VTP Pruning - VLAN Pruned, but Some Traffic Traverses Trunk Anyway

[eh2os]

Hello,

I have a metro ethernet connection with 3 switches (Switches A, C, and E) at 3 different locations each with a single interface into the metro-e cloud and each interface configured as an 802.1q trunk.  Switch A is in VTP domain xxx-xxx and Switch C and E are in VTP domain yyy-yyy.  VTP pruning is enabled on each of the 3 switches.  I am able to ping from Switch C to the real AND HSRP addresses of VLAN 251 which reside on a pair of Sup 720s in Switch A and its partner Switch B in another 6509 not connected to the metro e.  However, Switch C, while having an interface on VLAN 251, shows me that VLAN 251 is pruned on the trunk link into the metro ethernet.  Switch D, which is trunked to switch C, also has a VLAN interface on 251 and can ping the real addresses of Switch A and Switch B on VLAN 251, but cannot ping the HSRP address of VLAN 251 which is Switch D's default gateway.  I have no idea why this is.  There is a whole other set of interesting things going in the setup, but I'll leave that out for right now in the hopes of focusing the discussion.  I have attached a Visio diagram that hopefully helps clairfy what I have written above.

Thanks in advance for your help.

[Andrew Gossett]

Hi,

VTP prunning blocks broadcast and unicast flooded traffic on the link.  If the switch learns a MAC on a prunned vlan but still in the STP forwarding status, it will still send packets out on that port.  It will also accept packets on a prunned vlan (again, assuming in the port is in STP forwarding status for that vlan).  The biggest problem you will see is ARP since that is sent via broadcast.  However, if ARP is already resolved (possible by ARP request from site with vlan not prunned) and thw switch has the MAC in its CAM, then unicast traffic will work.  If we are trying to send broadcast traffic on the link or unicast flood (in the event that the destination MAC is not in the CAM) then prunning will drop the traffic.

I'm not sure of the exact problem that you were originally troubleshoot, but in either case you will likely want to ensure that the necessary vlans are not prunned on the metro link.  We can do this in two ways:

1) Disable prunning on the VTP server

   Switch(config)#no vtp prunning

2) Set the vlan in question to prunning ineligible for the uplink

  SwitchC(config)#interface g0/20

  SwitchC(config-if)#switchport trunk prunning vlan remove 251

-Andy