BGP "AS-PATH" manipulation with BGP Global cmds....
To the BGP Gurus,
I may be "flogging a dead horse" here but here goes........:
High Level Summary:
Are there any gloabl BGP neighbours cmds (i.e. NOT under VPNV4 or IPV4 address-family functionality) to perform a "AS-override" function?
PE1 (AS100) PE2(AS100)
\ VPN1 / VPN2
Basically the CE is dual-homed via different VPNs back the same AS100 cloud (please do not ask why this is being done this way, as I have only "extracted the bare minimal essence" to simplify the problem description and the issue- the real setup is quite complex....)
So the CE will learn the 10.100.10.0 subnet from PE2, likewise the CE will learn the 192.168.10.0 subnet from PE1. So far so good. Now when the CE advertises the 10.100.10.0 to PE1, of course PE1 will see it's own AS in the AS path info and delete/ ignore it. (BGP loop prevention). A similar scenario is undertaken on PE2 for the CE advertise 192.168.10.0 subnet.
[Yes, PE1 is required to have visibility of the 10.100.10.0 subnet and vice versa]
So two thoughts on this:
1) A couple of standard "nerd-knobs" to address this would be the bgp neighbor "as-override" or the "allowas-in" functions. However as this is a very one off / non-standard customer specific scenario, one would be relundant to apply changes on the PE side and if the cmds are global, may affects ALL BGP neighbors, not this specific one. There is also high-impedance to deviate away from a standard PE configuration...
What I would like to do is to remove the "AS100" from or manipulate the as-path information from the CE advertised routes to the PEs.
2) It seems that the only CE global BGP metric available to influence the best path selection is the autonomous system path length, which basically "prepends" an arbitray AS path string and not to modify one [i.e. I would like to remove the AS100 from the advertised CE routes and of course put the appriopriate filters in to prevent any potential loops etc]
I have investigated the use of route-maps to modify the advertise routes from the CE. Although one can used regualar expresssions to match on the as-path aspects, there seems to be no corresponding "set" cmd to modify it (probably because of the obvious implications! :-))
A temporary work-a-round using the "aggregate-address.......summary only" cmd does permit the PEs to accept the subnets, but with there origins stemming from the CE.
Is there any other way / method to manipulate / permit the AS-Path information that the CE is advertising to the PEs?
if R2 sumarize the network from R1 and R3 to 10.1.0.0/16 then both of R1 and R3 will receive the route with source AS of AS 2 because once the route aggregated it considered atomic aggregate and it will lose the route attributes such as AS-path
if you want this summary route to be sent only to AS 3
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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