From: stephen@dino.dnsalias.com   
      
   "luqs"  writes:   
   > I would like to know the significance of the IP address used in the GRE   
   > tunnel configuration.   
   > I thought that tunnel source XXX & tunnel destination XXXX   
   > are all that is required to get a tunnel to work. However, I did use IP   
   > address on both ends of the tunnel and kept it in the same subnet.   
   >   
   > Now, the issue is,   
   >   
   > In one setup, I changed the IP addresses to be of different subnet at   
   > the two ends of the tunnel for eg, at end A I would give 10.10.10.1 &   
   > at end B  I would give 192.168.10.1.   
   > It didnt work.   
      
   "It didnt work" isn't a lot to go on, but my guess is you did not have   
   the correct static route on either side directing the traffic to the   
   gateway at the opposite end of the GRE tunnel.   
      
   > However, in my live network where everything (connected & static) are   
   > reditributed into ospf the same setup worked!   
      
   If my theory about your static routes being incorrect then it would   
   work with OSPF (or any routing protocol) because once you tell it   
   about the interfaces you want it to run the routing protocol over, it   
   can dynamically learn the address(es) of peers and so populate the   
   routing table with the necessary routes to allow traffic to flow.   
      
   > which begs the question, what is the role of the IP address assigned to   
   > the tunnel?   
      
   One role is as the source IP address on any packets originating   
   routing traffic i.e. the OSPF daemon will (depending on the   
   implementation) either unicast or multicast its routing updates over   
   the GRE tunnel with the originating tunnel IP address as its source address.   
      
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