Re: VTP Pruning?

azmunnabi · ‎09-19-2013

Need some clarity on VTP Pruning.

What I understand:

When pruning is done on trunk interface of a switch, the data frames corresponding to pruned VLAN will not not pass through the Switch's trunk.

Here is some doubt on VTP Pruning:

Q1) Can VTP pruning of a VLAN be done on switch's trunk port before the VLAN creation.

Q2) Scenario:

If Switch A (VTP Server) is connected to Switch B (VTP Server),

Trunk Interface of Switch A is pruned for VLAN 20.

VLAN 20 is then created on Switch A.

Will VLAN 20 be created on Switch B by itself (through VTP)? (However data frames of VLAN 20 will not pass through the trunk of Switch A.)

If someone can explain: VTP pruning only blocks the data frames transmission or it also blaoks the creation of Vlan across trunk through the VTP.

Please explain the VTP pruning if I am getting it wrong.

Also if any one can suggest, on which switch in packet tracer does the VTP Pruning work.

I am using packet tracer, and there is no option of Pruning

On Switch 2960-24T:

Switch(config-if)#switchport trunk ?

allowed Set allowed VLAN characteristics when interface is in trunking mode

native Set trunking native characteristics when interface is in trunking mode

>>> Here there is no option of pruning.

1 more query: VTP Client and NVRAM

IN VTP Client mode, VLAN information is not stored in NVRAM and VLAN information is deleted when switch is reloaded. It is in temporary memory.

Please correct the statement if it is wrong.

I configured a VTP Client Switch, it read VLAN information, then I reloaded it.

No VLAN information was deleted. Please comment on the behavior.

Any help is appreciated.

Azmun

Rolf Fischer · ‎09-19-2013

Hi Azmun,

I think you're mixing up 2 differnt things: VTP pruning and manual pruning.

1) VTP pruning

is configured on a VTP server for the entire VTP-domain with the command vtp pruning
switches exchange special VTP join messages which contain the list of VLANs and their status (needed or not on that switch), (VTP) pruining is done automatically based on that lists.
when a pruning eligible VLAN is not needed on a trunk, no broadcast, multicast and unknown unicast is flooded for that VLAN on the trunk. That's the purpose of (VTP) pruning.
VLAN 1 is never pruning eligible (for obvious reasons), other VLANs can be configured on trunk-interfaces to be not pruning eligible
VTP pruning has no effect on spanning-tree protocol
VLANs can be added, renamed, deleted; this will be distributed in the domain by normal VTP operation
show commands: show interface [] trunk; show interface [] pruning

2) Manual pruning

is configured on a per-interface basis with switchport trunk allowed vlan
no frames will be forwarded for a manually pruned VLAN (exception: untagged control traffic)
the link doesn't participate in STP, often manual pruning is done to limit the number of STP instances.
show command: show interface [] trunk

To be continued ;-)

Rolf

[EDIT] : Attached 2 Wireshark Captures with VTP join messages

Rolf Fischer · ‎09-19-2013

Useful link: http://www.informit.com/library/content.aspx?b=CCNP_Studies_Switching&seqNum=26

azmunnabi · ‎09-19-2013

Thanks Rolf,

1) VTP pruning

Could not find the below command on my switch: Same is mentioned in the shared link.

Switch-A(config)# vtp pruning
Switch(config-if)# switchport trunk pruning vlan vlan-list

or command:

Switch#sh interfaces gigabitEthernet 1/1 pruning

is not running on my switch.

Please comment if I am running wrong command.

Also,

VTP pruning

is configured on a VTP server for the entire VTP-domain with the command vtp pruning

Isn't data frames is blocked on that particular trunk ports only and not in the entire domain.

But this will then be manual pruning as you are referring to.

Please see the link below:

http://www.orbit-computer-solutions.com/VTP-Pruning.php

In the given topology two ports of different switch is pruned. We can have run manual pruning there.

Manual Pruning is by: "switchport trunk allowed vlan ". That is OK.

I will give some time on the data and comments shared by you and will then come back with my understanding.

Many Thanks for responding.

Regards,

Azmun.

Rolf Fischer · ‎09-19-2013

Azmun,

is this a real switch or is it packet tracer? Remember that packet tracer is just a simulation with a limited set of commands.

On former platforms like the 2900xl and 3500xl there was a special command mode to configure VLANs and VTP settings ("vlan-database" in exec mode), but I don't think you have on of these, do you?

Isn't data frames is blocked on that particular trunk ports only and not in the entire domain.

Pruning is a VTP feature, and as a feature you enable or disable it on the VTP server and as part of the vlan.dat this setting will be distributed within the domain (like the VTP version).

If a VLAN will be pruned or not (by the protocol) on a particular trunk-port is the protocol's decision and it depends on

is the VLAN pruning eligible (VLAN 1 is not, other VLANs can be configured to be excluded from the pruning process on a per-link basis)
does the neighbor-switch report that the VLAN is not needed downstream (no ports in that VLAN)

1 more query: VTP Client and NVRAM

IN VTP Client mode, VLAN information is not stored in NVRAM and VLAN information is deleted when switch is reloaded. It is in temporary memory.

Please correct the statement if it is wrong.

In VTP server or client mode, neither the VLAN configuration nor the VTP configuration (domain name, password hash, etc.) is part part of the startup-config. Instead, this part of the switch's configuration is stored in a file called "vlan.dat" (the content of the vlan.dat is more or less the content of VTP subset advertisements). The default location of that file on 2k/3k platforms is flash:.

If you (manually) delete the vlan.dat and reboot the switch, beside the VLANs all the VTP configurtion will be lost as well (including domain name and password).

Regards

Rolf

MikeMongillo24 · ‎12-05-2014

That Wireshark capture brings up a new question.... "VLAN 0 " ? I didn't not know there was such a thing..

in VLAN truncking Protocol:

First VLAN ID: 0

Last VLAN ID: 1007