BGP inbound for Class C - Preferences Not Taking Effect

Hi all,

I have a class C public IP range that was allocated from Apnic.
ISP A = 40 Mb fibre connection
ISP B = 40 Mb fibre connection
Behind both ISP's connections we have a cisco 2801 router
Each router gets a default route only via BGP
Each router advertises our public IP range via BGP to the ISP's

We are attempting to preference ISP A over ISP B to get most of the traffic flowing over the preferred provider.

We have tried the following:

Item                                      ISPA                   ISPB
prepending                           None Set           6 x prepended paths
local pref                              Set to 300          Set to 100
local pref via community      Set to 100          Set to 90
MED                                      Set to 50            Set to 350

None of this works - we still get almost all traffic coming into ISPB connection.

Can anyone offer any suggestions on what we would need to do to get some of the traffic from ISPB to ISPA.
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rauenpcConnect With a Mentor Commented:
In my experience, many ISP's put a local preference to any routes they learn directly. This means that they could care less that you prepend your AS 6 times because the local preference wins out. In your scenario (and if the ISP B is doing the local preference thing), traffic goes out ISP A and hits a destination that is closer to ISP B. This destination could be on ISP B's network or simply closer. On the return route, traffic goes through ISP B and due to local preference ISP B drops it off to you directly ignoring your prepending.
There are one of two ways to get around this that I can quickly think of. Call the ISP and ask they they change that policy for your connection. Or, you could configure conditional advertisements so that ISP B doesn't know about your subnets until ISP A is down. The first option is without doubt the better of the two.
ISP B likely has more interconnections which will explain.
The other option is that ISPA is not pushing/advertising your IPs or those with whom they peer are not rebroadcasting them.

Look at and looking glass project/bgpplay to see BGP convergence for your network.
Just retread your post, why don't you terminate both ISPs on a single 2801?

And maintain the other as a backup.
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MED is not relevant when load balancing between ISP, so you can skip that.

Local pref via community, would mean to influence the local pref in each ISP network. Are you sure you use the right communities?

If ISP A buy IP Transit from ISP B, then it may be tricky to get this to work.
The reason local pref might not be working here because it is always for outgoing traffic for ibgp peers and incoming for ebgp peers. If you can satisfy this condition, local pref will/should work.

Since you have only stated that you have a public ip from APNIC and there is no mention about the AS(s) you are using, it will be difficult to visualize your scenario and offer a solution.
A jpg or a block diagram of what AS you are using and what is your hardware would point the answers more precisely towards a solution.

Testing ISPA feed using traceroute to experts-exchange.
and then sending the same traceroute via the ISP B interface
to see whether they take the same path as pointed earlier i.e. ISP B is the upsrteam/peering provider for ISP A.
Can you post your scrubbed bgp configs (and all related parts: ACLs, route-maps, etc.) please for both routers?
Dealer_SolutionsAuthor Commented:
ISP B just got back to me and stated that they do not forward the community string outside of their own AS.

They strip the community setting the local preference to 90 and so when it hits the next AS the local preference is back to 100.

I am going to ask them to relax the policy however they have stated that the only way they will allow what I am proposing is to use conditional advertisements.

So it turns out it's not the architecture but the vendor...
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