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I have 2 cisco 3825 routers connected to two cisco 3560 switches. One is currently our main router with 3 /22 public addresses and the usual slew of rfc1918 address space routed around for internal usage. I would like to take a /24 and route it from our main router over to an identical router in a different room so that we can basically begin to isolate our colocation/dedicated customers from our home subscribers.

I'm not sure exactly how to do this so I can really use someone cisco gurus to jump in, if possible. I currently have the 2 routers connected directly via a /30, say for example here on the main (core1) and (core2) on the new one, on a seperate vlan between core1's gi0/1.4 and gi/0.0 on core2, the newer router. From here, I could choose to do something like this from core1:

ip route

How do I not prevent loops or otherwise hose myself? The default route on core1 one is a

ip classless 
ip route

and I have the interface between the two setup up like this (with a switch providing vlan access between the 2 on vlan 9.

interface GigabitEthernet0/1.4
 description core1-to-core2
 encapsulation dot1Q 4
 ip address

I have a similar entry on core2,

ip classless
ip route


interface GigabitEthernet0/1.1
 encapsulation dot1Q 1 native
 ip address
 no snmp trap link-status
 no cdp enable
ip classless
ip route

When I traceroute to, I get a loop between the 2 core1 and core 2 interfaces.

Edit: Sorry but I made a few typo's in the IP addresses ... s/169/168 and the interface ip address is a 1, not a 0.

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There is no such thing as a Class C network any more. You've used the correct CIDR notation in your question, so I've removed it from the title. – Mark Henderson Aug 23 '11 at 4:52

1 Answer 1

The answer from Shane is correct that there is a typo in your config but he only fixes part of the issue:

ip address

There is a simple typo here, s/169/168/, but the larger problem is your trying you use .0 (the network address) as an interface. While you can use the default network/broadcast address between newish network equipment (a change that was allowed so /31s could be used on point to point links), this is asking for trouble on a subnet with hosts on it. I would recommend something like

ip address
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Heh, I missed the forest for the trees on that one - just fixing the typo fixes the loop, but not the actual connectivity. Thanks for the help :) – Shane Madden Aug 23 '11 at 14:32

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