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Are there any problems running BGP over a BT LES100 Circuit

Are there any problems running BGP over a BT LES100 Circuit?

I have been told conflicting things by different ISPS, I assume there id no problem as if one end of the link dies and one stays up that TCP will be dropped and thus down UDP 179 instantaniously!

The competing ISP has said that:

ne of my clients sent this to me I thought it may be of interest.

"the reason we don't like to use LES services is because the BGP

peer connection is not on the same wire. Unlike when you have say a E3 link

the equipment either end is a router running the BGP session and therefore

if the cable (wire) goes down the BGP sees this immediately and starts the

reconvergance process which would then route via are secondary link.

In the case of a LES connection the equipment either end is usually a

switch which then has routers either side of them, If the cable (wire) goes down on a LES service where the BGP is effectively not on the same wire as it goes through switches this causes a delay. This is OK on most networks where they just use the Internet for browsing as the user can put up with a 2 minute delay."


Re: Are there any problems running BGP over a BT LES100 Circuit

It sounds like you're discussing Frame Relay vs a straight wire connection of some type (?). The issue with any circuit where there is a switch in the middle is that the layer 3 protocols, such as BGP, don't always know when the link fails, since the link could fail someplace other than the local router to switch connection.

If this is frame relay, however, it's possible to use end-to-end lmi to resolve this on point-to-point links. You have to define the local router interface as a point-to-point link, possibly on a subinterface, to get the interface to fail when the end-toend lmi reports the link down. If it's some other type of link, then you could look into simply lowering the BGP keepalive interval, if your service provider will let work with you on this. There's a point at which you can't lower the keep interval any lower, though, probably about 1 and 3 seconds, so you will still only get convergence times in the seconds.


Community Member

Re: Are there any problems running BGP over a BT LES100 Circuit

He's absolutely right. For those who don't operate in the UK, a LES100 circuit is a 100Mbits/s circuit presented as 100BaseTX at the demarc. In effect, the circuit is presented to the customer as a switch port, and as long as there is power to the terminating device, the attached router gets link up. If the circuit drops out within the telco cloud, BGP only registers this when the keepalives stop coming in.


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