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Multilink loadbalancing

I would like to know how the multilink will behave if i have 3 links bundled together each of diffrent bandwidth.

Eg. i ahve 2 links of 1MB each and the 3rd link is a 512K link

I have been using multilink without any problems with same bandwidth links but it doesn't seem to work properly in the above scenario.

i would like to know how the links will be load balanced in the above scenario.

Is this methos recommended?

thanks

Narayan

21 REPLIES
New Member

Re: Multilink loadbalancing

narayan

can u plz explain little bit more which routing protocol are you using and what short of router do u have.....????

Re: Multilink loadbalancing

i have 3825 routers at both ends and iam not running any routing protocol.

all the routes are static routes.

Narayan

New Member

Re: Multilink loadbalancing

narayan

what sort of WAN link do u have and is it going to same destination or different.....like two isp's???? or same dest

New Member

Re: Multilink loadbalancing

This depends on a lot of things, but let's assume for sake of argument that you are running 2800 series routers and thus have a recent IOS image. As long as you have set your bandwidth statements on the interfaces correctly, this should be supported.

If this doesn't help, posting more details like the type of router, IOS version, and configuration snip would be helpful.

Re: Multilink loadbalancing

Hi Narayan,

Traffic across the links in a multilink should get equally loadbalanced as the packets are equally fragmented across each link rather than each packet on each link .

I think BW factor comes into affect when we use some routing protocols and as you are using static routing , it should not have problem , might be some ios bug kind of thing or a new ios feature.

regards

vanesh k

Re: Multilink loadbalancing

i think i need to rephrase the question.

say i have 2 links each of one MB bundled together and the traffic is 80%. The traffic is perfectly loadbalanced.

Now say if i add another link to the bandle whose capacity is just 128K, how the traffic will be loadbalanced here.

Will the traffic be loadbalanced between all the 3 links till the traffic exceeds 128K on the 3rd pipe and then distributes the remaining on the other links?

iam using 3835 routers with a 12.4 image (no routing protocols, & ofcourse since iam doing multilink it will be do the same destination)

Regards

Narayan

Re: Multilink loadbalancing

Hello Narayan,

when you do load balancing based on static routes (and CEF), adding a third link with lesser speed will cause that link to be equally utilised. The default for load-balancing is per-destination, you could change that to per-packet load-balancing, but that still would cause a bottleneck on the 128K link.

In order to use all 3 links, you would need a routing protocol such as EIGRP, which can do unequal cost load balancing.

How do you have your load-balancing configured now ? Can you post the config of your router ?

Regards,

GNT

Re: Multilink loadbalancing

Hi GNT

The configs are simple

The Bandwidth is restricted on the Mux to 128K on the 3rd pipe.

interface multilink 1

ip address 10.10.10.1 255.255.255.252

ppp multilink

ppp multilink group 1

int serial 0/0

bandwidth 1024

encap ppp

ppp multilink

ppp multilink group 1

int serial 0/1

bandwidth 1024

encap ppp

ppp multilink

ppp multilink group 1

int serial 0/2

bandwidth 128k

encap ppp

ppp multilink

ppp multilink group 1

ip route x.x.x.x x.x.x.x 10.10.10.2 (other side multilink)

Narayan

Re: Multilink loadbalancing

Hey Narayan,

I understood your question, and I dont think any of the previous posts answered it.

I'm looking forward to this answer as well,

Vlad

New Member

Re: Multilink loadbalancing

Hi,

If you are looking for a multilink solution based on bandwidth you can take a look at the Gateway Load Balancing Protocol (GLBP). This protocol offer different options to handle traffic as well as offers redundancy for your links.

I have it implemented on a lab enviroment and it works very nice.

Let me know if it helps.

Re: Multilink loadbalancing

Rafael

I know how GLBP works. THe post was meant to understand the behaviour of multilink when configured with different speed links.

As VLAD has already stated none of the posts have answered my question and the above post too deviates fron the original post.

Narayan

Re: Multilink loadbalancing

Hi Narayan,

As mentioned earlier , Traffic in a multilink is equally fragmented across the links and there is no per packet load sharing across the links .

