Router behaviour when proxy-arp is disabled.

[from88]

Hello guys, i have simple topo

[R1]--------------------[R2]--------------------[R3]

all routers connected with /30 subnets. R2 have loopback (8.8.8.8) and i disabled the PROXY-ARP on R2 interface pointing to R3.

The question is:

                    Why when im pinging from R3 to 8.8.8.8 the R3 sends ARP REQUEST ? Why he didin't use routing and create packet ?

Mar  1 15:06:52.500: IP ARP: sent req src 45.45.45.2 5433.5342.6764,

                 dst 8.8.8.8 0000.0000.0000 FastEthernet0/0.

*Mar  1 15:06:54.496: IP ARP throttled out the ARP Request for 8.8.8.8.

[John Blakley]

It shouldn't arp for something that isn't on the same subnet. Can you post the config of R2 and R3? I'm curious....I can't get mine to arp for it.

[from88]

here you are:

R2

interface Loopback88

ip address 8.8.8.8 255.255.255.255

interface FastEthernet0/1   (interface to R3)

mac-address 0033.0333.3333

ip address 45.45.45.1 255.255.255.0

no ip proxy-arp

duplex auto

speed auto

R3

interface FastEthernet0/0 (interface to R2)

mac-address 5433.5342.6764

ip address 45.45.45.2 255.255.255.0

duplex auto

speed auto

end

R3#ping 8.8.8.8

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 8.8.8.8, timeout is 2 seconds:

*Mar  1 15:43:06.868: IP ARP: creating incomplete entry for IP address: 8.8.8.8 interface FastEthernet0/0

*Mar  1 15:43:06.868: IP ARP: sent req src 45.45.45.2 5433.5342.6764,

                 dst 8.8.8.8 0000.0000.0000 FastEthernet0/0.

*Mar  1 15:43:08.864: IP ARP throttled out the ARP Request for 8.8.8.8.

*Mar  1 15:43:10.864: IP ARP: sent req src 45.45.45.2 5433.5342.6764,

                 dst 8.8.8.8 0000.0000.0000 FastEthernet0/0

[cadet alain · Accepted solution]

Hi,

when you configure a static route with the outgoing interface and this interface is multipoint then every destination for the router is connected out this interface and so it will arp for the destination even whan it is not on the same subnet.

if you disabled proxy-arp on the next-hop then it won't be able to reply with its own MAC and the router won't be able to encapsulate the icmp packet and so the ping will fail.

Regards.

Alain.

Don't forget to rate helpful posts.

[John Blakley]

Alain,

You taught me something today I've never noticed that the packet will arp if the interface is set as the next hop as opposed to the next hop ip itself. I just change my default route and then I started getting arp requests. Rated.

John

[from88]

It's a crazy information cadet alain    very big thanks.

So it means that static route with interface will be treated as directly connected. In this topology as you guessed i was using this command:

ip route 0.0.0.0 0.0.0.0 FastEthernet0/0.

but you mentioned that this works only if the interface is MULTIPOINT ? in my case fasthertnet0/0 is not multipoint ?

[cadet alain]

Hi,

of course it is multipoint even if you have the 2 routers directly connected.

Regards.

Alain.

Don't forget to rate helpful posts.