Question about routed ports on L3 switches

Neuromancer · ‎05-27-2007

Say you have a L3 switch. One of the ports is routed (no switchport), and another is a switchport. Is it possible for devices coming off both ports to be in the same subnet? Can the devices coming off the switchport use the routed port as their default gateway?

(I'm trying to work out some theory in my head while studying for BCMSN)

Thanks!

Anand Narayana · ‎05-27-2007

Hi Christopher,

NO, it can't fall under same subnet, because 1 is being the routing port & the other being a switch port, the PC which is connected on the switch port can't have the gateway ip address as the routing port interface ip address.

eg.

interface fas 0/1

ip address 192.168.1.1 255.255.255.0

interface fas 0/2

switchport mode access

Neuromancer · ‎05-27-2007

Thank you for replying. I am confused about how the architecture and flow of data within the switch happens in this scenario. Can you explain why this is possible?

Can a PC coming off the switchport do a successful ARP for a PC coming off of the routed port?

Thanks!

Amit Singh · ‎05-27-2007

Chris,

A PC connected to a routed port and a PC on a switchport cannot be in a same IP subnet. PC which is connected to a switchport has to be in a separate subnet which will be done by using the SVI L3 interface on the switch as the gateway for the PC. When a switchport PC ARP's for the routed port PC, the L3 SVI will proxy-arp for the routed port PC and that's how the communication will happen.

HTH,Please rate if it does.

-amit singh

Neuromancer · ‎05-28-2007

Thank you!