default route through DHCP interface?

Unanswered Question
Feb 29th, 2008

I have a router which is to connect to an ISP via DHCP. I'm needing to configure the default static route to exit the DHCP interface. The online help appears to indicate that I can specify this by:


# ip route 0.0.0.0 0.0.0.0 fa0/0


...where fa0/0 is configured to be a DHCP client interface, or:


# ip route 0.0.0.0 0.0.0.0 dhcp


Is one form any better than the other?


Thanks for any insight shared.

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Richard Burts Fri, 02/29/2008 - 12:28

Victor


It is an interesting link that you posted but it seems to me that it does not address the essential question here about what impact is there that the interface is learning its IP address via DHCP.


And there are several aspects of a static route pointed out a LAN interface that are problematic. First, the operation of the static route is dependent on the neighbor router enabling proxy arp. And increasingly ISP are not wanting to enable proxy arp. Second, if the neighbor router does enable proxy arp then this router must arp for every destination address (since it needs to learn what MAC address to use for each individual address that it forwards). This generates extra traffic over the link to the ISP and potentially becomes a bottleneck. Add to this the fact that the ARP table becomes very large consuming more memory and more CPU cycles to maintain it.


A static route pointed out a point to point serial interface works very well. But a static route pointed out a LAN interface is problematic.


So it seems to me that for this situation the second option:

ip route 0.0.0.0 0.0.0.0 dhcp

is the preferable choice.


HTH


Rick

ocicat002 Fri, 02/29/2008 - 12:57

Thank you for your comments. Granted, your description is much more articulate than my rationalization, but I had also decided to go with:


ip route 0.0.0.0 0.0.0.0 dhcp


Not to get mired down into hypotheticals, but am I correct in thinking that this solution doesn't address multiple interfaces configured as DHCP clients? eg. given a router with two interfaces fa0/0 & fa0/1 both receiving their configurations from upstream, would a default static route have to be defined as:


ip route 0.0.0.0 0.0.0.0 fa0/0


...if this is the desired interface defining the default static route?


Thanks to both of you for responding.

Richard Burts Fri, 02/29/2008 - 13:17

James


You raise an interesting question. And I do not know what would happen if a router had 2 interfaces learning addresses via DHCP. I have not seen anything in the documentation that addresses this aspect. And I do not have any real experience with it to go by. But I would guess that if you had 2 interfaces learning addresses via DHCP and if both ISPs included a default route in their DHCP information that the router would accept both and there would be 2 default routes in the routing table (similar to the result of having 2 static default routes configured).


As I indicated in my previous response there are issues with ip route 0.0.0.0 0.0.0.0 fa0/0 and I would look for a different alternative.


HTH


Rick

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