ISIS on breakout USB - no neighbor

Interesting issue encountered when trying to use USB ethernet to physical switches.  Dot1q works fine and I can get layer 3 end-to-end connectivity between hosts.  OSPF and EIGRP work.  However, ISIS will not form neighborhship when on a sub interface of each side.  To narrow it down to a hardware issue I set up the switch as layer 3, added a SVI to the ISIS process and then set up the GSN3 router with a sub interface and ISIS enabled.  Layer 3 okay but no ISIS.  Debugging the ISIS it never shows any IIH packets inbound.  If I switch the adressing to the interface itself and change the switch side to a routed port it works fine.  I thought this was a MTU problem, but both sides match.  Wireshark captures on the GNS3 side interface don't see incoming packets either.   I haven't tried adding another monitor session to the switch and see if packets are at least going one direction.  In other test environments I've had ISIS adjency issues due to MTU issues betwen IOS-XR and IOS, single vs. multi topology, etc, but not of that is present on this simple generic test.  Just a GNS3 router host and cloud connection to switch.  

Anyone experienced anything similar?  

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