As we all know, when we configure a static IP route, the next hop IP
address should be a host IP address of a directly connected network (or an
exit interface number). for example, in Windows NT or Netware systems, if
your system is connected to, say, network 192.168.50.0/24, and you try to
add a static IP route, say, 192.168.60.0/24 with a next hop IP address
192.168.53.x. you will get an error message. Am I right?
If you are thinking this will be the same in Cisco IOS, you could be wrong.
Try to turn on you router (just one router is enough), and leave all you
interface down. Then typing in the following lines
IP route 200.168.1.16 255.255.255.240 192.168.92.65
IP route 192.168.92.0 255.255.255.255 192.168.1.65
IP route 192.168.1.64 255.255.255.240 Null 0
Note that, none of the router's interface is connected to any of the network
numbers shown in the configuration (of course, except the null 0 interface).
Now try a 'show IP route'. You will get the following lines
s 200.168.1.16/28 via 192.168.92.65
s 192.168.92.0/24 via 192.168.1.65
s 192.168.1.64/28 is directly connected, null0
This experiment approved that IOS will keep a static route in its routing
table as long as it thinks there is a 'live' path to that network, even that
'live' path will lead to a dead null interface. and it really does not care
where the next hop IP address is located in the entire network as long as
that hop is 'reachable' according to its own routing table.
Is this a good thing, bad thing or an IOS Bug???
However, this explains John's problem very well. Even when the PVC is down
the route entry
'S 10.50.1.1 255.255.255.255 via 10.1.1.2'
will be still in routing table, because the next hop address 10.1.1.2 is
still reachable via
'S 10.0.0.0 255.0.0.0 is directly connected Null0'
Could anyone explain how John's problem would be fixed without turning off
'IP classless' and still keep the
'IP route 10.0.0.0 255.0.0.0 Null0' entry in the routing table?
JZ
"John Neiberger" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Ok, to recap, here was the problem. We have two PVCs from router A to
> router B. We have eigrp running on the first PVC but not on the second,
and
> there are static routes forcing certain traffic to use the second PVC.
> These are point-to-point frame relay links. Here are the relevant static
> routes affecting the second PVC, and let's assume the loopback address at
> the router B is 10.50.1.1:
>
> ip route 10.50.1.1 255.255.255.255 10.1.1.2 50
>
> We also have another static route for an entirely separate reason, but it
is
> involved here, as well:
>
> ip route 10.0.0.0 255.0.0.0 Null0
>
> We recently upgraded to 12.1 from 11.2. In 11.2, "no ip classless" was
the
> default. In 12.x, ip classless is the default.
>
> We thought that if the second PVC went down, since the next hop was
> unavailable, the router would pick the next available route with a higher
> administrative distance. Since there is a valid route to router B over
the
> first PVC, we thought it would take this route. The odd thing was, it
> wasn't and we didn't know why. Here's why:
>
> Even though 10.1.1.2 was at the other end of a point-to-point link and
shows
> in the routing table as directly connected, the router never realized that
> that route disappeared! When the PVC was down, if I did a show ip route
> 10.50.1.1, it would show as reachable via 10.1.1.2. Then a show ip route
> 10.1.1.2 would show that it was reachable thanks to the 10.0.0.0 route
> pointing to null0. Isn't that freaky??
>
> I assumed that the router would be smart enough to know that if a point to
> point link is down, the remote IP address is truly unreachable, even if
> there is valid supernet route; apparently, such is not the case. I did
> some testing and proved to myself that this is what was happening.
>
> So, since we don't require classless routing on this router, I've turned
it
> off. Now, the router will not use the 10.0.0.0 supernet route and it will
> correctly decide that the point to point link is down. Then, it will use
> the eigrp-learned route on the other PVC.
>
> If someone understands why a router would choose a supernet route to reach
a
> directly attached down interface, please let me know. I'm sure this will
> come up later!
>
> Thanks,
> John
>
>
>
>
>
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