Layer3 Switching Q.

Netstudent

OKay lets say I had a distribution switch that had layer2 links to my layer2 access module. Then it has layer3 IP conections to the core. Then lets say for some reason I have decided to cross the core to get to my DHCP.

Must I still use the helper-address command to create a unicast transmission from the distribution switch to the server farm?

I'm just wondering about the behavior of broadcasts like DHCP once the broadcast reaches a layer3 switch.

In the BCMSN book it states "cisco layer3 switches are able to convert broadcasts such as DHCP into unicasts before the packets leave the building distribution module, to minimize broadcast flooding in the backbone module."

This is the part that has me questioning. What would happen if the switch did not convert to unicast? Would it then broadcast the DHCP request packet over the core? That is what this statement seems to imply.

If there were no unicast conversion, would this packet have an destination address of 255.255.255.255 or would it have an IP-Directed broadcast address of the subnet between distro and core.

Any input on these questions is appreciated.

dtlokee

The part about the "are able" is not a default setting and is provided by the "ip helper-address" as you had mentioned. I guess what they're trying say is in contrast to the what a L2 switch would do, simply flood the broadcast onto the core. A L3 switch is no different than a router in respects to how it handles a broadcast packet. A SVI will not forward a broadcast to another SVI or routed (L3) interface, or a routed interface will not forward to another routed interface or SVI.

Netstudent

Thank you. you da man!