In this case because it's DHCP the router sees that the destination port matches UDP 67 and allows the packet to be "helped". When the router sees a packet arrive on an interface with the ip helper-address command configured it checks to see is it matches any of the protocols that are "helped" by default or configured with the global ip forward-protocol command. In effect, by assigning a unicast layer 2 address the frame is no longer a broadcast frame at all even though it has the 255.255.255.255 address at layer 3.Īs for your last question, DHCP relay is a router's way of "cheating" around the rule about not forwarding broadcast packets. Instead of sending the packet out all interfaces on the same vlan it would use the source address table (SAT) like it would with any other unicast destination address. The switches would only see the unicast layer 2 address. Where this would matter is with switches which don't care about the layer 3 address at all. All the router cares about is that 255.255.255.255 destination address and drops the packet. Having a unicast layer 2 destination address would not change the router's behavior at all since the router makes its decisions at layer 3. This would defeat the entire purpose of it being a broadcast frame and just not work outright. To answer your second question, it would make no sense to have a frame with a layer 3 broadcast address without a layer 2 broadcast address. This is also slightly off topic as to your original question, see this cisco forms page for more on this. This does not however allow the router to forward limited broadcasts, those are still blocked by default. This will allow it to forward directed broadcasts according to its routing table as if they were normal packets. 192.168.1.255/24) Normally by default this functionality is disabled but can be enabled by issuing the ip directed-broadcast command on the router. If routers forwarded broadcasts a single arp request would reach every single reachable host on the internet which would be terribly inefficient and rather silly.ĭirected broadcasts on the other hand are sometimes routed. That's why routers are said to be the boundary of broadcast domains. By default when a router receives a frame with a destination address that is broadcast at either layer 2 or layer 3, the router simply drops the frame. The first thing to understand to answer your questions is that limited broadcast frames are not routed. How Routers Handle Limited and Directed Broadcasts My questions are not from a perticular subject (DHCP ond Broadcasting ), so you might be angry, But Please do help me. When a DHCPDiscover ( which is a broadcast packet ) comes to that router, How does it proceed ? Because first thing its a broad cast IP also the network "0" is there, Does always relay-agent first checks the packet if the Packet is of DHCP ? Is there any protocols for this ? Does router prevents all broadcasting or only the Limited/Local broadcast ?Ĭan we send a packet with destination as a direct/limited broadcast IP without a broadcast MAC address ? If Yes, I think because of a perticular MAC address the packet will forward to a single Host, So the question, in which case this would be helpful.Ĭonsider a router is enabled by relay agent. How broadcast packet routes ? Because if the packet is intended for a different network than source, obviously the router will drop the packet. But could not find any RFC (RFC919,922 not much helpful) for broadcasting in details. But for Understanding DHCP in depth, I felt like I have to know Broadcasting and Relay Agent ( RFC 1542 ) in details. I was reading DHCP ( RFC 2131 ), I have basic knowledge about Relay Agent and Broadcasting.
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