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dual connections to internet - ip route with interface as next hop?


I have a C1800 with dual ISP connections. There is a default route via ISP A on fa0. I want to route traffic destinated to a specific host on internet via ISP B (fa1). However, because of the fact that fa1 has its ip assigned by dhcp, I cant just make a "ip route <specific host> <next hop ip>. The next hop IP is also dhcp assigned.

What is the catch with using the interface as next hop address (ip route <host> fa1)? What is the destination IP for outgoing traffic following that route?



Cisco Employee

Re: dual connections to internet - ip route with interface as ne

Hi Jimmy,

The destination ip will remain the specific host whatever you have specified on your ip route command. I mean when any client on your network want to reach that specific host the destination ip will remain that specific route only.

It will check that there is an exit interface specified for that specific host so it will send any request for that host out from interafce fa1.




Re: dual connections to internet - ip route with interface as ne

the catch with using the interface in your route instead of the next hop ip address is that when you use the interface, and Administrative Distance of zero (0) is assigned to the route.

(even though this is a static route that would otherwise normally have a default AD of one (1), if the ip address of the next hop was used)

when you use the ip address of the next hop in your route, an Administrative Distance of one (1) is assigned to the route.

when the interface is used in the route, the destination ip for outgoing traffic is technically the next hop MAC address used in conjunction with the destination ip address.

(it has to be, as the packet header will be changed by the router to include the MAC address of the router on the other end of the link; the destination ip address will remain the same as originally defined in the source packet)

also, it might be better if you can use static ip addressing on your router interfaces or at least, if they have to be dhcp, then use a dhcp reservation.

(this will prevent many issues that can be created by a routers interface address changing unexpectedly)

Hall of Fame Super Gold

Re: dual connections to internet - ip route with interface as ne


It is possible to do a static route specifying the interface rather than specifying the next hop IP address. And as you indicate there is a catch (really more than one). Doing static routes this way is dependent on the next hop router implementing proxy arp. If the next hop router does not implement proxy arp then the static route will not work. Increasingly security considerations regard proxy arp as a weakness and I see more places which do not enable proxy arp. Perhaps you can check with ISP B to verify whether they do enable proxy arp.

Another consideration is that static routes with outbound interface on a broadcast interface such as Ethernet will require the router to arp for every destination address. If you are routing a single address then this does not matter much. But I have seen people who do their default route this way. The result is much larger arp cache resulting in increased memory usage and more CPU cycles used to maintain the arp cache.

In your case where you are routing to a single remote address it is reasonable to use a static route with the outbound interface rather than next hop IP as long as the provider will enable proxy arp.

The part of your question that asks about the destination IP is easy to answer. The destination IP is the destination IP. Nothing in the static route changes the destination IP. What will change in the packet is the layer 2 address of the next hop (which is why the router must arp for it) but the destination IP address remains constant.



Re: dual connections to internet - ip route with interface as ne

The destination-ip (next-hop_ip)following the interface in the static route is intended to overcome a problem with static routing. What it does is, the router would forward the packets to next-hop-ip for that particular route and if the interface (fa1) through which the next-hop is supposed to be reached is down then it would purge the route from the routing table irrespective of the fact if there's another recursive route to get to that next-hop_ip.

I recommend you look at the following document w/examples to get a good understanding of the different flavors of static routing and the problems associated with some of them. This would help you choose the correct static route in your situation.

Since you don't know the next-hop IP the following should suffice. For this to work the ISP next-hop has to support proxy-arp, which is enabled by default on Cisco equipment.

ip route fa1



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