North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Disaster recovery using as-prepend?
On Feb 17, 2006, at 1:25 PM, Christopher L. Morrow wrote: Not really - horrendous ASCII art below:On Fri, 17 Feb 2006, Todd Vierling wrote:I might be crazy, but couldn't you just prepend the route enough toOn Thu, 16 Feb 2006, Warren Kumari wrote:If your primary is connected to ISP_A and the backup is connected to ISP_B,And in addition to that, even multihomed customers of ISP_B may choose the Customer / \ / \ ISP_A ---------ISP_B \ / \ / DC1 DC2 Assuming DC is AS_65530, ISP_A is AS_655301 ISP_B is AS_655302 and DC_2 prepends 5 (or some other "large" number) times: Under "normal" conditions: ISP_A sees: 192.0.2.0/24 -- 65530 i (direct from DC1) ISP_B sees 192.0.2.0/24 -- 65530 65530 65530 65530 65530 i (direct from DC2) 192.0.2.0/24 -- 65531 65530 i (ISP_A -> DC_1) <= Best due to AS_PATH Customer sees: 192.0.2.0/24 -- 65531 65530 i (ISP_A -> DC1) <=Best due to AS_PATH 192.0.2.0/24 -- 65532 65531 65530 i (ISP_B -> ISP_A -> DC1) If ISP_B sets Local-Pref on customer routers: ISP_A sees: 192.0.2.0/24 -- 65530 i (direct from DC1) ISP_B sees: 192.0.2.0/24 -- 65530 65530 65530 65530 65530 i (direct from DC2) <- Best due to Local-Pref 192.0.2.0/24 -- 65531 65530 i (ISP_A -> DC_1) Customer sees: 192.0.2.0/24 -- 65532 65530 65530 65530 65530 65530 i (ISP_B -> DC2) 192.0.2.0/24 -- 65531 65530 i (ISP_A -> DC_1) <- Best due to AS_PATH This means that any traffic that enters ISP_B (eg: Customer is singly homed to ISP_B, their connection to ISP_A goes down or they adjust local_pref to prefer ISP_B) will go to DC2. The problem is that Local-Pref trumps basically all other conditions in the BGP decision process - if ISP_B adjusts it it will be prefered in their network no matter how many times you prepend. Warren
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