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May 9th, 2008
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Can we have multiple routers on the same subnet....There is a LAB environment that I am working in and they are trying to re IP it. We have been given one Class A IP. So we would have to have to creat multiple subnets. We have . 10.X.X.0/21 network and we are planning to subnet that into /24 subnets. Each of these subnets would have multiple routers. Do we need to subnet it further based on network segments or we could just have all the routers and switches on be on that one subnet and just give them a .1, .2 , .3 etc.

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sundar.palaniappan Fri, 05/09/2008 - 13:40
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You can certainly have multiple routers in the same subnet. Just make sure the routing is setup correctly on the multi-access segment. The one common issue I have seen people often run into when multiple routers exists on a subnet is the 'ip redirect' feature, which is enabled by default on Cisco routers.


Let's say you have two routers, R1 & R2, in the same subnet. The hosts on the subnet use R1 as it's default gateway. To get to NetX R1 routes traffic through R2. When a host attempts to connect to NetX R1 sends 'ip redirect' message to the host telling it to send the traffic directly through R2. Sometimes the hosts ignore the 'ip redirect' message and that can cause some problems. You can disable 'ip redirect' on R1 to address this problem.


HTH


Sundar

uzmausmani Fri, 05/09/2008 - 14:51
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But if we have all the routers on the same subnet why do we need a router in the first place.. I thought a router is to route traffic between networks. We could just use switches instead apart from the gateway router.

sundar.palaniappan Fri, 05/09/2008 - 14:59
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I don't know your exact setup. If all you have is routers and everyone of them have all the routes in the routing table then you don't have to worry about 'ip redirect' problem.


HTH


Sundar

uzmausmani Fri, 05/09/2008 - 15:11
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We have routers and switches both. They are mainly used for software testing thats why we have all varieties and flaours of routers. But that doesnt matter. I was just not able to understand how could all routers and switches be on the same subnet and not be causing routing loops. I mean routing protocols forward a route to the interface where that network is supposed to be. So if the same network is connected to all its interfaces..wont it be forwarding the packet to all the interfaces and sometimes even cause loops?

sundar.palaniappan Fri, 05/09/2008 - 16:46
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It's hard to give a specific answer without knowing your topology. I don't know what routing protocol you are going to be running.


Most routing protocols use multicast packets to exchange routing information. In a multi-access segment, like the one you described, all the routers would get the same update from the router that's originating the route information.


With regard to your question about another router sending a routing update to a router that has an interface connected to the destination network. The router would choose the directly connected route over the route it learned from another router because the administrative distance of the directly connected route is preferred (lower) than any routing protocol (higher).


Again, it's hard to give any specific answer without knowing your topology, routing protocol information, the configuration you wish to use etc.


But, unless you are doing some kind of redistribution and your configuration is too complex most routing protocols would just work fine.


Let us know if you have any problems or present any specific concerns you may have before setting it up.


HTH


Sundar



Richard Burts Fri, 05/09/2008 - 13:41
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uzma


Perhaps there is some aspect of your question that I do not understand well. But on the surface it seems quite simple - yes you certainly can have multiple routers and/or switches be in the same subnet.


HTH


Rick

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