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sphughes
Beginner

3750 - Default Gateway to same subnet

Is it possible to set up a 3750 to act as a default gateway to forward packets within the same subnet?

Here is what I have in mind, any problems?

The 3750 switch has one vlan and it is on the 192.168.10.0 network.

ip route 192.168.50.0 -> 192.168.10.5

ip route 192.168.51.0 -> 192.168.10.5

ip route 192.168.60.0 -> 192.168.10.10

ip route 192.168.61.0 -> 192.168.10.10

I'm switching ISP's and need to be able to make the transition from the old T1 to the new T1.

Thanks!

2 ACCEPTED SOLUTIONS

Accepted Solutions

The networks you mentioned are on different subnets if you are using the natural subnet mask of 255.255.255.0. Instead, if you are using a shorter mask then they could be on the same subnet.

Anyway, to answer your question, yes you could route the traffic for the same subnet. See the example below.

int vlan 5

ip address 192.168.10.1 255.255.0.0

ip route 192.168.50.0 255.255.255.0 192.168.10.5

ip route 192.168.60.0 255.255.255.0 192.168.10.10

The router would forward the packets destined for 50 and 60 subnets to 192.168.10.5 and .10 respectively. This would work because the routers always use the longest match criteria to route packets.

HTH,

Sundar

*Please rate all helpful posts.

View solution in original post

You have stated there's only one VLAN interface configured on the 3750 and it's the gateway for the LAN. That means all the ports on the 3750 are assigned to that VLAN. All ports have to be switchports as long as the devices connected to them are on the same VLAN.

HTH,

Sundar

*Please rate all helpful posts.

View solution in original post

4 REPLIES 4

The networks you mentioned are on different subnets if you are using the natural subnet mask of 255.255.255.0. Instead, if you are using a shorter mask then they could be on the same subnet.

Anyway, to answer your question, yes you could route the traffic for the same subnet. See the example below.

int vlan 5

ip address 192.168.10.1 255.255.0.0

ip route 192.168.50.0 255.255.255.0 192.168.10.5

ip route 192.168.60.0 255.255.255.0 192.168.10.10

The router would forward the packets destined for 50 and 60 subnets to 192.168.10.5 and .10 respectively. This would work because the routers always use the longest match criteria to route packets.

HTH,

Sundar

*Please rate all helpful posts.

View solution in original post

Do I need to change the mode of the ports or anything?

My setup is as you suggest, I have 7 Access Layer switches sent to the 3750 Core Switch. Do I have to use the "no switchport" command on each port in order for them to route the packets?

You have stated there's only one VLAN interface configured on the 3750 and it's the gateway for the LAN. That means all the ports on the 3750 are assigned to that VLAN. All ports have to be switchports as long as the devices connected to them are on the same VLAN.

HTH,

Sundar

*Please rate all helpful posts.

View solution in original post

rwyates_2
Beginner

A couple of questions:

What are you connecting to the 3750 with?

Are trying to connect PC's with all of these subnets to the 3750? If so, you will have problem because the PC's will not see the default gateway if you are using the 10.5 as it. THat is unles you are using a 16 bit mask. then it would see the gateway.

a little more info and the switch config would help.

Thanks

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