I should have thought out my previous reply a little more thoroughly. It is not correct. Switch 4 e0 in the first instance is the Designated Port for its link with switch 3. Switch 3 e1 is the Non Designated Port and is blocking. Switch 3 is listening for BPDUs from switch 4. Switch 4 is sending BPDUs to switch 3.
Now when switch 5 e1 fails - switch 4 stops receiving BPDUs and thus doesn't pass any on to switch 3. After Max Age (20 sec) switch 3 discards the BPDU information that it has learned from switch 4 and goes into listening state. Eventually it transitions into forwarding mode and sends BPDUs to switch 4. That's how switch 4 learns that it has another path to the root bridge. So the explanation below doesn't describe how step one leads to step two. > -----Original Message----- > From: Pierre-Alex Guanel [mailto:[EMAIL PROTECTED]] > Sent: Wednesday, February 27, 2002 4:10 PM > To: Daniel Cotts; [EMAIL PROTECTED] > Subject: RE: Spanning tree Protocol Questions [7:36559] > > > So a blocked port can receive AND send BPDUs .... Correct? > > Pierre-Alex > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of > Daniel Cotts > Sent: Wednesday, February 27, 2002 2:54 PM > To: [EMAIL PROTECTED] > Subject: RE: Spanning tree Protocol Questions [7:36559] > > > 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? > Step 2. Yes, all along bridge 4 has been receiving CBPDUs on > both ports. > 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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36731&t=36559 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
