Part 1: What do you know about VLANs, VTP, and Trunking?
Guess nothing! Just joking there, but here is a reminder just in case if you needed to refresh your memory:
And let me start with a great saying by Dan Farkas that will remind you about the roles of Vlans:
If you can see something and it’s there, then it’s real
If you can’t see something but it’s there, then its transparent
If you can see something but it’s not there, then it’s virtual
If you can’t see something and it’s not there then it’s missing!
VLANs themselves are what you see individually at edge LAN switch ports, to which hosts connect. In general, when you interconnect more than one modern switch, the ports interconnecting them run a trunking protocol. Trunking protocols allow traffic from different VLANs to share a physical link. A single non-trunked switch still can contain VLANs, with different ports assigned to different VLANs.
The process of getting a packet from the edge port to a trunk involves tagging each frame with information that lets switches know what VLAN it belongs to.
VTP
Do you even know what VTP is? Nope, you are wrong. This is it: it is actually a VLAN trunking protocol. In other words, it is a Cisco proprietary mechanism that reduces the operational workload of making changes to VLANs. VTP travels over trunks, so any participating switch must have at least one trunk port for VTP to exist.. right? VTP can improve overall performance by preventing the propagation of VLAN traffic to switches with no port in the VLAN, reducing trunk bandwidth and switch processing. It can also prevent certain configuration errors and inconsistencies. And remember to configure your VTP before connecting the switch physically to the network.
Pause: Did you know that VTP mode is set to Server as default? Guess not!
There are three different modes for VTP, and they re the following: Server, client, transparent.
VTP Pruning
Now seriously, what the hell is Pruning? I didn’t know till I executed it on my lab, but here it is, I hope it makes sense to you:
Pruning is an important function in reducing VTP overhead on trunks, often more than offsetting the increased overhead induced by VTP itself. You avoid consuming trunk bandwidth and switch processing for messages the messages that will be discarded. In other words, why waste the bandwidth on processes that are going to be discarded, right?
Pruning recognizes when a downstream destination switch will actually be able to use certain VTP messages, and will flood them downstream only when they will be relevant to the destination. The term “spanning tree” is especially apropos, because this function prunes branches of the tree that don’t need to receive any sap from trunk of the tree — I mean, any frames from the conceptual all-VLAN-trunk. As opposed to garden pruning where you amputate the branches, VTP pruning can reattach branches when the relevant switches do include ports that include the previously pruned VLANs.
This feature is off by default in both VTP v1 and v2. When it is enabled, only the information for VLANs in the pruning-eligible list can be pruned. The contents of this list depend on software image and any manual configuration. With the standard image and default behavior, VLANs 2-1001 are pruning-eligible. VLANs 1 and the range 1002-1005 can never be eligible for pruning. Extended range where VLAN ID is greater than 1500 can be pruned only when running EMI software in all switches in the domain.
Last few words about VTP Pruning. VTP does consume RAM and sometimes NVRAM. It also not intended to affect switches in VTP transparent mode. If any of your switches are in VTP transparent mode, you have to make adjustments to avoid incompatibilities. One such adjustment is turning off pruning for the entire switches system. Alternatively, you can make sure that the transparent switches do not see pruning, by adjusting the pruning-eligible list of directly connected non-transparent switches.
And that is it.
Will cover configuration in part 1-2.
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Yes, by using the following command line:
[code]
Switch(config)# interface type mod/num
Switch(config-if)# switchport trunk pruning vlan {add | except | none | remove} vlan-list
[/code]
vlan-list is vlan ids that can be seperated by commas or by dashes.
and hey note: vtp pruning has no effect on switches in the VTP transparent mode.
I hope that helps, if not… let me know!
I got the command. But somewhat I’m not clear.
(server)swith 1 ————–switch 2(transparent)
| |
| |
switch 3————–switch 4 (clients)
/ | \ / | \
/ | \/ \/ | \
switch4 sw5 sw6 sw7 sw8 sw9
I have depicted an example network where all the switches have redundant connections.
1. Switch 1 is the root bridge for even vlans
2. Switch 2 is the root bridge for odd vlans
3. All the switches are in VTP domain except switch 2
I this case, I’m afraid enabling VTP pruning might disconnect the vlans that are through the switch 2 . How and where do I configure pruning in this case?
I know I’m bothering you too much but I’m a blunt head.
Morad Rayyan is IT professional specialized in New Media development and has special interest in networking "routing" as well as programming.
I’m in a situation where I have to configure VTP pruning and I’m faced with a doubt about working of VTP pruning with few Transparent mode swithes in the switching infrastructure. Your article (part-1) somewhat cleared my
doubt.
You mentioned “Alternatively, you can make sure that the transparent switches do not see pruning, by adjusting the pruning-eligible list of directly connected non-transparent switches”.
Can you give me example of this?
Thanks for the nice article.