Solved: Destination MAC when using IP route interface

John White · ‎12-04-2015

Hello,

When using ip route <network><mask> interface <interface> how does IOS determine the destination MAC to use when encapsulating the datagram?

Oviously on a serial link there is only one destination. However if a broadcasting protocol like ethernet is the outbound interface, how would a router forward a packet when only given that interface, not a logical address to check against ARP?

I have no intention of using this kind of configuration, but I am curious around the logic behind that function.

Bilal Nawaz · ‎12-04-2015

Hi John, my understanding is that the router ARP's out for the destination address.

e.g. if we had ip route 0.0.0.0 0.0.0.0 Gi0/0

Static routing to an interface where the next-hop address is ambiguous (i.e. NOT a point-to-point interface) will treat all addresses within the route as directly connected.

This means that every destination address that matches this route will be ARP’d. Depending on the environment, this could create an inordinate amount of ARP traffic on this segment, and increase the size of the ARP table on the router.

If proxy arp is enabled on the actual next hop it will respond back to the ARP request of the router. If not then the whole process fails.

Hope this helps

Bilal

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View solution in original post

Bilal Nawaz · ‎12-04-2015

Hi John, my understanding is that the router ARP's out for the destination address.

e.g. if we had ip route 0.0.0.0 0.0.0.0 Gi0/0

Static routing to an interface where the next-hop address is ambiguous (i.e. NOT a point-to-point interface) will treat all addresses within the route as directly connected.

This means that every destination address that matches this route will be ARP’d. Depending on the environment, this could create an inordinate amount of ARP traffic on this segment, and increase the size of the ARP table on the router.

If proxy arp is enabled on the actual next hop it will respond back to the ARP request of the router. If not then the whole process fails.

Hope this helps

Bilal

Please rate useful posts & remember to mark any solved questions as answered. Thank you.

Joseph W. Doherty · ‎12-04-2015

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Posting

Ditto, and additionally, it's not unusual for such a configuration (default route to Internet using an interface) ARP cache to grow so much, all the router's free RAM is used and the router crashes.

John White · ‎12-04-2015

Thanks very much to both of you.

Masoud Pourshabanian · ‎12-04-2015

Hello,

Both answers are correct. I am just adding more info. You use interface as a next hop when the next hop is unkown. You should avoid that if it is possible.

Take a look at the link below.

Nice explanation with visual sample,

http://keepingitclassless.net/2012/03/static-routes-to-an-interface-not-a-next-hop/

Masoud