Driver disk loading via NFS fails on Cisco switch, OK on Dell

I have a working Cobbler server, using it to load units with
CentOS 5.7.  It works fine with a simple Dell 2716 switch
connecting the server and targets.  Also with a LinkSys RV016.

Substituting a Cisco 2960 switch, it fails.  It network boots,
shows the PXE menu, I choose an entry, it gets a DHCP address on
its eth0, and starts loading including getting its kickstart via
HTTP.  But when it goes to load the driver disk image via NFS, it
hangs.  The Anaconda Alt-F4 screen shows it not seeing the NFS
server; after 2 minutes of trying, it times out and the Alt-F4
screen shows an error dump.

Tried the switch with and without RST (rapid spanning tree)
active, no difference.

The switch seems to work OK for normal network operations outside
the kickstart environment, for what we use.

Any idea what it might be that could be interfering with the NFS
disk driver loading step?