I'd like to set up a SPAN [port-mirror] port on our main router. The router is a Cisco 2821. The goal is to be able to troubleshoot Internet issues at the packet level, and it won't happen that often. Here are the ports:
GigabitEthernet0/0 - Connects to AT&T uplink equipment... some fiber converter box. GigabitEthernet0/1 - Connects to our Juniper firewall, then the rest of the network. FastEthernet0/0/0 ... FastEthernet0/1/3 - All of these FastEthernet ports are unused.
I was considering keeping the cables where they are and assigning a FastEthernet port as a SPAN port, but Cisco's IOS gives me the error that you cannot SPAN across multiple switch modules.
Then I was considering not using GigabitEthernet0/1, and instead going into FastEthernet0/0/0, with FastEthernet0/0/1 being the SPAN port. But you can't assign those ports IP addresses -- THEY ARE STRICTLY LAYER 2 PORTS.
So then I thought, I'll just keep the current setup but connect GigabitEthernet0/1 to FastEthernet0/0/0. Another FastEthernet port will connect to the Juniper, and yet another will be a SPAN port I can connect a laptop with Wireshark to.
But that looks so goofy, with a cable going from one port to another on the same router. And would that create some routing/switching loop? I've just never done something like that before.
Would this work, or is there a better way to do this? Thanks.