On Feb 17, 2006, at 1:25 PM, Christopher L. Morrow wrote:
On Fri, 17 Feb 2006, Todd Vierling wrote:
On Thu, 16 Feb 2006, Warren Kumari wrote:
If your primary is connected to ISP_A and the backup is connected
to ISP_B,
customers connected to ISP_B MAY still flow to your backup DC
(ISP_B will
probably set local preference on all customer routes - you should
be able to
override this behavior with communities but not all providers
support this (or
honor it 100% of the time!))
And in addition to that, even multihomed customers of ISP_B may
choose the
prepended route for a number of different reasons; for instance,
ISP_B might
be a cheaper pipe for them, or there may be a smart-ish routing
device or
scheme in play that overrides normal BGP decision making.
I might be crazy, but couldn't you just prepend the route enough to
effectively poison it at ingress to 'backup-isp' ? so they kept
chosing
the remote path and never really accept the route from local until the
remote path is gone?
Not really - horrendous ASCII art below:
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
