Imagine the following lab topology
In the diagram you have two routers R4 and R5 connected to each other over two separate serial interfaces. Easy enough right? Well imagine that these two routers are running back-to-back frame relay with the following requirements:
1) Two subnets (10.186.45.0/24 and 10.186.54.0/24) should be able utilize either physical circuit at any given point.
2) Broadcast and Multicast packets must be allowed over both PVC's
3) You are not allowed to use the "Broadcast" parameter on either router
Given this scenario we could tackle this in one of two ways, however in this example we will be using multilink frame relay sub-interfaces. Basically this works in the same way as PPP multilink where the physical interfaces point to the logical interface for it's configuration.
First let's configure the physical interfaces on both R4 and R5
R4
interface serial0
no ip address
encapsulation frame-relay
no keepalive
serial restart-delay 0
clock rate 2016000
end
interface Serial1
no ip address
encapsulation frame-relay
no keepalive
serial restart-delay 0
clock rate 2016000
end
R5
interface Serial0
no ip address
encapsulation frame-relay
no keepalive
clock rate 2016000
end
interface Serial1
no ip address
encapsulation frame-relay
no keepalive
clock rate 2016000
end
If you notice I have the clockrate configured on both sides, this is perfectly valid since the only side that will accept the clockrate command will be the DCE side. If you were to run a "show controllers" you could find out which side was acting as DCE and configure the clockrate on this interface only, this is a preference. You will also notice that I have used the "no keepalive" command, this is needed in order to disable LMI which isn't needed in back-to-back frame relay, because there is no frame switch in this configuration.
Now that we have the physical interfaces configured let's create our logical multilink interface and tie the physical interfaces to it.
Run these commands on both R4 and R5
interface MFR1
encap frame
no ip address
no keepalive
end
interface Serial0
encapsulation frame-relay MFR1
interface Serial1
encapsulation frame-relay MFR1
Since we don't have the ability of using the broadcast command the physical interfaces are out, so how do we fix this? For this scenario we will use logical sub-interfaces as follows
R4
interface MFR1.45 point-to-point
ip address 10.186.45.4 255.255.255.0
frame-relay interface-dlci 450
end
interface MFR1.54 point-to-point
ip address 10.186.54.4 255.255.255.0
frame-relay interface-dlci 540
end
R5
interface MFR1.45 point-to-point
ip address 10.186.45.5 255.255.255.0
snmp trap link-status
frame-relay interface-dlci 450
end
interface MFR1.54 point-to-point
ip address 10.186.54.5 255.255.255.0
snmp trap link-status
frame-relay interface-dlci 540
end
That's it!! Now run some simple ping tests and see if we are good
R4(config-subif)#do ping 10.186.45.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.186.45.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/47/72 ms]
and
R4(config-subif)#do ping 10.186.54.5
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.186.54.5, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 24/47/72 ms
Awesome!
So that's all there is to it, we were able to complete all tasks without breaking any rules. If you have any comments or questions please let me know
