Actually, the 5-4-3 rule has everything to do with detecting collisions. It's a limiting factor of distance so that a collision will be detected within the first 64 bytes of a frame's transmission (also known as Ethernet's minimum frame size). It's preferable to detect a collision before the frame leaves the buffer of the transmitting interface - so that retransmission can be accomplished at the data link layer rather than left to upper layers.
Several months ago, Priscilla and I debated the 7 switch rule. If you wanted to search the archives for the entire thread, it was titled "What's the diameter of your switched network? [7:17489]" and was discussed at the end of August, 2001. Here's an excerpt from one of my posts regarding the 7 hop limit: >From other statements I've read (Cisco published material) and from the original excerpt I published, I'd imagine that the placement of the root does matter. "Part of this restriction is coming from the age field BPDU carry: when a BPDU is propagated from the root bridge towards the leaves of the tree, the age field is incremented each time it goes though a bridge. Eventually, when the age field of a BPDU goes beyond max age, it is discarded. Typically, this will occur if the root is too far away from some bridges of the network. This issue will impact convergence of the spanning tree." I'd think that if a bridge were to be the third bridge away from the root, and another switch was the third bridge on the far side of the root, I wouldn't expect to see any problems with MaxAge because I can't see the root being too far from some of the bridges in the network. Now if a bridge were to be the seventh, I could see how that would impose a greater delay and possibly negatively impact the MaxAge parameter. Now my question would be... does this really apply in today's networks or is this more of a limitation of yesteryear's "software-based bridges"? And essentially, that's the conclusion Priscilla and I came to - that the 7 hop radius doesn't really seem to apply to today's switched environments... You might want to check with her again though - Priscilla just authored a new book on troubleshooting campus networks and may updated her thinking. -- Leigh Anne Chisholm (CCNP, CCDP) -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of > Steven A. Ridder > Sent: Saturday, May 18, 2002 5:16 AM > To: [EMAIL PROTECTED] > Subject: Re: STP and 7 hops [7:44408] > > > I believe the 5-4-3 rule is for repeaters, not switches. > > > ""Brian Hill"" wrote in message > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > Steven, > > > > The 7 hop limit is from the root bridge, as STP calculates the tree from > the > > root. Historically, I am not sure why it's 7, but Ethernet has a base hop > > "limit" of 4 switches (5-4-3 rule), so it doesn't really matter so much. > The > > reason for the 4 hop limit in Ethernet is simple: For 10 Mb or full duplex > > 100 Mb connections, the limit is mostly to reduce noise from the > > amplification of the signal as it passes through the switches/hubs, where > as > > in 100Mb half-duplex connections, it is mostly to keep the propogation > delay > > within specs. > > > > Hope this helps, > > > > Brian Hill > > CCNP, CCDP, MCSE 2000 (Charter Member),MCSE+I (NT4.0), > > MCSA (Charter Member), MCP+I, MCP(21), Inet+, Net+, A+ > > Lead Technology Architect, TechTrain > > Author: Cisco, The Complete Reference > > http://www.alfageek.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=44446&t=44408 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
