BGP MED Configuration Step by Step [Gns3 Lab]

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#
Bilel A

Bilel A

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