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Configuring BGP so a DS1 backs up a DS3?

We currently have 2 ISP's with equal bandwidth connections. Via BGP traffic fails over if a ISP connection fails. We are upgrading to a DS3 but want to keep a DS1 with a 2nd ISP as a backup to the main link. Will BGP allow me to force all traffic via DS3 until problems occur?

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Accepted Solutions
Silver

Re: Configuring BGP so a DS1 backs up a DS3?

Hi there,

Yes and no... :)

Yes to the outbound but both yes and no to the inbound..

Here are some pointers, while there exists others as well.. :)

You can use the "local-preference" parameter to influence which way to send your traffic out.. but incoming is more difficult. It all depends on several parameters:

- First of all; you can't control how the internet routes, but you can give hints and try to manipulate as much as you can.

What I would do is simply to prepend your own AS in the ASPATH a number of times on the peering towards ISP 2 over the DS1. Because routedecision is made by ASPATH in a early stadium in BGP, a longer ASPATH will be less attractive.

Another option is to simply announce more specific prefixes over the DS3 link:

- If you have f.ex the 169.254.0.0/16 prefix, announce this in addition to 169.254.0.0/17 and 169.254.128.0/17 on the DS3 links..

There is of course a practial limit to this method as some ISP's out there simply filters out prefixes less than a specific size (I think many uses /19 as a minimum size). This could then lead to some suboptimal routing for these ISP's, but it shouldn't matter too much (in worst case, you'll get some traffic on the DS1.. :)

Here is a link to a good document:

"BGP Best Path Selection Algorithm"

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

Did it help? If so, please rate it.

3 REPLIES
Purple

Re: Configuring BGP so a DS1 backs up a DS3?

Hi,

If both of those links are on the same router, what you can do is to apply a route-map to the BGP session over the DS1 that sets the local-preference of all routes learned via that session to something like 200. By default, all routes learned over the BGP session over the DS3 will get a local preference of 100 and will be installed as the preferred routes. If the DS3 goes down, the best path will now be over the DS1.

This can also work if the links are on different routers. You will then have to set up both these routers to conditionally inject defaults into your IGP, with the router with the DS1 injecting the default with a higher metric. The injection of the default has to be conditional so that it is only advertised if the router's BGP session is also up.

hope that helps.

Paresh.

Silver

Re: Configuring BGP so a DS1 backs up a DS3?

The local-preference parameter is respected in the whole local AS (no matter which router), the BGP Weight though is a Cisco-proprietary parameter and is local to the router itself.

Silver

Re: Configuring BGP so a DS1 backs up a DS3?

Hi there,

Yes and no... :)

Yes to the outbound but both yes and no to the inbound..

Here are some pointers, while there exists others as well.. :)

You can use the "local-preference" parameter to influence which way to send your traffic out.. but incoming is more difficult. It all depends on several parameters:

- First of all; you can't control how the internet routes, but you can give hints and try to manipulate as much as you can.

What I would do is simply to prepend your own AS in the ASPATH a number of times on the peering towards ISP 2 over the DS1. Because routedecision is made by ASPATH in a early stadium in BGP, a longer ASPATH will be less attractive.

Another option is to simply announce more specific prefixes over the DS3 link:

- If you have f.ex the 169.254.0.0/16 prefix, announce this in addition to 169.254.0.0/17 and 169.254.128.0/17 on the DS3 links..

There is of course a practial limit to this method as some ISP's out there simply filters out prefixes less than a specific size (I think many uses /19 as a minimum size). This could then lead to some suboptimal routing for these ISP's, but it shouldn't matter too much (in worst case, you'll get some traffic on the DS1.. :)

Here is a link to a good document:

"BGP Best Path Selection Algorithm"

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

Did it help? If so, please rate it.

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