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Re: Preserving established communications (was RE: about draft-nordmark-multi6-noid-00)
On 31 okt 2003, at 14:38, marcelo bagnulo wrote:
The problem is that to detect that there is no route to a certain,
all
the alternative routes with increasing AS path are tried, which is
inherently slow.
Only if there are no alternatives. If there are, the path for those is
probably not that long so they show up pretty quickly. Only when the
last/only path goes down you'll see the following problem:
Ok, but this is the case that we care, right?
From ISPA prespective, there is no longer a route to the destiantion,
so the
multi-homed site detects that and tries the alternative ISP (ISPB).
So the case that we care is the reconvergence of BGP within the portin
of
the network where a route to the destiantion address no longer exist
No, fortunately it works a bit better than that. Consider:
ISP A
/ | \
X | Y
\ | /
ISP B
If X gets transit from both A and B, both A and B will always see the
path to X over the other. (This is required because if X didn't get
transit from B but only from A, B would _have_ to reach X over A.) So
B's routing table would show:
prefix next hop as path
10.11.12.0/24 x x
a a x
If the direct connection to X now disappears, B's routing table shows:
prefix next hop as path
10.11.12.0/24 a a x
This happens within seconds of the BGP session between X and B going
down.
BGP only propagates the best path, so before the outage Y would see:
prefix next hop as path
10.11.12.0/24 a a x
b b x
and after:
prefix next hop as path
10.11.12.0/24 a a x
b b a x
Anyway, i guess that the reconvergence time is only part of the
problem.
Another part of the problem is the time required to detect the outage.
Only
this factor would render the solution inacceptable for many
applications.
Agree. Fortunately work is underway in this area so hopefully this will
get better in the future.