Port channel between Force 10 and Cisco switches goes down when one Force 10 in the VLT is rebooted.

Customer has a pair of Force 10 S4820 switches configured in a VLT domain. There is a port channel configured to a pair of Cisco C4510R+E connected via VSS. When one of the F10's is restarted the port channel on the surviving F10 goes down.

Looking at the default LACP timer settings a Cisco 4500, they are set to 30 seconds: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/XE3-7-0E/15-23E/configuration/guide/xe-370-configuration/channel.html#pgfId-1084922

(See “Configuring LACP Fast Rate Timer”)

On the Dell Force 10's they are set to short/fast (i.e. 1 second) already.

The options are stick with the static lag, or upgrade the IOS.

If you’re running a Supervisor Engine III or below then IOS v12 is the only option, therefore static lag.

Supervisor Engine IV or above will take IOS 15. 

The solution is this case was to upgrade the iOS on the C4510R+E and use the "LACP FAST RATE" on the port channel. If you can't upgrade the iOS then configure a static LAG.