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New Member

Load Balance Multiple E1s

I need multiple E1 connections to Internet. (10 * E1s) How do I load balance this links?

Thanks

Nimitt

  • Other Network Infrastructure Subjects
1 ACCEPTED SOLUTION

Accepted Solutions
New Member

Re: Load Balance Multiple E1s

Yes, a couple. The fact that you are aiming to create a single logical point-to-point link between you and your ISP means that clever traffic engineering using BGP does not need to be configured, but there is one major issue to be overcome. This is that IOS does not permit more than 6 routes to the same destination address (the default network, in this case) to appear in the routing table. Your 10 E1s, if configured separately, would break this limit, so we have to use a cleverer approach.

The very-simple and grossly-inefficient method is to combine all the E1s into a single PPP multilink bundle. This means there is only a single IP route in the table to the default network. I do not have the figures to hand, but I have seen test results showing that as the number of channels in the ppp bundle increases, link efficiency decreases alarmingly (by maybe 40%, if I remember right). By comparison, there is very little loss of efficiency by using equal-cost routing over separate IP paths up to the 6-path maximum (maybe 10% from memory), however you need to be careful to ensure that a suitable IP load-balancing method is used. You will need to agree this approach with your ISP as they will need to configure a mirror-image of your configuration.

A better approach than this would be to bundle all the E1s together using IMA. Sure, there is a loss of efficiency because of the cell tax overhead, but I expect performance would be better. You would need to agree this with the ISP, because of course they will need to do the same at their end, involving additional equipment.

A halfway-house solution might be to agree with your ISP to combine pairs of E1s into PPP multilink bundles. You can then have 5 equal-cost paths, with only a small performance reduction. You would do this by configuring virtual-templates on the router and bundling pairs of E1s into each virtual template. What you gain in perfomance you lose in complexity of configuration, and again you would need to agree this with your ISP because they will need to configure a mirror image.

A very straightforward and efficient approach, and one that involves no special configuration on your ISP's side, is to split the E1s onto two different routers (assuming they don't have the same problem you do!). Configure each E1 separately . Then connect both these routers to a third router which connects back into your network:

<============ ISP =============>

| | | | | | | | | |

| | | | | | | | | |

\ \ | / / \ \ | / /

Router 1 Router 2

| |

\ /

Router 3

|

|

Internal network

Routers 1 and 2 each have 5 equal-cost static default routes to the internet. Router 3 has 2 static default routes pointing at Routers 1 and 2. Router 3 does not need to be big or complex. This adds some latency to the links, but not much, but gains greatly in efficiency (and resilience). I would go for this given the choice.

Hope this helps.

3 REPLIES
New Member

Re: Load Balance Multiple E1s

The problem is not as simple as it sounds. Are your 10 E1s all going to the same ISP and POP? If so, you could achieve load balancing using a combination of PPP multilink and equal-cost load-balancing, but this will be inefficient in terms of router CPU. A better solution would be replace the 10 E1 circuits with a one or two fatter pipes, such as 2 fractional E3s.

If the E1s are going to different ISPs and/or different POPs you have a much larger problem. You can do anything you like to load-balance outgoing traffic from your networks, but you can only influence incoming traffic using BGP, which you will need to agree with your providers.

Hope this helps.

New Member

Re: Load Balance Multiple E1s

Hi,

U have got my problem right. In India I cannot get fractional E3s. All the links are to a single ISP, is there any other solution?

Thanks for your help.

Nimitt

New Member

Re: Load Balance Multiple E1s

Yes, a couple. The fact that you are aiming to create a single logical point-to-point link between you and your ISP means that clever traffic engineering using BGP does not need to be configured, but there is one major issue to be overcome. This is that IOS does not permit more than 6 routes to the same destination address (the default network, in this case) to appear in the routing table. Your 10 E1s, if configured separately, would break this limit, so we have to use a cleverer approach.

The very-simple and grossly-inefficient method is to combine all the E1s into a single PPP multilink bundle. This means there is only a single IP route in the table to the default network. I do not have the figures to hand, but I have seen test results showing that as the number of channels in the ppp bundle increases, link efficiency decreases alarmingly (by maybe 40%, if I remember right). By comparison, there is very little loss of efficiency by using equal-cost routing over separate IP paths up to the 6-path maximum (maybe 10% from memory), however you need to be careful to ensure that a suitable IP load-balancing method is used. You will need to agree this approach with your ISP as they will need to configure a mirror-image of your configuration.

A better approach than this would be to bundle all the E1s together using IMA. Sure, there is a loss of efficiency because of the cell tax overhead, but I expect performance would be better. You would need to agree this with the ISP, because of course they will need to do the same at their end, involving additional equipment.

A halfway-house solution might be to agree with your ISP to combine pairs of E1s into PPP multilink bundles. You can then have 5 equal-cost paths, with only a small performance reduction. You would do this by configuring virtual-templates on the router and bundling pairs of E1s into each virtual template. What you gain in perfomance you lose in complexity of configuration, and again you would need to agree this with your ISP because they will need to configure a mirror image.

A very straightforward and efficient approach, and one that involves no special configuration on your ISP's side, is to split the E1s onto two different routers (assuming they don't have the same problem you do!). Configure each E1 separately . Then connect both these routers to a third router which connects back into your network:

<============ ISP =============>

| | | | | | | | | |

| | | | | | | | | |

\ \ | / / \ \ | / /

Router 1 Router 2

| |

\ /

Router 3

|

|

Internal network

Routers 1 and 2 each have 5 equal-cost static default routes to the internet. Router 3 has 2 static default routes pointing at Routers 1 and 2. Router 3 does not need to be big or complex. This adds some latency to the links, but not much, but gains greatly in efficiency (and resilience). I would go for this given the choice.

Hope this helps.

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