On Tue, 2002-11-26 at 23:16, Larry Letterman wrote: > Hi Peter, > > In a simpler analogy..... > vlan 7 would be a seperate switch with a connection to the vlan 1 switch... > vlan 8 would be a seperate switch with a connection to the vlan 1 switch.. > > no loop exist between any part of the 3 networks....vlan 7 and 8 are > isolated > from each other. So stp sees no loops between vlan 1 and either of the > other > vlans. if the switches are capable, do a show spantree summ, and see what it > replies with....
Thanks. I'm pretty cool with the concept of vlans. My brain immediately went to the more intesting question of non native VLAN PVST in this case as should have been more evident from my questions. Assuming switch A was entirely VLAN 7 and switch B entirely 8 for example. This would be more interesting. Untagged traffic would flow freely, yet STP likely wouldn't catch loops were it encoded with VLAN IDs. > > Peter van Oene wrote: > > >At 10:41 PM 11/26/2002 +0000, Larry Letterman wrote: > > > >>switch A and B wont talk to each other or cause a loop > >>because you have switch B isolated. STP in your case is > >>set for 3 instances : STP for Vlan 1, Vlan 7 and Vlan 8. > >>A loop would be present if switch B were set for Vlan 7 > >>on both links and STP did not block one of the ports. > >> > > > >I'm curious here. Given Switch A and B don't emit tagged frames, traffic > >should flow freely despite A and B's disagreement on VLAN ID. I am not > >very familiar with Per VLAN STP encoding however. Are the BPDU's modified > >to carry a VLAN identifier? This would seem superfluous to me and I'd > >wonder where it would be needed. My take on 802.1q PVST+ is that only the > >common STP BDPUs are sent untagged and all other BPDUs are sent tagged with > >their appropriate VLAN making them easy to disambiguate. > > > > > > > >>pauldongso wrote: > >> > >>>Hi All, > >>> > >>>Please advise how STP participates in the following scenario and why STP > >>>fails to stop the loop? > >>> -------------------- > >>> |switch A | > >>> --------------------- > >>> |(vlan 7) | (vlan 8) > >>> | | > >>> | | > >>> |(vlan 1) |(vlan 1) > >>> ------------------- > >>> | switch B | > >>> -------------------- > >>> | | | > >>> vlan 1 hosts > >>> > >>> > >>>In short, switch A has two ports configured with vlan 7, vlan 8 > >>>respectively. Swtich B all ports are at default vlan 1. > >>>links between swA and swB are access mode. > >>> > >>>This scenario creates bridging loop. But just can't figure out why STP > >>>fails to stop loop. > >>> > >>>Thanks in advance. > >>> > >>>Paul Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=58170&t=58099 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
