[c-nsp] problem with VPC port-channel between Cisco 3550 and pair of Nexus 5020

[Tom Mikelson]

Physical links are up, cdp neighbor shows all devices
Nexus 5020 pair configured with working VPC link on VLAN 11.
Port-channel 64 on both Nexus 5020s show blocked for spanning-tree VLAN 11.

***** Nexus 5020s *****

interface port-channel64
  description TEST
  switchport mode trunk
  switchport trunk allowed vlan 11
  speed 1000
  vpc 64

interface Ethernet1/8
  description TEST
  switchport mode trunk
  switchport trunk allowed vlan 11
  speed 1000
  channel-group 64 mode active

***** Cisco 3550 *****

interface Port-channel64
 description TEST
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11
 switchport mode trunk
!
interface GigabitEthernet0/1
 description TEST_to_5020_A
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11
 switchport mode trunk
 channel-group 64 mode active
!
interface GigabitEthernet0/2
 description TEST_to_5020_B
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 11
 switchport mode trunk
 channel-group 64 mode active
!