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Static Routing with exit interface as the next hop

How does the Address Resolution happen when we mention a static route with wan interface as the exit interace

ex: ip route 0.0.0.0 0.0.0.0 s0

incase of Broadcast interface it is using the proxy-arp process that the address resolution happens.

e: ip route 0.0.0.0 0.0.0.0 e0

what is it in the case of Wan Interface ?

Thanks,

Vikram

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: Static Routing with exit interface as the next hop

Vikram

Yes you are correct. If you configure a default route like ip route 0.0.0.0 0.0.0.0 e0 and the other router does not enable proxy arp then the packets will not be routed and will be dropped. In this situation if you turn on debug you would see the router reporting encapsulation failure because it was not able to find the destination MAC address for the next hop.

HTH

Rick

7 REPLIES
Silver

Re: Static Routing with exit interface as the next hop

ARP is on a Ethernet segment on P2P like serials it is all with the encap (PPP/HDLC) so there are no ARP for tht

Hall of Fame Super Silver

Re: Static Routing with exit interface as the next hop

Vikram

I think that Gautam is on the right track. I would answer the question slightly differently though. Your question had to do with address resolution when a static route points to an interface rather than identifying the specific next hop address. As your question implies in this situation there must be some kind of address resolution. For LAN interfaces like Ethernet the address resolution is ARP/pro9xy ARP. For Frame Relay interfaces it is the Frame Relay mapping that provides address resolution. When the protocol is point to point like ppp or like HDLC and there is only a single neighbor on the connection then address resolution is authmatically built into the protocl and there is not a separate step of address resolution.

HTH

Rick

Re: Static Routing with exit interface as the next hop

Hello Rick,

Thanks for the reply, it cleared my doubt.

Thanks,

Vikram

New Member

Re: Static Routing with exit interface as the next hop

Hello Vikram,

We are talking about 2 thinks here. The routing table is used to forward packets. When you define a static route with a next-hop address:

ip route 0.0.0.0 0.0.0.0 10.10.10.2 (AD = 1)

the administrative distance is 1. The next hop network need to also be in the routing table as directly connected (AD = 0).

If you are using a static route

ip route 0.0.0.0 0.0.0.0 serial0

with an interface specification, it is considered a directly connected network.

HTH,

Nadine.

Re: Static Routing with exit interface as the next hop

Hi Vikram,

In the ethernet case we need a destination MAC address for the ethernet frame, which we can obtain through an ARP. The ARP contains a request for the destination IP in the IP packet to be sent. If there is a router answering it with his MAC (proxy-arp) we are fine.

In the case of a serial interface there is no need for address resolution on layer 2, because there is only ONE other device out there.

So as a sending router the layer 2 frame (f.e. HDLC) can be readily built as it does not contain a device specific layer 2 address like MAC we need to know.

Hope this helps

Martin

Re: Static Routing with exit interface as the next hop

Hello Martin,

Thanks for the reply, it cleared my doubt.

Referring to your reply, if there is isn't any router with proxy-arp enabled in a scenario with a route statement : ip route 0.0.0.0 0.0.0.0 e0

the packets will never be routed , isn't it ?

Thanks,

Vikram

Hall of Fame Super Silver

Re: Static Routing with exit interface as the next hop

Vikram

Yes you are correct. If you configure a default route like ip route 0.0.0.0 0.0.0.0 e0 and the other router does not enable proxy arp then the packets will not be routed and will be dropped. In this situation if you turn on debug you would see the router reporting encapsulation failure because it was not able to find the destination MAC address for the next hop.

HTH

Rick

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