the difference in dealing of Layer 3 Switch and Router with broadcast communication

[thsecmaniac]

How does L3 Switch deal with broadcast communication such as ARP,DHCP or Broadcast storm? Is it the same that router deal with such as blocking a broadcast communication or ip address "255.255.255.255"?

[Peter Paluch]

Hi,

An L3 swich is really a combination of Layer 2 switching and Layer 3 routing functionality. To answer your question comprehensively, we need to look at both these aspects.

From the Layer 2 switching perspective, all broadcasts - and that includes ARP and DHCP - will be flooded to all other ports in the same VLAN except the incoming switchport.

From the Layer 3 routing perspective, it is sufficient to limit ourselves to IP traffic because that is what routing is concerned with. If packets are sent to 255.255.255.255 (the so-called limited broadcast address) then this broadcast wil be flooded to all other hosts in the same broadcast domain (this is still the Layer 2 switching task) but it will not be routed to any other network, in other words, it will remain contained in its originating network. If packets are sent to a directed broadcast address (a particular network's broadcast address) then there are two options:

• If the directed broadcast corresponds to this network's broadcast address, it will be flooded just like a limited broadcast. It will not be routed to any other network.
• If the directed broadcast corresponds to a different network, it will be forwarded via the shortest path to the destination network.

Feel welcome to ask further!

Best regards,
Peter

[Richard Burts · Accepted solution]

Perhaps there are some aspects of your question that I am not understanding. But it seems fairly straightforward. A layer 3 switch in its routing/layer 3 processing treats broadcasts exactly the same way that a router does - a broadcast is not forwarded from the vlan/subnet where it originated to any other vlan/subnet. So an ARP request received by the layer 3 switch will not be forwarded to any other vlan/subnet. A DHCP request received by the layer 3 switch will not be forwarded to any other vlan/subnet (note that ip helper-address does provide the ability for the layer 3 switch to forward the DHCP broadcast request to some address in another vlan/subnet).

If this does not address your question then please provide clarification of what the question is asking.

HTH

Rick

[thsecmaniac]

Thank you so much for your answer . It answer my doubtfulness about a function on L3 Switch and Router.

[Richard Burts]

I am glad that our answers were helpful. Thank you for using the rating system to mark this question as answered. This helps other readers of the forum to identify discussions that have helpful information.

HTH

Rick