How to configure PPP Multilink

Multilink is a way of bundling more than one PPP WAN links and bundle them together into one logical interface. This is one of the method you can implement when you have a primary and secondary links in place and create a logical conduit where both links are present.

This method can be used to load balance data across the links and at the same time it will support redundancy when one one link fails.

I have tested this configuration on 2 of my 2500 Routers and I haven’t tested these on more recent IOS so some commands may vary. I will be checking them on a pair of 3845s/2691s on a later date and will add my findings to this post.

As you can see the Diagram, I am using R4 and R5 which are connected via Serial 0 and Serial 1 respectivly.

R4 Configuration

Multilink Interface
interface Multilink1
ip address 172.16.1.1 255.255.255.252
ppp multilink
ppp multilink links maximum 2
ppp multilink links minimum 1
ppp multilink group 1

Serial 0

interface Serial0
no ip address
encapsulation ppp
clock rate 64000
ppp multilink
ppp multilink group 1

Serial 1

interface Serial1
no ip address
encapsulation ppp
clock rate 64000
ppp multilink
ppp multilink group 1

As you can see, since this is a lab scenario, I have set the clock rate on the link. If you are setting this up on a WAN link, you don’t need to do this as the circuit provider would be setting this up for you.

Also another thing to watch out for is to create the Multilink interface first then move onto configuring the Serial interfaces. If you do the other way around, it will not allow you to set the multilink group as it is not present on the router (See the error message below).

R4(config-if)#ppp multi gr 1
% Cannot set multilink group. Interface Multilink1 does not exist

That is all to watch out for and you can see my R5 configuration below…

R5 Configuration

Multilink

interface Multilink1
ip address 172.16.1.2 255.255.255.252
ppp multilink
ppp multilink links maximum 2
ppp multilink links minimum 1
ppp multilink group 1

Serial 0

interface Serial0
no ip address
encapsulation ppp
ppp multilink
ppp multilink group 1

Serial 1

interface Serial1
no ip address
encapsulation ppp
ppp multilink
ppp multilink group 1

Here is how it should look…

R4#sh ip int br | i up
Multilink1    172.16.1.1    YES   manual  up  up
Serial0       unassigned    YES   manual  up  up
Serial1       unassigned    YES   manual  up  up

R5#sh ip int br | i up
Multilink1    172.16.1.2    YES   manual  up  up
Serial0       unassigned    YES   manual  up  up
Serial1       unassigned    YES   manual  up  up

R4#ping 172.16.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 40/44/52 ms
R4#

Now I will bring Serial 0 interface down and test the Multilink…

R4(config)#int se0
R4(config-if)#shut
R4(config-if)#

*Mar  1 01:19:41.123: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down
*Mar  1 01:19:42.123: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down

R4(config-if)#do sh ip int br
Interface     IP-Address   OK?    Method  Status              Protocol
Ethernet0     unassigned   YES    NVRAM   administratively    down  down
Multilink1    172.16.1.1   YES    manual  up                  up
Serial0       unassigned   YES    manual  administratively    down  down
Serial1       unassigned   YES    manual  up                  up

R4(config-if)#do ping 172.16.1.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 32/32/36 ms
R4(config-if)#

As you can see, it works even when one interface is down…

Tagged as: Mu1, multilink, ppp, ppp multilink