Help needed - 3750G Routing Issues

Unanswered Question


We use a fixed wireless PTP link for our small business internet connection. We were using a cheap cable router connected to the PTP radio, and connecting the LAN side to a VLAN the 3750 for the LAN to get out to the net.

The cable router has now died, so I set up the 3750 to route for the LAN directly to the PTP radio. Here are the details:

1) The problem is that the switch will not route from VLAN 300 ( to the PTP gateway

2) Ping to from .45 works

3) Routing VLAN 300 to VLAN 200 works

4) PCs on VLAN 300 can ping, but not .1

5) Debug IP packet shows no output when trying to ping from VLAN 300, but shows packets when pinging .45

6) Source ping from to works

7) A cable router or laptop configured like g1/0/24 and connected directly to the PTP radio gets out fine.

Hopefully it's something simple I missed...but I have to get this up ASAP. Config is attached.

Big thanks for the help everyone!



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scheikhnajib Fri, 05/30/2008 - 06:27
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A small diagram will make it easier to understand.


scheikhnajib Fri, 05/30/2008 - 07:44
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Looks right ...

Try to create a new VLAN with a new IP address (i.e. VLAN400,, change GE1/0/24 back to a switched port (change the segment between the wireless router and the 3750G to an L2 segment). Keep the default route pointing to the wireless router and see if this works.



rchicks Fri, 05/30/2008 - 07:16
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The configuration looks correct to route out to the wireless device. Since everything works with a cable router, I have to assume this is a routing problem with the wireless device, or you will need to replicate the NAT functionality the the cable router would have done to make this work.

Thanks rchicks. That makes sense.

The thing that bothers me is that I see no packet output in the debug when pinging 88.1 from VLAN300. If the issue is the 88.1 device I think I would see packets out .45 and nothing back in.

But thinking more about that, perhaps NAT is the answer as I believe it was enabled in the cable router before it died.

I'll try NAT. In the mean time, I'm still open to ideas...

Thanks again,


scottlivingston Fri, 05/30/2008 - 12:27
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Does the PTP gateway know how to get back to the network (either w/ a 0/0 default or explicit route for that /24)?


Since the device is managed by the provider, I can't be 100% sure of that. But I'm leaning away from the radio because:

1) It worked with the cable router connected and no changes to the radio config.

2) There are no packets displayed in the 3750 debug when sending from VLAN300 to 88.1. Only when packets are sent from VLAN300 to 88.45 are there packets in the debug.

But it's true there could be other reasons for it on the radio or provider side. I'll start looking closer at the radio once I get back over there and try the previous suggestions to no avail.

Thanks Scott!

rchicks Fri, 05/30/2008 - 12:45
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which debug are you doing? Debug ip icmp? remember debug ip packets doesn't always show everything especially when route-caching is enabled.


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