Cisco Router PPP Multilink Setup and Configuration

This article provides an in-depth explanation of PPP Multilink and shows how to configure a Cisco router to use PPP Multilink between two serial interfaces (WIC-1T). We'll show you the commands that are required for the configuration, plus how to ensure the PPP Multilink is working correctly.

The Point-to-Point Protocol (PPP) is an encapsulation protocol that allows the transporting of network layer traffic over point-to-point links. Examples of point-to-point links are ISDN and Synchronous serial links. PPP is also described in RFC1661.

PPP Multilink is used to  take multiple PPP links and 'bond' them together, making them act as a single PPP link.  Examples of PPP Multilink can be usually found at service providers, who will bond multiple links for a customer in order to provide increased bandwidth.

For example, a company can order two E1 connections from their service provider: each E1 providing a speed of 2.048Mbits/sec. With the use of PPP Multilink we can bond these two links into one and effectively obtain a single link with the speed of 4.096Mbit/sec.

It is important to understand that PPP Multilink requires both ends to be configured the same, which means that both you and your ISP must have PPP Multilink configured otherwise the link will not work.

Configuring PPP Multilink is a pretty straightforward process, however, configuration steps can differ depending on the interfaces and protocols you are dealing with.  

Our example uses two WIC-1T interfaces, installed on our customer's router, which need to be bonded with the service provider's links. Of course, the customer has previously requested the two E1 lines to support PPP Multilink.

Configuring PPP Multilink On A Cisco Router

 First step is to configure the multilink interface on our router. The multilink interface is assigned to a multilink group:

As you can see, the configuration of the Multilink Interface is similar to that of a physical interface. The multilink interface is configured with its IP Address and ip nat outside is enabled, indicating that this virtual interface is the public interface of our router - but of course, you probably figured that out just by looking at its IP address :)

Next, we need to configure each physical serial interface and bind them both to the multilink group 1:

Verifying Our Configuration

Once the above configuration has been applied to the router, we can view the serial ports and check to see they are up and connected to the provider's DCE:

In most real-life scenarios, you are bound to see a few errors on the interfaces as it's unavoidable, however, this number should remain relatively small. The above output is from a working environment and the statistics haven't been reset for over 1.5 months.  The most important areas of the output are highlighted.

If you would like to check the clocking, but also confirm the DTE/DCE ends, simply use the 'show controllers' command. The output data from this command is quite a bit, so we've omitted the rest, keeping just the information we are interested in:

As a last step, we can view the multilink interface to ensure it's up and running as expected. Remember, we are expecting to see an interface with a bandwidth of 4096Kbit/sec:

The output of our command indicates that everything is running fine. Of course, to access the internet, NAT Overload must be configured. More information on Cisco Router NAT Overload can be found on our NAT Overload page.

Summary

In this article we covered the configuration of PPP Multilink on a Cisco router. We covered the necessary commands to ensure the selected serial interfaces are part of the Multilink bundle and verify that the Multilink interface is working correctly.