At 03:53 PM 2/27/02, Daniel Cotts wrote: >When you have an opportunity to read Clark and Hamilton the explanation will >fit. A blocked port is not physically cut. Data traffic is blocked while >BPDUs are allowed. Else, how would it know that a change in topology has >taken place?
Topology Change Notification BPDUs travel in the opposite direction from Configuration BPDUs, however. >Step 2. Yes, all along bridge 4 has been receiving CBPDUs on both ports. No, it hasn't. B3's e1 is blocked. (It's the one that connects to B4). B3's e1 is receiving but not sending Configuration BPDUs. According to Clark and Hamilton anyway. I'll check IEEE 802.1D too, but I've never found C and H to be wrong. Priscilla >Since e1 had the lower root path cost, it became the root port. There is no >need to forward the CBPDUs it receives on e0 out e1 because it has already >determined that a lower cost path exists out e1. Once the e1 link fails >bridge 4 already knows that it has another path to the root bridge via >bridge 3. >Step 5. Logically follows. It takes the CBPDUs that it has always been >recieving on e0 and now forwards it out e1. It has no clue why it no longer >receives CBPDUs on e1. >Now think about what happens when bridge 5 e1 becomes functional again. > > > -----Original Message----- > > From: Pierre-Alex Guanel [mailto:[EMAIL PROTECTED]] > > Sent: Wednesday, February 27, 2002 1:46 PM > > To: [EMAIL PROTECTED] > > Subject: Re: Spanning tree Protocol Questions [7:36559] > > > > > > Priscilla, > > > > In the attachment (Cisco CCNA Exam Guide #640-507 > > Certification Guide: ISBN > > 0-7357-0971-8), page 167 - 168, the authors seem to indicate > > that CBPDUs are > > sent from blocked ports! The ability for CBPDUs to be sent > > out of blocked > > port seems to a determinant factor for the Spanning Tree to > > be recomputed > > after a topology change .... > > > > > > Here is a word by word copy of the passage. The part I have > > problem about is > > the one with (!!!!!!) > > > > Thanks! > > > > Pierre-Alex > > > > --(e0)Bridge-1 (e1)-- (e0)Bridge 5(e1)---(e1) Bridge 4 > > (e0)---(e1)Bridge 3 > > (e0)---(e1)Bridge 2 (e0)---> (To bridge 1 e0) > > > > Cost advertised by B2 is 100 > > Cost advertised by B3 is 200 > > Cost advertised by B5 is 10 > > Blocked port: Bridge 3, port e1 > > > > Bridge 5's E1 port fails...Only Bridge 4's MaxAge expires. > > The other bridges > > are still receiving CBPDUs on their root ports. After MaxAge > > expires, Bridge > > 4 will decide the following: > > > > Step 1 My E1 port is no longer my root port > > Step 2 The same root bridge is being advertised in a CBPDU on > > my E0 port > > (!!!!!) > > Step 3 No other CBPDUs are being received > > Step 4 My best path (and the only path, in this case) to the > > root is out of > > my E0 port; therefore, my root port is now E0.) > > Step 5 Because no other CBPDUs are entering my E1 port, I must be the > > designated bridge on that segment. So, I will start sending > > CBPDUs on E1, > > addming my E0 port cost (10) to the cost of the CBPDU > > received in the CBPDU > > entering E0 (200) for a total of 210 (!!!!!!) > > > > >>>>> > > > > > > Kind Regards, > > > > Pierre-Alex ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36730&t=36559 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
