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VTP pruning: switchport trunk pruning remove command.

Steph1963
Level 1
Level 1

Hi to All,

I am having some difficulty with the concept of VTP pruning.

I would like to know at first if SW 3 will have some instance of spanning-tree for VLAN 7. SW3 does not not have any access port using VLAN 7 but is aware of this VLAN with VTP. Is it enough to find some spanning-tree instance for VLAN 7 on SW3.

I am also trying to understand the concept of removing a VLAN on the pruning list. If I am using the command switchport trunk pruning vlan remove 7 under fa0/1 of SW2, does it means that this port will then send VLAN 7 broadcast even if SW 3 only request for VLAN 3.

Sans titre.JPG

Thanks for your help

Stephane

1 Accepted Solution

Accepted Solutions

The concept that I have a lot of difficulty to understand is the fact that the pruning list can be different depending  on which port on the switch you look at. The switchport trunk pruning vlan remove can be applied under an interface.

You are right, it can be confusing. The best advice is to keep it simple.

Does this means that different port on the same switch can have a different pruning list? Is it like saying that I can have a switch sending broadcast for VLAN 7 on one port and not sending this VLAN on another port?

Yes, this could be the case but when a vlan is not pruned, a lot of traffic is forwarded, not just broadcasts.

Remember pruning only applies to trunk links.

Can I also say that modifying the pruning list will only have an effect on the downstream switch?


Yes, this is correct but there may be more than one switch downstream. This makes the effects of (manual) pruning sometimes hard to comprehend. Personally, I am reluctant to use it for this reason and because the net-advantage or performance gain is not that substantial in medium sized networks.

regards,

Leo

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3 Replies 3

lgijssel
Level 9
Level 9

Hi,

A trunk can be regarded as a member of all vlans. As such, the trunk port on the switch will show up as 'forwarding' when using "sh spanning-tree", even when the switch has no other ports in this vlan.

When pruning a vlan, the traffic for that vlan is not sent over a trunk on which there are no recipient ports for that vlan.

This is actually a form of optimalization.

The traffic sent over a trunk includes broadcast, multicast and unknown-mac address as the destination.

When a trunk is not pruned, all this traffic is forwarded over trunk links, so the answer to your second question is a yes.

regards,

Leo

Hi Leo,

Thanks a lot for your reply,

The concept that I have a lot of difficulty to understand is the fact that the pruning list can be different depending  on which port on the switch you look at. The switchport trunk pruning vlan remove can be applied under an interface.

Does this means that different port on the same switch can have a different pruning list? Is it like saying that I can have a switch sending broadcast for VLAN 7 on one port and not sending this VLAN on another port?

Can I also say that modifying the pruning list will only have an effect on the downstream switch?

Thanks for your help

Stephane

The concept that I have a lot of difficulty to understand is the fact that the pruning list can be different depending  on which port on the switch you look at. The switchport trunk pruning vlan remove can be applied under an interface.

You are right, it can be confusing. The best advice is to keep it simple.

Does this means that different port on the same switch can have a different pruning list? Is it like saying that I can have a switch sending broadcast for VLAN 7 on one port and not sending this VLAN on another port?

Yes, this could be the case but when a vlan is not pruned, a lot of traffic is forwarded, not just broadcasts.

Remember pruning only applies to trunk links.

Can I also say that modifying the pruning list will only have an effect on the downstream switch?


Yes, this is correct but there may be more than one switch downstream. This makes the effects of (manual) pruning sometimes hard to comprehend. Personally, I am reluctant to use it for this reason and because the net-advantage or performance gain is not that substantial in medium sized networks.

regards,

Leo

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