So , even with the BW configured differently traffic would be distributed equally and if link capacity is less when compared with others , there would be drops and it would in turn affect others as the packets are fragemented equally across the links.

regards

vanesh k

Re: Multilink loadbalancing

That is what i was exactly looking for. Whether packets will get dropped once the threshold is reached.

Can you give me any link that confirms the same

Thanks

Narayan

Re: Multilink loadbalancing

Hi Narayan,

I tried searching the net , but Iam not able to find the exact doc .

I have experienced the same kind of problem with one of our customer.

regards

vanesh k

Re: Multilink loadbalancing

Let me see what TAC has to say about this although iam very skeptical on their response

Narayan

New Member

Re: Multilink loadbalancing

narayan,

Once you have the response from the tac, please post it here.

thanks

arav

Re: Multilink loadbalancing

Hi, this in an interesting question so thought I'd quickly Lab it up. I configured Multilink ppp over frame-relay. Without any changes to BW per-packet loadbalancing occurs and the multilink interface has BW of 200000 (correct BW for 2 virtual-templates). When I change the virtual templates to 128k and 512k the aggregated multilink interface drops to 256k.

So, to me it looks like we are still doing per-packet load balancing (as suggested in an earlier post) and the router is making the aggregated bandwidth to be the lowest cost interface multiplied by number of interfaces in the group.

interface Virtual-Template1

bandwidth 128

no ip address

ppp multilink

ppp multilink group 1

!

interface Virtual-Template2

bandwidth 512

no ip address

ppp multilink

ppp multilink group 1

r3#show int multilink 1

Multilink1 is up, line protocol is up

Hardware is multilink group interface

Internet address is 1.1.1.3/24

MTU 1500 bytes, BW 256 Kbit, DLY 100000 usec,

Re: Multilink loadbalancing

Mark,

I know that manipulating the virtual template bandwidth affects the bandwidth on the multilink. I have experienced the same earlier. But still how did you manage to restrict the bandwidth on one interface?

I think manually changing the bandwidth on the interface will affect only the routing protocols.

But again what we want to know is how the loads get distributed when the true physical bandwidth of the link is different

Can you try to restrict the bandwidth to 56K by policing the traffic on one interface and try pumping traffic and see what happens

I will surely post the TAC response

Narayan

Re: Multilink loadbalancing

Hi all,

Below is the response from TAC

Multilink PPP, standardized in Request for Comments (RFC) 1990, is similar to load balancing techniques in that it sends packets across the individual links in a round robin fashion. However, Multilink PPP adds three significant capabilities.

Firstly, because Multilink PPP works at the link layer, it makes an Nx ISDN bundle appear as one logical link to the upper layer protocols in the router. Thus, only one network address needs to be configured for the entire Nx ISDN bundle.

Secondly, Multilink PPP keeps track of packet sequencing and buffers packets that arrive early. With this ability, Multilink PPP preserves packet order across the entire Nx ISDN bundle.

Thirdly, packet fragmentation can be enabled to split large data packets into smaller packet fragments that are individually transmitted across the links. In many circumstances, fragmentation can increase the efficiency of the N x ISDN link, and is often required for links less then 768Kbps carrying VoIP.

Multilink does not round-robin packets. It give data to the links on demand.

Several small frames may land on one link to balance a large frame on another link. A fast link draws more data than a slow link. Over time, the amount of data sent on each link is proportional to its bandwidth.

MLPPP do make a point of spreading frames across all links as much as possible, and keeping all the links equally utilized. Hence if each link takes about the same amount of time to transmit the frames given to it, the results will approximate round-robin distribution.

Narayan

New Member

Re: Multilink loadbalancing

Hello Narayan,

Thanks for providing such a helpful information. I have a query regarding Fragmentation that what should be the MTU size of a packet for fragmentation to be applicable to that packet in MLPPP implementation.

Regards,

Mujeeb

Hall of Fame Super Gold

Re: Multilink loadbalancing

One must be careful in mentioning theoretical support of unequal cost load balancing in protocols like EIGRP, because nobody is known to use it.

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