Static Routing

Unanswered Question
Sep 11th, 2007
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I have a CE to PE link on Fastethernet. I see a strange behaviour that my static routes with next hop as ip address work, but next hop as interface don't. Can somebody help please.

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Richard Burts Tue, 09/11/2007 - 10:43
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Nitin


A static route pointed to a LAN interface will have to ARP for every destination address. This will work if the next hop router has enabled proxy arp and will not work if the next hop router has disabled proxy arp. From your description I would guess that the next hop router does not enable proxy arp.


HTH


Rick

shrikant4u Tue, 09/11/2007 - 10:49
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Rick


May be stupid Q ...If PE-CE are directly cross connected this problem should not come up. Right?


thanks

shri


Richard Burts Tue, 09/11/2007 - 13:22
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Shri


Whether the routers are cross connected or not has no bearing on this problem. If a static route points at an outbound interface (especially an LAN interface like Ethernet) then it needs to ARP for every destination address.


the reason for this is that to build the Ethernet framing to forward the packet it needs a destination MAC address. So for each destination it needs a MAC address and must ARP for each one. And cross connected or not has nothing to do with this.


If the next hop router does enable proxy arp then it works (I would say that it works poorly because of the extra work that the router must do - but it works). And if the next hop router does not support proxy arp then it does not work - regardless of cross connect.


HTH


Rick

shrikant4u Tue, 09/11/2007 - 10:43
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are CE and PE cross connected or they are on switch?


thanks

Shri

Richard Burts Wed, 09/12/2007 - 02:49
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Aleksey


Thanks for the interesting article. It makes the same point that I had made which is that a static route pointed to an Ethernet interface will cause the router to ARP for each destination address which is not reachable by a more specific route. But beyond the impact on router CPU (and the impact on router memory because the ARP table is likely to become VERY large) there is the risk that if the next hop router does not enable proxy arp then the ARP request will fail and the traffic will be dropped.


The solution is to use static routes with next hop addresses when the outbound interface is Ethernet.


HTH


Rick

chopra.nitin Wed, 09/12/2007 - 05:44
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Tanks everyone for the help...I now have a more crystal idea:)

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