""Peter van Oene"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > 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. > CL: my quick look here in my own lab indicate the switches don't like it if one side of an access link states one vlan, and the other side states another.
6d09h: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEther net0/22 (7), with Switch_48 FastEthernet0/46 (1). CL: that's beside s the point however. I think what you are asking is how the 802.1d headers are marked? CL: the current standard takes what used to be the bridge id number ( 16 bits ) and separates that into two fields. The bridge i.d. portion is now 4 bits, and bridge i.d.'s are now in multiples of 4096. 0001 = 4096 0010 = 8092 etc CL: the remaining 12 bits is now the vlan i.d. field. So vlan information is carried in tagged and untagged frames alike. 802.1q encapsulation inserts a few more bytes into the frame. > > > >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=58165&t=58099 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
