Time for routing change to take effect?

We have a /24 of PI space that's currently being routed to leased line A
from ISP A.

We're moving ISPs so have leased line B from ISP B in place and working
just fine with private IP's/NAT on the trusted side.

ISP B has been assigned with RIPE as the MNT object on the network block
though for now nothing else has changed as we're using the IP's on our
live connection.

I'm struggling to get a clear answer on how long it will take for the
change in routing to take effect once they do whatever they have to do?

TIA,
Paul

Paul Hutchings wrote:
> We have a /24 of PI space that's currently being routed to leased line A
> from ISP A.
>
ok..

> We're moving ISPs so have leased line B from ISP B in place and working
> just fine with private IP's/NAT on the trusted side.

ok..
>
> ISP B has been assigned with RIPE as the MNT object on the network block
> though for now nothing else has changed as we're using the IP's on our
> live connection.

ok..
>
> I'm struggling to get a clear answer on how long it will take for the
> change in routing to take effect once they do whatever they have to do?
>

A fairly difficult question to answer. IIRC everyone still used
BGP..between ISPs..and your network will have to be deleted from the
original ISP's advertised routes, and added to the new ISP's.

Experience suggest that this will not happen exactly synchronously, and
you may experience a period of instability with both, or neither, ISP
advertising the block.

Which can lead to 'route flapping' as boundary routers pick up two
different routes..

This shouldn't matter too much as longs as the original ISP has the
correct route to the new ISP so that wherever the packets go, they
ultimately end up in the right place.

If the techies in both ISPS are on the mark, it should be almost
seamless..for most local traffic within the UK, experience suggests that
*some* remote ISPS in far flung corners of the world may be unreachable
for a day or two. Because basically moving global routing tables around
takes bandwidth and some people dont do it as promptly as they should.

When I've done this in the past, its been well worth while talking to
the top techies at BOTH ISPs, being there, on line and on the phone at
switchover time, and checking routes to far flung places as it happens.
In case they cock it up.

However, even if they don't, other ISPs can, and you are dependent on
your ISP to contact them to get things sorted out.

So the short answer is it should be seconds only. The more realistic
answer is that you may get instability for some days to some places..do
it on saturday morning and hope its over by Monday.

> TIA,
> Paul

> I'm struggling to get a clear answer on how long it will take for the
> change in routing to take effect once they do whatever they have to do?

It'll be fairly instant that you'll see traffic but it's worth running both
lines for 24 hours to be sure.


--
Regards

John Lyons

Netserve Consultants Limited
www.netserve.co.uk