Cisco Support Community
Community Member

Fundamental broadcast question

When a workskation sends a request to a server that happened to be on a different VLAN, does it send it as a broadcast or a unicast. Furthermore, if it's a broadcast request, why does a router pass it?


Cisco Employee

Re: Fundamental broadcast question

It depends on what kind of request you are talking about. Let's look at two different cases

Scenario 1:

A workstation sends a DHCP request. Since it currently has no IP address, it has no idea where the DHCP server is, so it sends out a broadcast DHCP request. All stations in the same VLAN will receive this broadcast. If the DHCP server is on the same VLAN, it will respond to the request. Now, the router interface on this VLAN will also receive this request. By default, it will simply drop the packet. However, if you have a DHCP server on a different VLAN, in order for the server to receive DHCP requests from other VLANs you have to configure DHCP relay (aka IP helper) on the router interface, which will specify an address(es) to forward broadcasts to. With this configured, the router will take the broadcast packet and send it as a unicast to the specified IP address(es).

The following link explains IP helper in greater detail:

Scenario 2:

A workstation with an IP address sends a unicast request to an FTP server. When the router receives the packet, it will do a routing table lookup on the destination IP address, and simply route the packet out the interface based on it's routing entry.

hope that helps.

CreatePlease to create content