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New Member

ospf routing problem

This may not be the right forum. its just a lab simulation question.

I have 5 2501 routers connected back to back via serial cables. all routers have loopbacks and some have ethernet interfaces which are up. any router can ping any interface on any other router - am running ospf between them all.

BUT, a PC connected to any of the ethernet segments (when default gateway is properly configured on the pc) can ping any of the serial interfaces (all /30 masks but put into ospf as network 172.16.x.x area 0). All the loopbacks and ethernet segments are 192.168.x.1 The PC can ping any of them that were entered into OSPF as 192.168.x.1 area 0, but it can't ping any of the ones that were entered 192.168.x.0 area 0.

I think I'm having a brain freeze. Is there any particular reason it would be better or worse to use the as opposed to a The routers all communicate fine, its just the PC that is not working.


Re: ospf routing problem

If you trace to those Loopbacks that are not working, where does it stop.

New Member

Re: ospf routing problem

Earlier the tracert from Windows would just stop at the first hop. Then I'd tracert to a serial interface on the same desitnation router and it would work fine. Strange.

Just now I shut everything down...again...and only booted up two routers. now for reasons unknown, the PC at can ping the loopback on both routers - and Maybe it was a fluke. While I'm asking, below is the show IP route of the 2nd router. On the first router, is the e0 and is the loopback0. Both are entered on their respective interfaces with 24 bit mask, both are entered into OSPF at net 192.168.x.0 area 0. How come in the route table of the 2nd router it displays one with the /24 and one with /32 & subnetted remark?

Is there a specific forum for questions like these or this one as good as any? I'm new to the forums.

Gateway of last resort is not set

O [110/74] via, 00:01:08, Serial1 is subnetted, 1 subnets

C is directly connected, Serial1

C is directly connected, Loopback0 is subnetted, 1 subnets

O [110/65] via, 00:01:08, Serial1


Hall of Fame Super Silver

Re: ospf routing problem


This forum is fine for questions like this one (or the WAN Routing and Switching forum would also be good).

The answer to your question is that OSPF treats a loopback interface slightly differently when OSPF is enabled on it. By default when OSPF is enabled on a loopback interface it will treat that loopback as a host route (/32) no matter what netmask was used in configuring the interface (and no matter what mask was used in OSPF configuration). If you want OSPF to advertise the subnet of the loopback then you can use this command under the loopback interface:

ip ospf network point-to-point

This will cause OSPF to advertise the subnet rather than the host route.