Question - 1 Explain BGP MED attribute.
- Always lowest MED will be preferable.
- It is Optional non-transitive attribute.
- MED is exchanged between AS.
- It can be used to advertise to your neighbors to influence how they should enter in your AS
- Always MED is advertised to all router within the neighboring AS, note that it will not advertised to any other AS
Question - 2 How can we configure BGP MED attribute ?
Router3 and Router2 are both in AS 20 and advertising same network (5.5.5.0/24) to Router1.
Initially we will configure the BGP between three routers.
- Router1 Configuration -
Router1#show running-config
interface Ethernet1/0
ip address 192.167.23.1 255.255.255.0
half-duplex
!
interface Ethernet2/0
ip address 192.167.24.1 255.255.255.0
half-duplex
!
router bgp 10
no synchronization
bgp log-neighbor-changes
neighbor 192.167.23.2 remote-as 20
neighbor 192.167.24.3 remote-as 20
no auto-summary
- Router2 Configuration -
Router2#show running-config
!
interface Loopback1
ip address 5.5.5.2 255.255.255.0
!
interface Ethernet1/0
ip address 192.167.23.2 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
network 5.5.5.0 mask 255.255.255.0
neighbor 192.167.23.1 remote-as 10
no auto-summary
- Router3 Configuration -
Router3#show running-config
interface Loopback1
ip address 5.5.5.3 255.255.255.0
!
interface Ethernet2/0
ip address 192.167.24.3 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
network 5.5.5.0 mask 255.255.255.0
neighbor 192.167.24.1 remote-as 10
no auto-summary
Output -
You can see in this output, Router1 prefer 192.167.23.2 as its path to reach network 5.5.5.0/24, right now MED is 0.
Now we will change the MED value
Router2#configure terminal
Router2(config)#route-map MED-VALUE permit 10
Router2(config-route-map)#set metric 800
Router2(config-route-map)#exit
Router2(config)#router bgp 20
Router2(config-router)#neighbor 192.167.23.1 route-map MED-VALUE out
Router3#configure terminal
Router3(config)#route-map MED-VALUE permit 10
Router3(config-route-map)#set metric 700
Router3(config-route-map)#exit
Router3(config)#router bgp 20
Router3(config-router)#neighbor 192.167.24.1 route-map MED-VALUE out
Output -
You can see in this output, now Router1 prefer 192.167.24.3 as its path to reach network 5.5.5.0/24, because it has lower MED - 700
- Router1 Configuration -
interface Ethernet1/0
ip address 192.167.23.1 255.255.255.0
half-duplex
!
interface Ethernet2/0
ip address 192.167.24.1 255.255.255.0
half-duplex
!
router bgp 10
no synchronization
bgp log-neighbor-changes
neighbor 192.167.23.2 remote-as 20
neighbor 192.167.24.3 remote-as 20
no auto-summary
- Router2 Configuration -
Router2#show running-config
!
interface Loopback1
ip address 5.5.5.2 255.255.255.0
!
interface Ethernet1/0
ip address 192.167.23.2 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
network 5.5.5.0 mask 255.255.255.0
neighbor 192.167.23.1 remote-as 10
no auto-summary
- Router3 Configuration -
interface Loopback1
ip address 5.5.5.3 255.255.255.0
!
interface Ethernet2/0
ip address 192.167.24.3 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
network 5.5.5.0 mask 255.255.255.0
neighbor 192.167.24.1 remote-as 10
no auto-summary
Output -
You can see in this output, Router1 prefer 192.167.23.2 as its path to reach network 5.5.5.0/24, right now MED is 0.
Now we will change the MED value
Router2#configure terminal
Router2(config)#route-map MED-VALUE permit 10
Router2(config-route-map)#set metric 800
Router2(config-route-map)#exit
Router2(config)#router bgp 20
Router2(config-router)#neighbor 192.167.23.1 route-map MED-VALUE out
Router3#configure terminal
Router3(config)#route-map MED-VALUE permit 10
Router3(config-route-map)#set metric 700
Router3(config-route-map)#exit
Router3(config)#router bgp 20
Router3(config-router)#neighbor 192.167.24.1 route-map MED-VALUE out
Output -
You can see in this output, now Router1 prefer 192.167.24.3 as its path to reach network 5.5.5.0/24, because it has lower MED - 700
Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.
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