10-16-2001 06:05 AM - edited 03-01-2019 06:55 PM
We are a multihomed ISP with two full BGP peer.
One is connected to a 10Mbit line, the other one is only E1.
We are using AS-path prepending to announce our AS longer over the E1, this way we only get traffic which is "really" good on this upstream provider.
The question is how do we set paths for outgoing traffic?
Right now I see almost every network 1 AS-hop shorter on the E1, but we want the majority of the traffic going out on the 10Mbit link.
So I use local weights to push *every* traffic on the 10Mbit.
Is there a possibility to prepend the *incoming* AS-path? So that I can add one AS-hop to every AS learned via E1?
tia
Christian
10-17-2001 06:44 AM
You can't define the policy that how to enter the traffic in your network.
you can use the med attribute to configure your router EBGP paths
10-18-2001 03:04 PM
Hi
Manipulating the AS-PATH and the MED is to
influence the entrance (inbound traffic) .
To influence the exit (outboud traffic) You could
use the "local_pref" attribute .
Presume You are using single router to terminate
the 2 link , so "weight" is almost as good as
"Local_Pref" . In case 2 routers are used U side so
the use of "Local_Pref" is mandatory ("weight" is
local to the router and not propagated within
the AS ).
IF U still need to influence the exit by using
the AS-PATH attribute U could manipulate the
incoming AS-PATH but U have to prepend the provider
AS nbr not your's .
Brgds
10-18-2001 06:32 PM
You can either prepend the last ISP AS as needed and advertise out to IBGP peers or you can set the local preference for routes which would also be communicated to IBGP peers within your AS.
I would use the local preference. If you have other EBGP peers that you also need to provide routing policy for, you may want use the path prepending to span the ASs
10-25-2001 07:37 PM
Have you tried using a route-map to set local pref higher on the 10MB. Example:
router bgp "Your AS"
neighbor "10Mb AS#" route-map LOCPREF in
ip as-path access-list 90 permit "use reg exp to list AS path"
route-map LOCPREF permit 10
match as-path 90
set local-preference 200
!
route-map LOCPREF permit 20
This route-map will set local pref to 200 for all matching AS pathes listed in the "ip as-path access-list 90". Default is 100 so your out bound traffic will perfer the higher local pref.
Also check out "Internet Routing Architectures 2nd Edition"
10-25-2001 08:12 PM
One thing you might consider is asking your providers (especially the one with the smaller connection) to send you bgp community information.
Most providers can give you "customers-only" communities. Since this minority of routes will likely perform better through this provider, you'd want to take advantage of that. You'd do this by using a route-map to increase the localpref of those routes.
You'd set
all customer routes from provider A to localpref of 200,
all other routes from them to localpref of 90
and all routes from provider B to localpref of 100.
The other choice would be to select all routes that terminate in the provider's AS to a higher localpref. This is generally a smaller subset than all customer routes.
I'm pretty sure there's a good example on cisco's web page.
Most reasonable-size and up ISP's can support this configuration.
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