OSPF neighborship thourgh a third router

[thomastrupel]

I started  wondering if it was possible to get a functional OSPF neighborship between two routers without a PVC connecting them in a one-subnet-three-routers-over-frame-relay topolgy (first attachment, GNS3 topology and configurations files using c3745-adventerprisek9-mz.124-25d.image). Even I was able to ping the second router via the third router with static frame relay mapping, no Hello OSPF packet were received by the second router. Apparently, the third router dropped Hello packets and sent back an ICMP type 11 (Time-to-live exceeded).

Pursuing my reasoning, I started an other configuration with a three-routers-two-subnet-over-ethernet topology (second attachment). Again, I tried to get a functional OSPF neighborship through a third router using static IP route and configuring "non-broadcast" network type. But in this case, routers not even sent OSPF Hello packet. I supposed that IOS was dropping these Hello packets.

So my question is why Cisco IOS is dropping packets in each topology and how I can observe these drop on Cisco IOS ?

[pascalfr0]

I guess you can't do that unless configuring an OSPF virtual link between R1 and R3 (R1 and R3 being in backbone area), R2 being in another area. Twisted :x

Maybe I missed something though.

RFC 2328         

15.  Virtual Links

[thomastrupel]

Hello pascalfr0,

I'm not looking for a workaround but why it is not possible (if this is the case) and how I can see it in Cisco IOS.

[pascalfr0]

FrameRelay:

R2 have no FR to IP mapping so shouldn't be able to ping R1 nor R3.

R3 sees R1 thru DLCI 301, ok.

R1 sees R3 thru DLCI 101, ok.

OSPF should go up between R3 and R1. Shouldn't between R2 and any other router (no connectivity in R2 configuration with either R1 or R3 unless I've missed something in the configuration);

Ethernet:

For me, on NBMA all neighbors should be in the same subnet.

One NBMA domain = one LSA type2 advertised by the DR (and better configure the hub router as the DR if you want it to work, as it is the only router with full connectivity to the spokes).

[thomastrupel]

Frame-Relay:

As you can see below, R2 can ping R1 and R3. IP-DLCI mapping is done dynamically with InARP.

Ethernet:

If it's not possible how I can see errors or alerts on Cisco IOS ? (routers does not send a single Hello packet even with the "neighbor" command correctly set on both R1 and R3)

Thank you for helping me.

[pascalfr0]

FrameRelay  :

Ok for the inarp discovery at R2, I'm not familiar with frame relay mumbo jumbo, seems to work fine  =)

Then if you configure NBMA on all routers, add neighbors manually, and give highest priority to R2 (hub router), lower to spokes R1 and R3 with "ip ospf priority 0", it should work (assuming that R2 as effective connectivity to R1 and R3)

If each router can see every other one thru a specific DLCI, every router could be used as DR (as the topology is full meshed).

But not seeing any DLCI definition in R1 or R3 to reach R2 confuses me (not familiar, etc...).

On Ethernet, being by nature a broadcast media, dunno if configuring NBMA could cause side effect.

Anyway in a NBMA domain, all your routers should be in the same subnet.

I'm not sure I understand what you're trying to do on Ethernet, as R2 doesn't run OSPF. Kinda trying to set an OSPF adjacency on a multihop model between R1 and R3 thanks to static routes ? For me it doesn't match OSPF design : each router must be advertised as a LSA type 1, and on multiple access networks one router, the DR, advertise the network with its type (NBMA, p2p, ...), and other attributes, in a LSA type 2.

2 neighbors must talk through the same layer 2 direct topology,the only exception I know being virtual links.

Dunno where to look in the router log (debug ospf <options>), a good place to start if you haven't yet would be "http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/12151-trouble-main.html".

Reaching my limits here, maybe u could hope for a Guru to look at your topic for more valuable input :-)