There's nothing in the STP frames to enforce a 7 hop diameter. But there is the Message Age field in the BPDUs. Each bridge (switch) adds one to the Message Age when the switch propagates the BPDU downstream.
The Maximum Age threshold is 20. If a BPDU gets to a switch with the Message Age already at 20, it will think that the tree needs reconverging. This would get ugly if switches on the edges were always trying to reconverge. So, the max size from that viewpoint is 20 from the root. But 7? I really think DEC threw that in as a precaution. It's interesting that IBM was saying the same thing about source route bridging at the time (max bridges is 7). (But try finding 7 in IEEE 802.1D Annex C, the official standardization of source-route transparent bridging. The RIF can actually hold info for 14 rings and 13 bridges.) Back to the real subject at hand, the 7 max for STP is mentioned as a recommended value in Table 8.2 Maximum Bridge Diameter of IEEE 802.1D and is defined as "The maximum number of Bridges between any two points of attachment of end stations." Then it's discussed again in Appendix B "B.3.1.2 Basis of choice." This section is pretty incomprehensible, but, as far as I can tell, the main reason for the choice of parameters is to minimize the lifetime of a data (user) frame travelling across the switched network. Regarding gigastack, it sounds like the answer that Steven got from Cisco is that each switch counts as a hop, so if STP is enabled, each counts toward the _theorectical_ 7 hop count limit. But I bet you're right also that STP could be disabled with gigastack. It sounds like the topology is already a single linear branch (stack) with no loops. There's no need to prune it into a tree. But I'm way out on a limb now. ;-) Priscilla At 06:34 PM 5/19/02, Chuck wrote: >you know, it suddenly occurs to me that I have been barking up the wrong >[spanning] tree, so to speak. > >Let me guess - there is no maximum STP diameter in actuality because there >is mechanism for enforcing a max diameter. The BPDU's apparently contain a >field which shows distance from the root, and this value is incremented each >time it crosses a bridge. If that field is the root path cost field, then >this is a four byte value and that means a spanning tree could theoretically >have a max distance from the root of 64000 or so? > >It's just that the recommendation in terms of "best practice" is diameter of >7. > >thanks to Marty A. for providing the link that was the spike that finally >began to sink through this thick head. > > >""Chuck"" wrote in message >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > STP is really not an issue in the kind of application where gigastack >makes > > sense. For example, take an office of 400 users plus servers and printers, > > occupying a contiguous space. Basic file and print sharing plus an >internet > > connection. Rather than buy a honking 65xx, you throw in a few 3550-48's >and > > gigastack them. The electronics work in conjunction with the switch OS to > > create a half duplex bus between the switches. ( The interesting thing is > > that electronics are apparently smart enough to determine if there are >only > > two devices stacked, in which case the bus is full duplex. ) > > > > That's the question about gigastack - whether the entire stack is treated >as > > one switch, the way it is for management purposes, or if standard STP > > applies. We had a thread on this a few weeks ago, but none of us could >find > > an answer in the Cisco documentation. that's why I asked Steven ( who >asked > > Cisco ) what Cisco had to say about spanning tree over a gigastack setup. > > I'm willing to bet that in a gigastack situation, that STP is disabled ( > > where it would be enabled if you stacked the switches using the 10/100 > > ports, or using regular gbics and daisy chaining the gig ports. ) But I > > can't find documentation that clarifies. > > > > This does give me some food for thought. I'm working with a very large > > university on a switch design. I am pretty sure that they have several > > instances where their diameter is greater than seven ( building to >building > > to building to core ). However, being the sophisticates they are, I am > > certain they are running spanning tree per vlan, and in that case the tree > > would go back only to the core, and so would have a diameter no greater >than > > 4 in any place I can see from their setup. > > > > this seems to be another one of the great discussions that make this group > > valuable. > > > > > > > > ""Priscilla Oppenheimer"" wrote in message > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > At 03:51 PM 5/19/02, Chuck wrote: > > > >were the people you asked able to address the max STP diameter of 7 >issue > > > >then? > > > > > > Maybe they blew off the max STP diameter of 7 issue. ;-) It is just a > > > recommendation. Even IEEE 802.1D just uses that word (recommendation). > > > There's nothing in the protocol that would stop you from having a larger > > > network, is there? The Max Age timer defaults to 20 seconds, so that > > > doesn't limit you to 7 hops. > > > > > > Also, with gigaswitch, do all those 9 switches remain in the spanning > > tree? > > > I don't know anything about gigaswitch, as you can probably tell, but I > > > would think the 7 limit would only apply to the logical topology, if it > > > applies at all. > > > > > > Priscilla > > > > > > > > > > > > >""Steven A. Ridder"" wrote in message > > > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > > > I asked Cisco, and STP treats the stack as separate switches. > > > > > > > > > > > > > > > ""Chuck"" wrote in message > > > > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > > > > in your travels have you ever run into the gigastack, and its > > relation > > > >to > > > > > > STP? > > > > > > > > > > > > the question came up a short time ago. The Cisco gigastack > > > documentation > > > > > > does not cover STP - only how gigastack works. > > > > > > > > > > > > essentially, using daisy chained gigastack connections, one can > > connect > > > >up > > > > > > to 9 switches. If I understand correctly, the Cisco electronics >and > > > >switch > > > > > > OS consider this stack a single entity for management purposes - > > i.e. > > > >can > > > > > be > > > > > > managed from a single IP address. However, Cisco does not say one > > way > > > or > > > > > > another if the electronics and the switch OS treat this stack as a > > > >single > > > > > > device for STP purposes. > > > > > > > > > > > > I'm guessing, based on this and other discussions that it would >have > > to > > > >be > > > > > a > > > > > > single device for STP purposes. It just would be nice if Cisco >would > > > > > provide > > > > > > the specific information. > > > > > > > > > > > > That brings up the corollary question - if you have on two >switches > > > > > > gigastacked, then how does STP come into play. In a configuration > > such > > > >as > > > > > > this, again if I understand the documentation correctly, the > > > electronics > > > > > and > > > > > > the switch OS behave differently. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > ""Leigh Anne Chisholm"" wrote in message > > > > > > [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > > > > > 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 > > > ________________________ > > > > > > Priscilla Oppenheimer > > > http://www.priscilla.com ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=44544&t=44408 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
