ip address 2
set metric 200

* R3 configuration:

interface Ethernet0/0
 ip address 10.0.0.6 255.255.255.252
 half-duplex
!
interface Ethernet1/0
 ip address 10.0.0.13 255.255.255.252
 half-duplex
!
router bgp 277
 no synchronization
 bgp log-neighbor-changes
 redistribute connected
 neighbor 10.0.0.5 remote-as 266
 neighbor 10.0.0.5 route-map MEDI out
 neighbor 10.0.0.14 remote-as 277
 no auto-summary
!

access-list 1 permit 4.4.4.2
access-list 2 permit 4.4.4.3

!
route-map MEDI permit 10
 match ip address 1
 set metric 200
!
route-map MEDI permit 20
 match ip address 2
 set metric 100

Verification:

R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     4.0.0.0/32 is subnetted, 2 subnets
B       4.4.4.2 [20/100] via 10.0.0.2, 00:01:32
B       4.4.4.3 [20/100] via 10.0.0.6, 00:01:21
     10.0.0.0/30 is subnetted, 2 subnets
C       10.0.0.0 is directly connected, Ethernet0/0
C       10.0.0.4 is directly connected, Ethernet1/0


R1#show ip bgp
BGP table version is 35, local router ID is 10.0.0.5
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  4.4.4.2/32       10.0.0.6               200             0 277 i
*>                  10.0.0.2               100             0 277 i
*> 4.4.4.3/32       10.0.0.6               100             0 277 i
*                   10.0.0.2               200             0 277 i
*> 10.0.0.0/30      0.0.0.0                  0         32768 i
*> 10.0.0.4/30      0.0.0.0                  0         32768 i
R1#