Layer 2 Technologies – VTP Eligible List, VTP Transparent, Trunk Allowed List, Extended Vlans.

Lessons learned:

VTP Eligible List:

Vlans 2-1001 are “prune Eligible” by default.

Note: When you compare the pruning list VS the trunk allowed list – they actually accomplish the exact opposite things.

Prune eligible list: If a Vlan is NOT in the list it must be forwarded.

Trunk allowed list: If a Vlan is NOT in the trunk allowed list if cannot be forwarded.

Note: Vlan 1 cannot be removed by prune lists.

Verification's:

# Sh interface FA x/x switchport

Shows if pruning is enabled and the Vlans enabled to be pruned

Note: To remove a Vlan from the prune eligible list

# Switchport trunk pruning vlan remove (Vlan #) Example:10

This will make Vlan 10 not eligible to remove

This will also make both ends of the trunk send and receive.

Problems with Pruning:

If all devices do not support pruning, (VTP is a Cisco Proprietary protocol) each device would have to be in transparent mode.

For example, you have an end host that is a router or a VMware server. These devices will not support pruning, This will automatically undo everything that pruning tries to fix. At least from a uni-directional perspective.

This issue can be resolved be editing the allowed list on the trunk link to the router (if router on a stick) or VMware server and only allowing only the Vlans needed over the link.

Note: VTP Pruning is ONLY allowed if none if the devices in the transit path are in Transparent mode.

Another issue – if you have a device in the transit path configured in transparent mode. The device must at least share the same VTP domain name. If not DTP will try to correct this and drop the frames across the trunk link.

Manual Trunk Pruning:

Note: Anytime you have transparent switches in the path you run the risk of losing reach ability between the Vlans. The solution is basically to not run VTP pruning.

Trunk allowed lists:

This controls what Vlans will forward over the link. By default all Vlans (1-4094) are forwarded.

Allowed lists can be edited for manual pruning,

# switchport trunk allowed vlan

Verification

# Sh interface trunk

# sh interface Switchport

Extended Vlans:

Normal Vlan range is 1 – 1—5

Extended Vlan range is from 1006 – 4094

 This requires all devices be in VTP transparent mode. This also mean you cannot use VTP pruning or VTP authentication.

Unless you’re running VTP v3.

This also implied that Vlan changes are locally significant. Vlans can still span multiple devices. VTP does not define the broadcast domain.