Strange VPN issue

[nikonsean]

I'm having a very strange Cisco VPN issue. I connect to a Cisco VPN every day for work from my macbook. Currently I prefer to turn off the Airport when I'm at the desk and use a wired connection. If I try to connect to the VPN with only the wired ip up, the VPN times out and says it failed to negotiate. If I unplug and turn on the AirPort it works fine. Any ideas?

Sean
2010 MPB 15 i7, 8GB, Intel 160 SSD, OS X 10.6.5

 

[tshort]

Make sure you disable the interface you are not using.

Also, are you connected to the same router? Can you access the Internet over the wired connection?

 

[nikonsean]

Make sure you disable the interface you are not using.

Also, are you connected to the same router? Can you access the Internet over the wired connection?

Thanks for replying,

I have disabled the AirPort while using the wired interface. Every other network access (local machines and web browsing) all work. It's just the VPN that refuses to work via the wire.

 

[tshort]

Thanks for replying,

I have disabled the AirPort while using the wired interface. Every other network access (local machines and web browsing) all work. It's just the VPN that refuses to work via the wire.

Check the MTU of the interfaces. The interface MTU needs to be dropped to take into account IPSec/IKE overhead.

Go to terminal, and do 'ifconfig -v' to see the MTUs. You can change the MTU via ifconfig as well, do 'man ifconfig' to get the correct options.

 

[nikonsean]

Check the MTU of the interfaces. The interface MTU needs to be dropped to take into account IPSec/IKE overhead.

Go to terminal, and do 'ifconfig -v' to see the MTUs. You can change the MTU via ifconfig as well, do 'man ifconfig' to get the correct options.

Well I dropped the MTU to 1442 (1500 default) and it works. Thanks for the tip. Not sure why 1500 works on the AirPort but don't have time to research right now.