Understanding Switches and Broadcast

[Tuff]

Hi

   There is something that is bothering me and i would need help to clarify it. We all know that each vlan is its own broadcast domain. For example, broadcast for VLAN 10 does not extend to VLAN 20.. So if a pc connected to VLAN 10 send a DHCP broadcast message looking for DHCP SERVER which is located on VLAN 20, how does the switch handles it? Does the switch know to send the DHCP broadcast to the Router/DHCP only without broadcasting it to all ports? 

[balaji.bandi]

Take an example : if the DHCP Server Located in VLAN 10, Cient request for DHCP IP, and user PC will get.

but if the VLAN 20 user request for IP, the DHCP  Server in VLAN 10, this is not going to get DHCP IP.

That is the reason you need to add DHCP helper address for VLAN 20, so VLAN 20 aware where DHCP Server Located.

For this underlay, routing should be reachable from VLAN 20 to VLAN 10, if both are in the same switch ip routing should be able to deliver this.

[Tuff]

Thanks for the reply. I understand that a DHCP Relay needs to be configured for the clients in different VLANS to be able to obtain IP.. So assume the DHCP relay is already configured for all VLANs... So if the PC in VLAN 10 sends a DHCP broadcast message does the switch forwards it to only the Router/DHCP or does it broadcast it on all ports? What i really want to understand is how switches handles broadcast messages when there are multiple VLANS.

[balaji.bandi]

here is the detailed explanation document for reference :

https://www.cisco.com/c/en/us/support/docs/ip/dynamic-address-allocation-resolution/27470-100.html

[Joseph W. Doherty]

"So if a pc connected to VLAN 10 send a DHCP broadcast message looking for DHCP SERVER which is located on VLAN 20, how does the switch handles it?"

Are you asking about L2 or L3, if the switch supports the latter?

If L2, broadcasts are limited to just the original/source VLAN.

If L3, and a "helper" has been configured, on a routed interface, specific/selected broadcasts are repackaged as unicast and sent to the helper IP.