Please help our simple bgp

Justin M. Streiner streiner at
Tue Jan 31 03:12:25 UTC 2012

On Mon, 30 Jan 2012, Ann Kwok wrote:

> Our router is running simple bgp. "one BGP router, two upstreams (each 100M
> from ISP A and ISP B)
> We are getting full feeds tables from them

Are you sure you're getting a full table from each provider?  A full IPv4 
feed is close to 400,000 prefixes and a full IPv6 feed is getting close to 
8,000 routes.

It's also important to understand what the desired behavior is.  Do you 
want to use both upstream links, or do you want to use provider B only 
when provider A is down?  Based on your description above, I'm guessing 
you want to use both links at the same time.

> We discover the routes is going to ISP A only even the bandwidth 100M is
> full

BGP doesn't know or care about link utilization.  If all of your outbound 
traffic is using only one link, then it sounds like one (or more) of a 
few things is happening:

1. Provider B is only sending you a default route, or something less than 
a full table.  If that's the case, then you need to get provider B to send 
you a full table, or verify that your BGP import policy isn't rejecting 
most of what provider B is sending you.  Most specific route wins.
2. Provider B's routes are less preferred by your router for one or 
more reasons, with a longer AS path probably being the most common reason.
Check if provider B is doing anything like prepending routes before they 
send them to you (generally a bad idea, but I've seen stranger things 
3. You are taking some action on provider B's routes to make them less 
preferred, such as lowering the local-preference.  It might be helpful to 
post the whole "router bgp XXXX" section of your config, with any related 
items (route-maps, access-lists, prefix-lists, AS-path access-lists (if 
any, etc).

> Can we set the weight to change to ISP B to use ISP B as preference routes?
> neighbor description ISP B
> neighbor remote-as 111
> neighbor weight 2000

If you are receiving a full table from both providers, you can write a 
policy to reset the local-preference on some of the routes you get from 
provider B to higher than the same routes you get from provider A.

> If this works, how is ISP B upstream connection is down?
> Can it still be failover to ISP A automatically?

If you receive a full table from both providers, you should be able to use 
either provider's link when the other one fails, with little to no 
intervention on your part.


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