Link Aggregation - Multilink PPP (MLP)

Multilink PPP (MLP)

The MLP feature provides a load-balancing functionality over multiple WAN links while allowing for multivendor interoperability. It offers support for packet fragmentation, proper sequencing, and load calculation on both inbound and outbound traffic.

MLP allows packets to be fragmented and then sent simultaneously over multiple point-topoint links to the same remote address. It can work over synchronous and asynchronous serial types.

MLP combines multiple physical links into a logical link called an MLP bundle, which is essentially a single, virtual interface that connects to the remote router. None of the links inside the bundle have any knowledge about the traffic on the other links

Load balancing MLP provides bandwidth on demand, utilizing load balancing on up to 10 links and can even calculate the load on traffic between specific sites. You don’t actually need to make all links the same bandwidth, but doing so is recommended. Another key MLP advantage is that it splits packets and fragments across all links, which reduces latency across the WAN.

Link fragmentation and interleaving The fragmentation mechanism in MLP works by fragmenting large packets, then sending the packet fragments over the multiple point-topoint links. Smaller real-time packets are not fragmented. So interleaving basically means real-time packets can be sent in between sending the fragmented, non-real-time packets, which helps reduce delay on the lines. So let’s configure MLP now to get a good feel for how it actually works now.

Configuring MLP

But first, I want you to study the configuration of the two serial interfaces on the Corp router that we’re going to use for making our bundle:

Did you notice that each serial connection is on a different subnet (they have to be) and that the encapsulation is PPP?
When you configure MLP, you must first remove your IP addresses off your physical interface. Then, you configure a multilink bundle by creating a multilink interface on both sides of the link. After that, you assign an IP address to this multilink interface, which effectively restricts a physical link so that it can only join the designated multilink group interface.
So first I’m going to remove the IP addresses from the physical interfaces that I’m going to include in my PPP bundle.

We can see that a link joins an MLP bundle only if it negotiates to use the bundle when a connection is established and the identification information that has been exchanged matches the info for an existing bundle.
When you configure the ppp multilink group command on a link, that link won’t be allowed to join any bundle other than the indicated group interface.

Verifying MLP

We can see that the physical interfaces, Se0/1 and Se1/1, are members of the logical interface bundle Multilink 1. So now we’ll verify the status of the interface Multilink1 on the Corp router: