Ospf neighborship - not directly connected

[vinothnil]

Hi All,

Is there any way to make a ospf neighborship between two routers which are not directly connected.

Say example, R1-->R2--->>R3

Only R1 and R3 need to run ospf and form neighborship. 

Can we use gre tunnel? If so, how. Please advice

[Julio E. Moisa]

Hi

Yes, you can create a L2TPv3 VPN, recommended because it is based IP. Also you can use GRE tunnels as you mentioned. There is other way and it is configuring Bridge group between the routers but I suggest test first with GRE and then L2TPv3.

Hope it is useful

:-)

[Joseph W. Doherty]

"Can we use gre tunnel? If so, how. Please advice"

Yes, you can use a GRE tunnel.

You configure the GRE tunnel much like you would for any other GRE tunnel. (R1 should be able to ping R3's external interface and the converse should be true too.)

(Before bringing up OSPF, on the tunnel, you should be able to ping one tunnel interface from the other tunnel interface. A traceroute, between R1 and R3 should "show" a hop through R2 "outside" the tunnel, and a traceroute "inside" the tunnel shouldn't show any R2 hop.)

For OSPF purposes, you configure the GRE tunnel interfaces, and OSPF, much like you would any other p2p interfaces.

Once you enabled OSPF across the tunnel, it should come up and establish full adjacency.

Something to keep in mind, the underlying IPs being used to support the tunnel (not to be confused with the tunnel's IPs) should not be included in the OSPF topology.

[cofee]

I just tested with GRE tunnel in gns3 and it worked without any issues. Like the other member stated just make sure that tunnel source and destination are not advertised through the same ospf domain as the tunnel addresses.

[vinothnil]

Thank you. I will try in gns and let you know .

Can you please confirm what protocol you configured in R2 to make tunnel
interface reachable between R1 and R3

[Julio E. Moisa]

Hi

You could have something like:

interface tunnel 0

ip address x.x.x.x 255.255.255.0

tunnel destination <neighbor interface or IP>

tunnel source <local outside interface or IP>

router ospf x

network x.x.x.x 0.0.0.255 area 0

[cofee]

Adding to Julio's comment. I simply used static routing for IP reachability for r1 to reach r3 interface and vice versa through r2. I didn't have to use any routing on r2 since it was directly connected to both r1 and r3. But you can use any dynamic routing protocol to advertise IPs assigned to physical interfaces.

[vinothnil]

Thank you . I will configure as per your advice and update you .

[Julio E. Moisa · Accepted solution]

Hi

tunnel destination and tunnel source IP addresses can be used with static routing or dynamic like RIP, EIGRP, or OSPF with different process ID if you are going to use OSPF for the tunnel IP address. 

Please don't forget to rate or mark as answered if the comments were useful.

:-)

[Julio E. Moisa]

Hi, Coffee's answer is correct otherwise you will have flapping on the tunnel. Also I suggest take in consideration the overhead, the following config should be added to the tunnel:

ip mtu 1476

ip tcp adjust-mss 1436

GRE includes 24 bytes on the header. So to avoid any trouble the configuration is recommended.

:-)

[vinothnil]

Thank you

[vinothnil]

It is working fine now. Thanks for all your help.

My only doubt is now , why we should not allow the tunnel source and destination ip in ospf.

[cofee]

Running a different process over GRE tunnel (not including real interfaces) and a different process over real interface (not including the GRE tunnel)  provides the necessary prevention against recursive routing loops, as these two processes will not leak one to another, causing the real endpoint of a tunnel to be discovered through the tunnel itself.

[Julio E. Moisa]

Hi, Coffee's answer is correct otherwise you will have flapping on the tunnel. Also I suggest take in consideration the overhead, the following config should be added to the tunnel:

ip mtu 1476

ip tcp adjust-mss 1436

GRE includes 24 bytes on the header. So to avoid any trouble the configuration is recommended.

:-)