HDLC and Multilinking T1 lines

Unanswered Question
Sep 24th, 2007

We have an existing T1 line and cisco 2620 router. The serial interface is using HDLC for L2 encapsulation. We are getting another T1 line that goes to different NOC within the same ISP.

We would like to setup failover so that if one line fails and the other one will still keep the client connections. The two T1 (1.5mb/s each) lines will have to be bundled so that we have 3Mb/s bandwidth. The two T1 lines will be connected to cisco 2651XM with 256MB of RAM.

I would like to know if we should go with PPP or HDLC? or does it even matter?

What's your take on the routing protocol? Should I go with BGP or use static route? I read BGP can't load balance by default. I need to play with Max_path and other options.

We have a number of users who needs VPN in from their home. So the link layer protocol should handle the VPN connections better.


I have this problem too.
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Richard Burts Mon, 09/24/2007 - 13:22

First let me deal with the easy question. If you want to bundle the links then you are talking about multilink PPP. And multilink PPP does not run over HDLC.

However you can not run multilink PPP over these lines if they terminate in different NOCs. In general multilink PPP expects the lines to terminate in the same chassis. There is an extension for multi chassis multilink to terminate lines in different chassis. But I do not think that this runs between different NOCs.

There are several ways to achieve what you need. But first we need some clarification about what the real requirements are. Is the requirement for a primary link and a backup link to take over if the primary fails? It sort of sounds that way in your description. Or is the requirement to use both lines together sharing the load and have one take the full load if the other fails? when we understand the requirements better then we can make suggestions about how to achieve that.



pie8terrr Mon, 09/24/2007 - 13:56


Thanks for your response.

Our requirment is to use both lines together sharing the load and have one take the full load if the other fails.

Another requirment is that each line's WAN IP block should be available to both lines. For example, if line 1 routes subnet and line 2 routes subnet, line 1 should be able to route traffic intented for to gateway on line 2 and vice versa. For all outbound traffic from my network, my firewall will use 2651XM's fe0/0 as its default gateway.

I like to terminate both T1 lines in my 2651XM as this router has 256MB of RAM. If I can, I want to avoid BGP due to routing table size, flapping, etc.



Richard Burts Tue, 09/25/2007 - 03:25

Here are several points in response to the additional information that you have provided.

- I see no problem in terminating both T1s on 2651XM other than the service interruption when you move the existing interfaces (T1 and LAN) from the 2620 to the 2651XM.

- HDLC will work very well as the encapsulation for both T1s.

- Since both T1s are to the same ISP (different NOC) I see no reason to run BGP.

- from your perspective the simple solution is 2 static default routes - 1 pointing to each T1. That will load balance outbound traffic and if either T1 fails the other T1 will carry all the traffic. (Be aware that this implies that if the T1s get above 40% utilization then you are in danger of saturating the T1 if there is a T1 failure and a single T1 must carry all the traffic.)

- the 2 static default routes will take care of outbound traffic. You will need to talk to the ISP about what they will do for inbound traffic. They may do corresponding static routes or they may suggest some other solution.

- I am puzzled about the requirement for the IP block of 1 T1 to be available to the other T1. To use your example, if a T1 is and the other is, what do you send to



pie8terrr Tue, 09/25/2007 - 08:16

-We currently have say subnet and it's routed on the existing T1 line. Our plan was to acquire a second T1 line and route the aforementioned subnet on the second line as well. This way both lines will share the incoming and outgoing load. Our ISP told us that the new T1 line comes with a block of IPs say We wanted to take advantage of the extra available IP numbers. That's why we would like to have both IP blocks routed on the two T1 lines evenly (or based on whatever the load balancing algorithm is used).

-We are fine with one line carrying the full load until the second line comes up.

So when I use two static default routes, does the router create a single logical WAN interface from the two physical interfaces and sets the logical IF as a default gateway? What I am trying to achieve is something similar to multilink PPP.


Richard Burts Tue, 09/25/2007 - 08:42

The extra information gives me some different understanding of the questions. I thought that you meant that 1.1.1 and 50.50.50 were the subnets of the serial interfaces themselves. It is now apparent that these are not serial interface addresses but are address blocks assigned to you by the provider as part of the leased line package.

I still believe that 2 static default routes will work the best for you. It will load share traffic over both links and provide failover if one link fails. It will carry both 1.1.1 and 50.50.50 over both links just fine. It is up to the provider how they route traffic back to you. They may choose to route both address blocks over both links or they may choose to route 1.1.1 over one link and route 50.50.50 over the other link. You will need to discuss this aspect with the provider.

When you use two static default routes the router does not create a single logical interface. You are trying to make it into multilink and as I said in my first response multilink does not run to different NOCs. What you need is 2 independent default routes using your 2 independent links.



pie8terrr Tue, 09/25/2007 - 13:39

I just spoke to my ISP. They said the T1 lines will terminate in two different NOCs, but we will be switching to multilink PPP from HDLC and use static routing.

From what you said, that's not possible as both Ts have to terminate in one router at the same location. I think I need to get more details from my ISP. Next time I will talk to a tech rather than a sales person. Will keep you posted.

Richard Burts Tue, 09/25/2007 - 18:28

I have seen multilink operate when the serial links terminated on different chassis in the same location (multichassis multilink) and this is good. I have not seen multilink operate between routers in different NOCs. If your ISP knows something that will make this work, then more power to them. Till I actually see someone make it work when terminated in different NOCs I do not believe that it will work.



paul.matthews Wed, 09/26/2007 - 00:52

The multichassis *should* work, but it is not something I would recommend - normally both termonating devices would be in the same lan (I suppose that within the ISP space, they could use an ATM cloud and LANE!) (that is probably a requirement) and one becomes the master for the link. Take this example, and call the NOCs NOC-A and NOC-B.

That means the ISP would need a LAN between the two NOCs and the router at each NOC would be on that LAN. The real fun would be that one of the routers would be the master for the multilink cluster, Let's say NOC-A has the master. ALL traffic for the link would have to go to the router at NOC-A. NOC-A would make the decision which router would handle a particular packet, and if a packet was to go via NOC-B, then it would be sent across the LAN to the router at NOC-B. Similarly all trafic returned via NOC-B would be sent to the master at NOC-A and then forwarded appropriately.

That is not soomething I would propose!

There is also another possibility - that when hitting the ISPs NOC, the PPP is not going to be terminated immediately, but using some form of channel emulation service both links are presented to a single router, with the multilink effectively terminated on a single router, and some procees offering an alternate router if the primary fails.



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