MED (or metric) is the sixth BGP attribute:
- MED can be used to advertise to your neighbors how they should enter your AS.
- MED is exchanged between autonomous systems.
- The LOWEST MED is the preferred path.
- MED is propagated to all routers within the neighbor AS but not passed along any other autonomous systems.
Consider the below topology:
R1 has to reach the Loopback address of R4 (4.4.4.4/32) through R3 router. By default, it passes through the R3 router.
Lets start our configuration part:
R1 Interface Configuration:
R1#show ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.12.1 YES manual up up
FastEthernet0/1 192.168.13.1 YES manual up up
Loopback0 1.1.1.1 YES manual up up
R1#
R2 Interface Configuration:
R2#show ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.12.2 YES manual up up
FastEthernet0/1 192.168.24.2 YES manual up up
Loopback0 2.2.2.2 YES manual up up
R2#
R3 Interface Configuration:
R3#show ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.13.3 YES manual up up
FastEthernet0/1 192.168.34.3 YES manual up up
Loopback0 3.3.3.3 YES manual up up
R3#
R4 Interface Configuration:
R4#show ip int brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.24.4 YES manual up up
FastEthernet0/1 192.168.34.4 YES manual up up
Loopback0 4.4.4.4 YES manual up up
R4#
Now, we can start configuring BGP in all our routers.
R1 BGP Configuration:
R1#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router bgp 1
R1(config-router)#nei
R1(config-router)#neighbor 192.168.12.2 remote-as 23
R1(config-router)#neighbor 192.168.13.3 remote-as 23
R1(config-router)#redistribute connected
R1(config-router)#end
R1#
R2 BGP Configuration:
R2#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#
R2(config)#router bgp 23
R2(config-router)#neighbor 192.168.12.1 remote-as 1
R2(config-router)#neighbor 192.168.24.4 remote-as 4
R2(config-router)#redistribute connected
R2(config-router)#end
R2#
R3 BGP Configuration:
R3#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#
R3(config)#router bgp 23
R3(config-router)#neighbor 192.168.13.1 remote-as 1
R3(config-router)#neighbor 192.168.34.4 remote-as 4
R3(config-router)#redistribute connected
R3(config-router)#end
R3#
R4 BGP Configuration:
R4#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R4(config)#router bgp 4
R4(config-router)#
R4(config-router)#neighbor 192.168.24.2 remote-as 23
R4(config-router)#neighbor 192.168.34.3 remote-as 23
R4(config-router)#redistribute connected
R4(config-router)#end
R4#
- From the below R1 BGP table, we can see that the R1 prefers the path through R3 (Next Hop IP as 192.168.13.3) to reach the Network 4.4.4.4/32.
- Symbol ">" in-front of the network indicates the best path (router prefers to use this path to send the traffic)
Now, our task to change the path through R2 to reach the Network 4.4.4.4/32.
Lets start our Metric attribute configuration,
Here, I have created a route-map named as MED-TEST. Next, I have used the command "match ip address 1" to match the access-list 1, and then if it matches I have set the Metric (MED) value as 500. I have created a second permit statement for the same route-map AS-PATH-TEST.
Creating a Route-Map:
R3#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#
R3(config)#route-map MED-TEST permit
R3(config-route-map)#match ip address 1
R3(config-route-map)#set metric 500
R3(config-route-map)#exi
R3(config)#route-map MED-TEST permit 20
R3(config-route-map)#end
R3#
You can also view the created c-map using the below command:
R3#show route-map
route-map MED-TEST, permit, sequence 10
Match clauses:
ip address (access-lists): 1
Set clauses:
metric 500
Policy routing matches: 0 packets, 0 bytes
route-map MED-TEST, permit, sequence 20
Match clauses:
Set clauses:
Policy routing matches: 0 packets, 0 bytes
R3#
Next create a access-list as below:
R3#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#access-list 1 permit host 4.4.4.4
R3(config)#end
R3#
You can view configured access-list using the below command,
R3#show access-lists
Standard IP access list 1
10 permit 4.4.4.4
R3#
Applying the created route-map to bgp configuration.
R3#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#router bgp 23
R3(config-router)#neighbor 192.168.13.1 route-map MED-TEST out
R3(config-router)#end
R3#
Here, I have mapped the route-map MED-TEST as "OUT" to the neighbor R1 (192.168.13.1). so that it will advertise the configured Metric value (value: 500) its neighbor.
You need to clear the BGP session to take the newly applied configuration to take effect.
In-order to clear the bgp session, use the command "clear ip bgp * soft in"
You can see that,
- The metric for the neighbor 192.168.13.3 (R3 router) has now changed to 500.
- Best path changed to R2 (Next hop 192.168.12.2 ) as the metric has been changed.
Note:
- The lower the metric is preferred.
- MED will compare the configured value only if it receives from the same AS number. In this article, R1 is receiving the network 4.4.4.4/32 from R2 and R3 routers, but both the routers belongs to the same AS (AS 23)
- If received from the different AS number, by default it will not be compared, but there is a command by which we can make our device to compare. That feature is called as "BGP Always compare MED". We will discuss about in a different chapter.
That's it about MED (Metric) BGP Attribute.
You can also look into my video tutorial in Tamil:
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