What does a router do with a packet if it can't find the destination IP address in its routing table?

[arsalanpti]

If a router receives an ARP request in which the destination hardware address is a broadcast address, and if the destination IP address is an address that the router cannot find in its routing table. What will the router do in that situation? Is it going to cache pair IP/hardware source address and send ICMP message "Destination network unreachable" back to the source?

[aamirmurtaza67]

ARP requests are broadcasts, and routers do not forward broadcasts. Unless the request is for the host (including a router), an ARP request will be dropped by the host, and the host doers nothing else. olansi china purifier

[Giuseppe Larosa]

Hello @arsalanpti ,

if you are referring to a Proxy ARP enabled scenario the router will answer only if it is able to route to the destination with its own MAC address on the RX interface.

If it is not able to find a way to route the packet ( including using a default route) I think it does not answer to the ARP request.

There is a special case here :

at global level a router can be configured for classless and in this case it can use a default route to send packets for an unknown subnet of a connected major network ( Class A, B,C) or classful where in case a packet is for an unknown subnet of a connected major netwotk it is discarded.

The classless is the default since 12.0S.

Hope to help

Giuseppe

[MHM Cisco World]

BlackHole where the packet will end in router and host not known what is issue.

[Richard Burts]

The title of this post asks one question and the text of the post asks a different question. So perhaps 2 answers are called for.

1) the title asks what a router would do if it received a packet and can not find the destination address in its routing table. If there is a default route then the router would forward the packet using the default route. If there is not a default then the router would return an unreachable to the source.

2) the text asks what a router would do if it received an arp request with a broadcast hardware address and can not find the destination layer 3 address. There are several aspects of this question to address:

- in an arp request the hardware destination address is always a broadcast. So this is expected and does not change anything in how the router processes the request.

- if the destination IP address is in the local subnet it does not matter whether the router can find the address or not. The router does not send any response when the destination is local.

- if the destination IP address is remote then the issue becomes whether the router has enabled proxy arp or not. If proxy arp is enabled and if the router believes that the destination address is routable (either through a matching subnet in the routing table or through a default route) then the router sends an arp response and in the response the router supplies it own mac address. If proxy arp is enabled but the router finds that the destination address is not routable then the router does not respond to the arp request (there is no unreachable) and the arp request just times out.