EIGRP specific route reachability

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Jun 11th, 2007
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We are bringing up a new non-trunking core. Our current core has a 10.0.0.0/16 network management VLAN. The new core has a 10.0.20.0/24 network management VLAN. With the current setup I am unable to reach the /24 from the /16 VLAN. I believe this is due to the fact that EIGRP sees the /24 belonging to the /16 VLAN. I can reach the /24 from a server if I add a static route on the server itself. I have included an attachment that hopefully explains the setup. It is hard to include all the information so please let me know if you need more info. Thank you!



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Edison Ortiz Mon, 06/11/2007 - 07:45
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This is not an EIGRP problem since the default gateway 10.0.0.1 knows how to get to 10.0.20.0/24


The problem is with the hosts in 10.0.0.0/16 not using the default gateway for 10.0.20.0/24 since ,as you stated, the route is local - so unless you change the subnets, you will have to manually input static routes on each host.


anandramapathy Mon, 06/11/2007 - 07:45
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How difficult will it be for you to change the subnet mask for the int vlan 910 to a /24 & all machines associated with this ?

Richard Burts Mon, 06/11/2007 - 07:49
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EIGRP is certainly able to do this. It will track the different prefix lengths and both subnets should be in the routing table. I believe that the issue is that the server is sending an ARP request for the destination address of 10.0.20.x instead of forwarding to the default gateway. I believe that if you were to enable proxy arp that this network would work. As it is the server believes that 10.0.20.x is within the local subnet of 10.0/16 and so it arps for the address directly instead of sending it to the default gateway. If proxy arp were turned on I believe that the gateway would respond to the ARP request and the packet could be forwarded.


HTH


Rick

r-tyrell Mon, 06/11/2007 - 07:56
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You guys rock! I turned on ip proxy-arp for the 910 VLAN and now I can successfully reach the 920. Thanks you!!


-Rick

Richard Burts Mon, 06/11/2007 - 08:07
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Rick


I am glad that our discussion was able to help you resolve your problem.


HTH


Rick

r-tyrell Mon, 06/11/2007 - 07:51
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Thanks for the reply. So by changing the 910 to /24 that should solve my problem? It would be somewhat painful to change all of our 910 devices to the /24 but if that is what we have to do. I wasn't sure if there were some fancy tricks with EIGRP that could produce a workaround.

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