12-04-2015 07:20 AM - edited 03-08-2019 02:57 AM
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.
Solved! Go to Solution.
12-04-2015 09:46 AM
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
12-04-2015 09:46 AM
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
12-04-2015 10:34 AM
Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
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.
12-04-2015 02:01 PM
Thanks very much to both of you.
12-04-2015 04:44 PM
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
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide