Cisco VPNClient crashing your Mac again

The MacOS 10.5.1 update seems to have included bugs related to
networking performance. When 10.5.1 and Cisco's VPNClient 4.9.01 (100)
are combined, the problem can become fatal. The first symptom is that
the Network control panel can not set the TCP MTU. It remains stuck at
1356. iTunes will lock up dead if your music is on a remote volume.
Mail and web browsers may stall or lockup in the middle of downloading.

Here's a temporary workaround that you can run as the superuser in
Terminal:

kextunload /System/Library/Extensions/CiscoVPN.kext
ifconfig en0 mtu 1500

MacOS 10.5.1 network is still prone to stalling but it's much improved.
You will regain control of the MTU in the Network control panel. The
Cisco kernel extension reloads the next time you connect. Some Macs may
not have 'en0' as the main network interface - adjust as needed.

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["Followup-To:" header set to comp.sys.mac.comm.]
On 2008-01-21, Kevin McMurtrie wrote:
> The MacOS 10.5.1 update seems to have included bugs related to
> networking performance. When 10.5.1 and Cisco's VPNClient 4.9.01 (100)
> are combined, the problem can become fatal.

I have no functional problems with this combination. You're right that
some of the low-level ethernet settings are not available, which
should be reported to Cisco as a bug. But an MTU of 1500, which is
how mine is set, is fine by me. Functionally all the networking I use
at home -- mail, web, chat, ntp, etc -- appears to work flawlessly. I
have not tried iTunes sharing (if this is what you mean by "music on a
remote volume") or any other Bonjour multicasting, and in fact I have
no ready way to do so in the configuration in which I use the vpn. So
I can't comment on that. Or are you trying to multicast to other
machines on the vpn, as opposed your lan? That's unlikely to work,
though of course it shouldn't lock up. I'll test that case next time
the vpn is on.

Given the problems you're reporting with web and mail, sounds to me
like some factor in addition to 10.5.1 + Cisco VPN might be coming
into play here. For reference, I'm running the Cisco client on a Core
Duo MacBook Pro (_not_ Core 2 duo).

Btw none of what you described qualifies as a 'crash', and it's not
happening on my Mac, it's happening on yours. A less sensational but
also less misleading Subject would have been "Cisco VPNClient locking
up my Mac again".