Arp reply direction

[from88]

                   Hello, when user sends an ARP REQUEST, the router gets it, what is the logic of sending it back to user ? Do he send back (same interface) (from where ARP REQUEST came) ? Or it consults routing table where to send arp reply ?

[Parvesh Paliwal]

I did not come across any of such document in near past but i can say that the reply should go back from the interface it received a request not all of the interfaces.

I 'll check it out and confirm for my sack too.

Regards.

Parvesh

[from88]

Thank you. in standard situation there are no difference cause almost all sitautions will have directly connected interface. /24 or /30 (nevermind)

but in my case i have this configuration:

ip route x.x.x.x/32 fa0/0.135

interface fa0/0.135

vlan id 135

ip unnumbered loopback 0

all interfaces have source as unnumbered interface, and the users working by directing at them static routes. So i have an question do any user in that interface can get ARP with any IP addres or IP address in static route.

[Giuseppe Larosa · Accepted solution]

Hello Kajatonas88,

ARP messages are not routable by definition and ARP reply is sent out the same interface where the ARP request was received.

In other words ARP packets are not IP packets and they have a different ethertype in the ethernet header

IPv4 0x0800

ARP 0x0806

Hope to help

Giuseppe