Two default gateways in Cisco 1811 router...how does it work?

[Grand666]

Hey guys,

Let me start by saying that I'm not a computer networking expert or something, I'm studying networking from free materials I got on the net, CCNA etc.
I'm helping the lady in our town's library to make a schematic of the 11devices on the network, and to find out where all their cables go coz they have, for three years now, not had any drawings or paperwork on the network whatsoever. They have a Cisco 2800series router that connects to the T1 connection, then from their it connects to a Cisco 1811 wireless router, then to an Allied Telesyn switch, and then to a linksys switch. Now some of the computers are hard wired to the Allied Telesyn switch and Linksys switch.(they have two switches coz some computers are far from the Allied Telesyn switch, and implemented it so they just have one cable running to the linksys and then the computers near the linksys are connected to it. i guess the person who intalled it didnt want to run an additional of 6 cables all the way to the Allied Telesyn switch).

When we did an IPconfig on all the devices, we saw that all the hard wired devices have a default gateway different from the wireless ones. Is this something that you configure inside the Cisco 1811 router? To have a default gateway of lets say 192.168.1.1 for the hard wired devices, and 192.168.5.1 for the wireless devices? I can't seem to understand this, yet all the devices have a connection to the internet through the Cisco 1811 router. The subnet mask of the whole network is 255.255.255.0 for all devices, wireless or wired. So its not subnetted then?
I hope I make sense.

[clandestine_j]

You can definitely have multiple default gateways in a single subnet. First, are you 100% sure there is a single WAN connection? Second, you said this is a town library - do you know if they are VPN'd back to a town run datacenter or other IT department? Often this is the case for my municipal clients. It may be a matter that they are restricting, via firewall rules, what wireless clients can access. Often times when there is an open or somehow publicly accessible WiFi network we will set them up on a separate set of rules. One possible implementation of this would be:
1. Assign DHCP over the WiFi setting the default GW differently than the wired.
2. Have an access-list that match traffic to that GW
3. Filter based on that access-list

Do you have access to the router? It is all speculation at this point and without seeing the config of the router you likely won't discover more about the setup than you already know.

~j

[Grand666]

hey clandestine,
no i dont have access to the router. all im doing is just mapping the network out on paper. drawing it on autocad and labelling the cables so the old lady knows exactly where their cables go an what is connected to it. i was just curious coz when i did an ipconfig to see who is connected where, i saw two different gateways. thanks for you answer.

[Sophia Sweeden]

A Cisco router can, and often does, have more than one IP address. In this case it's likely that 192.168.1.1 is the IP address of the 1800 series router's physical ethernet interface that plugs into the Telesyn switch. 192.168.5.1 is likely the IP assigned to it's wireless service. The router has an IP address in each subnet, and both addresses point to the same router. The Cisco 1811 probably has yet a third IP address pointing to the Cisco 2800 as well. You can check this by examining the configuration file on the Cisco 1811.