From: Brad Hedlund (BHedlund@xxxxxxxxxxxxxxxxxxx)
Date: Mon Feb 14 2000 - 12:36:56 GMT-3
Russell,
In the real world, using 'backup interface' on a point-to-point F/R
interface is not a safe call.
You are relying on the F/R carrier to send your routers the proper LMI
status messages if the PVC is broken somewhere in the cloud. I can tell you
from experience, it doesnt always happen.
In a situation where your PVC traverses multiple carriers (you dont have
end-to-end LMI) the chances get even worse.
The safest way to backup a F/R circuit is not to depend on the F/R carrier's
status messages at all. Instead, (heres a novel idea) lets backup the
circuit if we stop recieving data from the remote router. That is the
ultimate indication that something is wrong.
We can achieve this by running a dynamic routing protocol over the PVC's and
configure floating-static routes. Simply, we would say, "if my more
preferable dynamic routes disappear from the routing table, use a static
route that will initiate dialing on the ISDN link".
Once the router has dialed the ISDN line and connected to the remote site,
lets run the routing protocol over the ISDN line and get all of my routes
back.
If/When the PVC comes back up, I will begin recieve routes via the more
preferable PVC (due to metrics). I will then send all traffic over the PVC
again, and my ISDN line will timeout and disconnect.
For fast convergence, I would recommend using EIGRP or OSPF as the routing
protocol.
This type of configuration works execellent on physical and multi-point F/R
interfaces. Infact, it works great with any interface. Why? Because we
are backing up Layer 3, not Layer 2.
Hope that helps,
Brad Hedlund
CCIE #5530
Network Guidance Co.
>
> Hi to everyone,
>
> Can anyone in the group clarify something for me please. I
> don't have access to ISDN so can't test any configs involving it.
>
> I want to back up a F/R line so that an ISDN kicks in on
> failure of the frame relay. I know that this works with a
> normal backup interface command with a point to point sub
> interface but how about with a point to multi point ?
>
> I'm using OSPF for the cloud so I'm guessing its an OSPF
> demand circuit. Do you have to manipulate costs ?
>
> Thanks,
>
> Russell
>
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