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8473 Views 13 Replies Latest reply: Sep 28, 2009 2:45 PM by Bourbon, CCNP, CCIE Candidate RSS

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VLAN Pruning

Sep 27, 2009 6:06 AM

Hello CLN

 

I have a quick question.  What is the difference between VTP pruning and the switchport trunk allow vlan command?  Don't they do eactly the same thing when it comes to a functional standpoint?

 

I did some research on the internet, but different opinions are given on this.  Can someone clear this up?

 

Thanks,

 

Erick

  • xuhu 3 posts since
    Sep 25, 2009
    Currently Being Moderated
    1. Sep 27, 2009 6:23 AM (in response to Erick)
    Re: VLAN Pruning

    vtp pruning is a dynamic mechanism ,when the downsream routers do not need the vlan traffic ,it will send pruning message .

     

    vtp trunk allow is a static mechanism ,it allows the specific vlans pass through the trunk .

     

    The two have the same result ,that is reduce the unnecessary traffic through the trunk .

     

    all  the above is  my personal opinion ,thanks ~~

  • Kristy 118 posts since
    Dec 3, 2008
    Currently Being Moderated
    2. Sep 27, 2009 9:07 AM (in response to Erick)
    Re: VLAN Pruning

    I concur with the previous reply.  VTP will auto-magically prune your trunks.  Among other wonderous functions it performs. 

     

    Manually pruning a switchport, SVI, or channel group with the 'switchp trunk allowed vlan' command, is the manual way.

     

    It is always good to prune your trucks.  So I think the manual way was provided for those who don't really need to run VTP.

  • Freecell 11 posts since
    Sep 25, 2009
    Currently Being Moderated
    4. Sep 27, 2009 6:55 PM (in response to Erick)
    Re: VLAN Pruning

    They do the same, but VTP pruning is the easy way to k1ll your network. If you have VTP servers everywhere (mode that vlan pruning requires the switches to be in), then you could have a complete disaster in your network. Here's why: let's say a company hired 20 more people and does not have any more ports on their switch. Someone could ask a co-worker if there was another switch, and the co-worker could reply by saying, there's one in the lab. We could hook that one up until we get a new switch. If that switch has a VTP revision higher than the ones on the production network, and does not have the same vlan numbers, guess what's going to happen? All the pre-existing vlans in the production network are going to be erased and the switches are going to be in a black hole and unable to communicate with anyone. You see what I mean? If you have only one VTP server and the rest clients, then you'll have no trouble. Hence, even though it's a hastle to type all the vlans in with the command vtp trunk allow, you'll greatly save your sanity from having to recreate all the vlans again if your network gets destroyed. Hope this helps.

     

    Freecell

  • xuhu 3 posts since
    Sep 25, 2009
    Currently Being Moderated
    5. Sep 27, 2009 7:03 PM (in response to Freecell)
    Re: VLAN Pruning

    your instance is a good one .the vtp information will only be  updated by a large configuration number ,it is not be  independent of trunk or pruning .

     

    if only you use trunk ,the vlan information can be updated ,but the vtp pruning or  vlan trunk allow command is a strategy optimization ,they two have no realtionship .

     

    it is my personal opinion ,hope is helpful for you ~~

  • Dave Owens 15 posts since
    Sep 21, 2009
    Currently Being Moderated
    6. Sep 27, 2009 7:23 PM (in response to Freecell)
    Re: VLAN Pruning

    The condition Freecell described could be avoided without the overhead of explicitly allowing vlans on every trunk.

         A.  Reset your VTP rev number on the lab switch by: changing the domain name to a temp then back again -OR- cycle VTP transparent, server.

         B.  Enforce a unique VTP domain name/password in your lab.

         C.  If you're really worried, both!

     

    Dave

  • Freecell 11 posts since
    Sep 25, 2009
    Currently Being Moderated
    7. Sep 27, 2009 7:24 PM (in response to xuhu)
    Re: VLAN Pruning

    xuhu,

    I'm not sure what you mean by no relationship. I do agree with Dave, but in a perfect world, no one would make mistakes, but we're all human. Things can happen. An admin could use the same VTP pw in the lab because he was at a crazy party and came to work with a hangover the next day. I'm just giving my opinion on precaution and optimization.

     

    Freecell

  • gio 27 posts since
    Jun 27, 2008
    Currently Being Moderated
    8. Sep 28, 2009 1:39 AM (in response to Freecell)
    Re: VLAN Pruning

    Not necessarily you need all your SWs to be in Server mode,if they are in the same domain a single pruning configuration on one server will be distributed to the other switches by VTP.

  • Freecell 11 posts since
    Sep 25, 2009
    Currently Being Moderated
    9. Sep 28, 2009 9:36 AM (in response to gio)
    Re: VLAN Pruning

    Well, my understanding is that vtp prunning works only on server mode.

  • Keith Bogart 42 posts since
    Apr 18, 2008
    Currently Being Moderated
    10. Sep 28, 2009 9:51 AM (in response to Freecell)
    Re: VLAN Pruning

    Hi everyone,

     

    If you wish to enable VTP pruning with the global command, "vtp pruning" your switch needs to be in VTP Server mode.  However the other switches in your network could be in VTP Client mode and they will also have VTP pruning dynamically enabled when the VTP Server tells them to do so.

     

    Hope that helps!

    Keith

  • Kristy 118 posts since
    Dec 3, 2008
    Currently Being Moderated
    11. Sep 28, 2009 11:41 AM (in response to Keith Bogart)
    Re: VLAN Pruning

    Freecell - Don't forget that a switch running in VTP client mode can bring devestation to a network as well, it does not have to be in server mode to overwrite the database.  If it has the same domain name and a higher configuration revision number the database will be overwritten.

  • Freecell 11 posts since
    Sep 25, 2009
    Currently Being Moderated
    12. Sep 28, 2009 1:12 PM (in response to Kristy)
    Re: VLAN Pruning

    Kristy,

     

    Yep, didn't forget about that. I thought that when using vlan pruning, you had to have servers everywhere (per cbtnuggets) which isn't true. http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/rel_4_2/config/vlans.htm#xtocid79807

     

    Freecell

  • Bourbon, CCNP, CCIE Candidate 148 posts since
    Dec 11, 2008
    Currently Being Moderated
    13. Sep 28, 2009 2:45 PM (in response to Erick)
    Re: VLAN Pruning

    I'm studying this for CCNP myself, and I could be wrong, but I think the big difference in these that everyone is missing is that:

     

    1.)     With VTP pruning, the broadcasts are blocked from reaching the downstream client switches.  However, the VLAN is still available to be used, at which point the client switches would be "un-pruned" automatically.

     

    2.)     With VLAN "dis"allowed from the trunk, the downstream client switches would NOT have the vlan available, or if they did get it through VTP, if that VLAN were still disallowed from the trunk, it would be degraded - not being able to reach back to the core of the network (assuming that's where the servers and Internet connection) would be.  The VLAN would have to be manually added back to the trunk.

     

    So the differences are really the difference between automation and manual granular control.

     

    Hope this helps,

     

    Bourbon

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