Arp Question

Unanswered Question
Sep 20th, 2007

When a router receives a frame and notices that it has a directly connected route to the IP, it then needs to rebuild the frame with the destination MAC address. If the Arp Table does not contain the Mac address of the destination host, the router sends an Arp request.

Does this Arp request get sent to all direclty connected hosts on that interface the router has the particular route for?

Or does this arp request only go to the Vlan this IP address is on?

For ex. if you have a trunk to a router interface for 10 different vlans, will the router send the arp request into all the vlans the trunk is on, meaning will the hosts not on that particular IP segment hear the Arp request?

I need some clarity about how this arp request is seen/used, etc.

I have this problem too.
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scottmac Thu, 09/20/2007 - 07:26

ARP has a broadcast-based initiation .. i.e. it sends "Who is 10.10.10.10" and that host responds back "that's me, my MAC is a.b.c.d.e.f"

So, the router will broadcast the "Who is.." message to the network/subnetwork to which is believe itself to be attached.

Since VLANs are defined as a broadcast domain, and ARP is a broadcast-oriented message, it would only be sent on the VLAN / SUBNET / BROADCAST DOMAIN that is associated with the address and mask of the destination (directly-connected) segment (regardless of whether it is a physical broadcast domain (one Ethernet interface) or logical (a VLAN or BVI).

Good Luck

Scott

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