Please help our simple bgp
Justin M. Streiner
streiner at cluebyfour.org
Mon Jan 30 21:12:25 CST 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
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
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
> Can we set the weight to change to ISP B to use ISP B as preference routes?
> neighbor 126.96.36.199 description ISP B
> neighbor 188.8.131.52 remote-as 111
> neighbor 184.108.40.206 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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