Need some help configuring a defualt route from a static IP pool

Unanswered Question
Oct 11th, 2007

I am setting up a router for a remote office with an Ethernet WIC on a 1721 going to a DSL modem. I have a pool of 6 usable Ip addresses. My question is, on my default route, do I use the Interface name? The Ethernet interface will have an IP of *.*.*.169 (first usable address)

ip route Ethernet0

Or do I use another IP from the pool for the default route?

ip route *.*.*.170


I have this problem too.
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Pavel Bykov Thu, 10/11/2007 - 14:47

You can do it either way you want! Or even together :)

There were a lot of debate when to use IP or INTERFACE static. Difference are of a system nature - e.g. ARP processing, or that another interface cannot be used for outgoing traffic.

You could use just an interface.

To use your allocated pool, you need to correctly setup NAT. More IP addresses are for your internal devices to be available on the internet, not for routing redundancy.

erik.doss Thu, 10/11/2007 - 15:06


The reason I was asking is because we have a few routers out in the field using that same setup.... a DSL modem going into an Ethernet WIC and the Ethernet interface configured with the IP address assigned. I just wasn't sure if I could do it both ways or one or the other.

Richard Burts Thu, 10/11/2007 - 19:29


I am one of the ones who frequently post on the issue of static routes. So let me explain it as I see it.

First let me make the point that there is a significant difference in doing static routes that point to interfaces when done on serial point to point interfaces as compared to Ethernet type interfaces. On serial point to point interfaces a static route specifying the interface works well and there are basically not issues with it. But with static routes specifying the interface on Ethernet type interfaces there are issues. The first issue is whether it will work or not. Doing static routes this way will work if the next hop router enables proxy arp and will not work if the next hop router does not enable proxy arp. Traditionally routers have enabled proxy arp but increasingly people (especially public providers) are disabling proxy arp because of potential security concerns. It sounds like the places that you have implemented do support proxy arp and perhaps this place will also.

Beyond the question of whether you CAN configure the static route pointed to the Ethernet interface are questions about whether you SHOULD configure that. When you configure a static route (especially a static default route) it will require that the router ARP for every destination address. This is likely to result in a very large ARP table. It will consume more memory to hold it, it will require more CPU cycles to maintain it, and it will require more bandwidth to handle the reuests and responses.

So I believe that the best practice is that static routes using Ethernet type interfaces should specify the next hop rather than specifying the interface.



erik.doss Thu, 10/11/2007 - 19:53

So what if I can't get the next hop address from the ISP? They have been pretty difficult so far. Thats why I was asking about one of the IP's from the pools that some of our other devices have. Can't I just use one of those?

Richard Burts Fri, 10/12/2007 - 04:43


If the ISP considers that this block of addresses is yours to use they are not likely to accept or process anything that you attempt to forward with that address as the next hop.

They should be able and willing to tell you what address they will use - and what should be your default gateway. If that is difficult then I would suggest that when you bring up the router that you do show arp. There should be an address in the arp table which will show what IP address they are using. You could then put that into the static route. Or if they enable proxy arp you could use just the interface form of static route. I do not like the performance implications of it, but if it works you certainly have the option of using it.




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