Cisco CCNP and CCIE Study Notes

If you were turn on Link Fragmentation and Interleaving (LFI) using the ppp multilink interleave in the multilink PPP example given in the last post, there would be a problem with the ordering of packets that get interleaved and sent over the pair of PPP links that are part of the multilink bundle. The reason is that the small packets that would get interleaved between the large fragmented PPP packets would be straight PPP packets lacking the multilink PPP header. Since they would be lacking sequence number information, they could get out of order at the remote end. So to use LFI, using more than one link in the MPPP bundle could lead to re-ordering issues.

This is where multiclass multilink PPP comes to the rescue. Cisco implements this using the ppp multilink multiclass configuration command. When turned on, the sequence number is maintained per class and so the interleaved small packets also would have the multilink header and so could be ordered correctly. This was defined in RFC 2686 and Cisco too has a good document that explains this.

I thought I knew multilink PPP well but in looking into the feature this morning, I realized that I had never configured it for serial interfaces; I had always used it in the past with ISDN or in the context of async dial-up. When I tried to see how I could exploit multilink PPP with synchronous serial interfaces, I realized that I didn't know how to configure the bundle interface. I fooled around with virtual-template and virtual-access interfaces for the MPPP bundle interface and then finally had to research the web and find out about the multilink virtual interface that had to be used. There is always more to learn even regarding topics you think you know well!

So, with two routers connected back-to-back via a pair of serial interfaces, I brought up multilink PPP with the following config

Router1

interface Multilink1
 ip address 10.10.10.1 255.255.255.0
 ppp multilink
 ppp multilink group 1
!     
interface Serial2
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
!     
interface Serial3
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1

interface Multilink1
 ip address 10.10.10.2 255.255.255.0
 ppp multilink
 ppp multilink group 1
!     
interface Serial2
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1
!
interface Serial3
 no ip address
 encapsulation ppp
 ppp multilink
 ppp multilink group 1

Of course this is very basic config and one could add on PPP authentication and a host of multilink features to this as needed.