Berkeley CSUA MOTD:Entry 33119
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2025/07/09 [General] UID:1000 Activity:popular
7/9     

2004/8/24 [Computer/Networking] UID:33119 Activity:nil
8/24    To those with Cisco/router fu:
        My co-worker walked over and asked me what the "set default interface"
        command does if you don't specify a next-hop IP address.  I said
        you always need a next-hop IP address, unless you expect the
        destination IP address in the IP header to be on the directly connected
        network.  He said it will broadcast on that interface.  I'm googling
        now, but please help if you happen to know.  Thanks.
        \_ I guess the answer is somewhere in here.  Thanks. -op
           http://www.experts-exchange.com/Hardware/Routers/Q_21093447.html
           To me this means I'm right, he's wrong.  I'm skeptical, though,
           that Router B will pretend to be Host C, even if Host C is on a
           directly connected network of Router B.  Anyways.
2025/07/09 [General] UID:1000 Activity:popular
7/9     

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Cache (3039 bytes)
www.experts-exchange.com/Hardware/Routers/Q_21093447.html
sambha03 on 08/13/2004 04:13PM PDT If while using policy, I use the command: set default interface ethernet 0 Then packets for whom no route is present in routing table are sent ethernet interface 0 Now will each node in ethernet try and fwd the packet and the destination may get duplicate packets? celsmk Date: 08/14/2004 09:33AM PDT Comment The answer is unless you have misconfiguration or loops in your ethernet network,there will be no duplicate packets. Bare in mind that the router will ARP to find out the MAC address of the node before sendind the actual packet. Therefore, either a router in your ethernet, or the actual node with matching IP address of the packet will answer the ARP and receive it. sambha03 Date: 08/15/2004 10:40AM PDT Author Comment since in policy routing "set default" interface is used only when the router does not have a route to the destination, whom will the router ARP for? I would think the packet would be sent on the braodcast mac address and each rotuer on the ethernet will try and send it to destination. celsmk Date: 08/15/2004 12:50PM PDT Comment No, rule #1 for a router is to never propagate ethernet broadcasts, because this would lead to broadcast storms everywhere, making TCP/IP networks not viable. A non-multicast or non-broadcast IP addressed packet will always be ARPed before being sent through a ethernet interface. If there is no answer (no host with that IP address, or no other router with matching entry in its routing table), the packet will be simply dropped. sambha03 Date: 08/15/2004 02:56PM PDT Author Comment celsmk: Let me explain in more detail. According to me scenario 2 should happen since scenario 1 does not serve any purpose. Scenario 1 ------------ Router gets packet for desination C which it does not have the route. Since "set default interface ethernet 0" is used, it sends an ARP for destination C No one responds since the Router does not have a route to C The packet is now dropped rendering "set default interface ethernet 0" as futile. Scenario 2 ------------ Router gets packet for desination C which it does not have the route. Since "set default interface ethernet 0" is used, it sends it on ethernet 0on broadcast mac address. It then either drops it or tries to send the packet to the destination if it has route(this is not same as forwarding a broadcast). celsmk Date: 08/15/2004 05:47PM PDT Grade: A Accepted Answer In Scenario 2, this is what it will happen: Router A gets packet for destination C, for which it does not have the route. Otherwise, it will broadcast ARP REQUEST (not the real packet) and listen for ARP responses. sambha03 Date: 08/15/2004 10:12PM PDT Author Comment Thanks celsmk . Could you also describe what happens if in point 3 there are multiple routers like router B that have the route? celsmk Date: 08/16/2004 04:03AM PDT Comment In case you have more than one router with route to destination C, each of them will respond with an ARP response, but router B will accept only the first ARP response, discarding other.