At 06:21 AM 5/18/02, Brian Hill wrote: >Steven, > >The 7 hop limit is from the root bridge
No. It's from edge to edge. The root is at the top of a hierarchy of switches. Think of the president's spot in an org chart, for example. The 7 hops is from one worker at the bottom of one branch talking to another worker at the bottom of another branch, with data travelling across the entire hierarchy, up to the top and then back down. IEEE says that 7 is recommended. They don't explain it very well though. It comes from the olden days when DEC invented STP. According to one of my mentors Marty Adkins (CCIE low number), DEC engineers said it allowed for conservative BPDU propagation, but also for serialization delay and WAN latency. Their spec allowed one (or two?) bridge hops to have a slow serial link, as low as 56 Kbps. With more than seven bridge hops, a frame might take longer than one second to travel end-to-end. That would cause DEC's LAT protocol (which can only be bridged) to time out and retransmit. If the max retry count was exceeded, the session was dropped. >, 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), The 5-4-3 rule has nothing to do with switches. It's for repeaters. It restricts the max size of a collision domain. >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, The rule doesn't have to do with amplification and switches don't come into this discussion at all. > where as >in 100Mb half-duplex connections, it is mostly to keep the propogation delay >within specs. Yes. The rule keeps propagation delay within specs. Specifically, the 5-4-3 rule exists to make sure that a sending station is still sending when a collision reflects back from the max size of network when the sender is sending a minimum-size frame (64 bytes). >Hope this helps, > >Brian Hill >CCNP, CCDP, Were at the pool drinking beers with that other guy when you wrote your reply? ;-) Priscilla >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 Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=44451&t=44408 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
