VTP and VTP Pruning vs allowed VLAN list

[SJ K]

Hi all,

I am reading and testing on VTP and in order for VTP work, trunk links must be setup between switches.
So i am thinking, if VTP is setup, it means that every switch (client) will have the same VLAN setup as the Server.

q1) With VTP pruning, it means that even though there are same VLANs setup across the switches, if there are no participating ports in the VLANs replicated, broadcast traffic will not be send over -> is my understanding correct ?

q2) when we want to remove a vlan that is eligible for pruning, do we set it across all the trunk links ?
(E.g. switchport trunk pruning vlan remove 6) do we issue this command 2 times on each end of the trunk interface , and must do this for all the trunks that vlan6 will pass through

q3) can 2 switches (in VTP server mode) having the same domain and password, updated each other configurations ?

q4) with no VTP turn on,  If i have a L3 switch with (VLAN 10,20,30) trunk to a layer 2 switch with (VLAN10,20),  can i still set pruning on the trunk to prevent traffic from VLAN30 for crossing over (just in case) ? or I should just disallow VLAN30 on trunk ?

Can i say the only difference between pruning and disallow VLAN is that there is a possibility for the prior (pruning) that ports might be allocated to the VLAN pruned ?

Regards,
Noob

[Mark Malone · Accepted solution]

Yes thats correct the point of VTP is less manual administration so server can distribute the change of the vlan database to everyone in its VTP domain

q1)yes no requirement to pass traffic to switch where the vlan has no active ports

q2)why do that pruning is automatic turn it on globally and it will decipher whats required where.

q3)yes they can update each other when a change is made on either server switch

q4)diss-allowing the vlan is manual pruning its basically telling switch no matter what dont allow that vlan pass the trunk , pruning only reacts if there is no need for it to pass traffic for that vlan

difference is manual and dynamic and you should only use 1 or the other,  using pruning and then manually specifying vlans allowed list may cause  issues at layer 2 , some people want full control of what is allowed and not whether currently in use or not , some just want to know that using pruning unnecessary flooding is cut out saving resources, it all depends on your network environment

q1)

I am reading and testing on VTP and in order for VTP work, trunk links must be setup between switches.
So i am thinking, if VTP is setup, it means that every switch (client) will have the same VLAN setup as the Server. - See more at: https://supportforums.cisco.com/discussion/12504776/vtp-and-vtp-pruning-vs-allowed-vlan-list#sthash.KYBukJcm.dpuf

[SJ K]

Hi Mark,

Thanks for the answers and reverting. 

With regards to

q2) No reason why actually.. I am just following the books around. Saw this command "switchport trunk pruning vlan remove", so i am thinking where should i issue this, on both interfaces on a trunk or just 1 will do ?

Regards,

Noob

[Mark Malone]

if your using that command you put it on a switch and it effects downstream traffic , so if you enable it on side a trunk side A , side B trunk will be the one effected and you only have to enable it on 1 side to see the result

Its a bit confusing why cisco uses this command when pruning is already doing this at a dynamic level i suppose it can give you manual pruning control rather than the switch doing it but it defeats the pyurpose then of less administrative overhead in large switched networks , the vlan would still need to be allowed on the trunk for this command to work though , if the vlans already dissallowed the pruning does not take any effect as the vlans not there

[SJ K]

Hi Mark,

Thanks for reply.

I believe the command switchport trunk pruning vlan can be use even if no vtp pruning is enabled. http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4500/12-2/25ew/configuration/guide/conf/vtp.html 

->  To configure VTP pruning on a trunking LAN interface, use the switchport trunk pruning vlan command. VTP pruning operates when a LAN interface is trunking. You can set VLAN pruning eligibility regardless of whether VTP pruning is enabled or disabled for the VTP domain, whether any given VLAN exists, and regardless of whether the LAN interface is currently trunking.

However, i believe that this command will need to be issue on both side of the trunk.

Think on this scenario whereby I have VLAN (e.g vlan10) spread physically spread across 2 switches

host a (vlan10) <--> switch 1 (fe0/1) <trunk--link> (fe0/2) switch 2 <---> host b(vlan10)

if I issue switchport trunk pruning vlan add vlan10 on fe0/1 -> when host b got disconnected from switch 2, vlan10 will be pruned from fe0/1.

But host a is still connected, so vlan10 is not pruned from fe0/2.

If we did not issue switchport trunk pruning vlan add vlan10 on fe0/2, when host a get disconnected from switch1, will vlan10 be prune on fe0/2 ?

I hope you get my meaning -- not sure if i explain correctly.

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With the assumption above,  i believe this command must be issue on both sides to take and have full effect.

My packet tracer cannot simulate pruning.  I will try it on a live lab tomorrow and let you know.

Thanks!

Regards,
Noob