From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Wed Jan 07 2004 - 20:17:42 GMT-3
Pruning stops the sending of broadcast and unknown unicast frames
out trunk links. Unicast traffic is never a problem, because the CAM table
knows which port to flood it to.
Suppose a frame comes into the switch destined for host X. The
switch looks in the CAM table to see what the outgoing interface is for host
X. If there is an entry, it sends the frame out that interface. If there
is not an entry, it sends the frame out all interfaces in that VLAN (except
that which it was received on), including any trunk interfaces that are
forwarding that VLAN.
Broadcast traffic uses the same method. When a broadcast frame
comes in the switch sends the frame out all interfaces in that VLAN (except
that which it was received on), including any trunk interfaces that are
forwarding that VLAN.
What pruning prevents is the sending of broadcast frames (unknown
unicasts are effectively broadcast) by determining whether the adjacent
switch either has the VLAN in question locally assigned, or is in the
transit path for the VLAN. Suppose you have the following:
SW1---SW2---SW3
SW2 has trunk links to both SW1 and SW3. SW1 has VLANs 10, 20, and
30 assigned, SW2 has VLAN 20 assigned, and SW3 has VLAN 30 assigned.
SW1 receives a broadcast (or unknown unicast) frame in VLAN 10.
Without pruning, this traffic will be forwarded to SW2, and SW2 will forward
it to SW3. With pruning on, SW3 will send a prune message to SW2 for VLAN
10. SW2 will in turn send a prune message to SW1 for VLAN 10. SW1 will no
longer send SW2 broadcast traffic for VLAN 10.
SW1 receives a broadcast (or unknown unicast) frame in VLAN 20.
Without pruning, this traffic will be forwarded to SW2, and SW2 will forward
it to SW3. With pruning on, SW3 will send a prune message to SW2 for VLAN
20. Since SW2 has VLAN 20 assigned, it will report to SW1 that it has it
assigned. SW1 will therefore forward broadcast traffic in VLAN 20 to SW2,
but SW2 will not forward it on to SW3.
SW1 receives a broadcast (or unknown unicast) frame in VLAN 30.
Without pruning, this traffic will be forwarded to SW2, and SW2 will forward
it to SW3. With pruning on, SW3 will send a prune message to SW2 reporting
that VLAN 30 is assigned. Although SW2 does not have VLAN 30 assigned, it
is in the transit path for it. Therefore SW2 cannot send SW1 a prune
message for VLAN 30. SW1 will therefore forward broadcast traffic in VLAN
30 to SW2, and SW2 will forward it on to SW3.
Regarding bugs in pruning, it's always a possibility. New features
should always be put into a test environment before implementing them. If
this is not possible you should at least check the bug toolkit for any bugs
relating to the feature in question.
I *would* however recommend turning pruning on in your switch block.
Pruning can significantly reduce the risk of a broadcast storm on an
isolated segment of the network bringing the entire switch block down.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 708-362-1418 (Outside the US and Canada)
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Kurt Kruegel
> Sent: Wednesday, January 07, 2004 12:38 PM
> To: Rajagopal S; phase90; Nathasha Aleyevka; ccielab@groupstudy.com
> Subject: Re: Pruning!
>
> pruning removes broadcast,unicast and unknown traffic on pruned vlans from
> trunks ....
> simply because it's not there.
> without pruning all trunks carry all traffic at all times
>
> therefore trunks only carry that traffic to switches that particpate in
> certain vlans.
> spanning tree remains ...
>
>
> ----- Original Message -----
> From: "Rajagopal S" <raj_ccie@yahoo.com>
> To: "phase90" <phase90@comcast.net>; "Nathasha Aleyevka"
> <naleyevka@yahoo.com>; <ccielab@groupstudy.com>
> Sent: Wednesday, January 07, 2004 12:02 PM
> Subject: Re: Pruning!
>
>
> > Hello Natasha,
> >
> > Pruning increases available bandwidth on a trunk ??
> >
> > Pruning basically reduces the VTP broadcasts on the trunk.. You cannot
> remove traffic flowing on a trunk by pruning. You need to use access lists
> in this case...
> >
> > use the command : vtp pruning on the global mode.
> >
> > YOu can also selectively prune the broadcasts on the trunk by the
> command
> : switchport trunk allowed vlan x,y,z on the trunk interface.
> >
> > Cheers
> > Raj
> >
> >
> > phase90 <phase90@comcast.net> wrote:
> > Nathasha,
> >
> > I would prune your trunks manually on as needed basis per
> > interface. Unless all your switches in you VTP domain are running images
> > where the pruning code is stable, and proven
> > reliable, you could break your network. How do I know this? Because it
> > happened to me once
> > although it was a while ago on some catalyst 6509s. I used the global
> > command "set vtp pruning enable" and 1 of the switches had a bad image
> in
> > it. This caused the vlan to break - dhcp and other
> > network services. Good Luck.
> >
> >
> >
> > Jerry
> > ----- Original Message -----
> > From: "Nathasha Aleyevka"
> > To:
> > Sent: Monday, January 05, 2004 5:23 PM
> > Subject: Pruning!
> >
> >
> > > Hi,
> > >
> > > I know that Pruning increases available bandwidth on a
> > > trunk link, now if I have to remove traffic from a
> > > VLAN that is not locally assigned to it, would I
> > > enable prunning(global configuration mode) on both
> > > sides of the link and be done with it.
> > >
> > > Is there any additional configuration needed to
> > > accomplish this?
> > > Thank you
> > >
> > >
> > >
> > >
> > >
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