How many LAN switches such as 3550 can be cascaded?

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Nov 10th, 2009
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I have read 7 hop BPDU limitation on a article? How many 3550 or 2960 can be cascaded without affecting STP?


What is the benefit to have distribution switch for STP management?


Thnaks


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Giuseppe Larosa Tue, 11/10/2009 - 10:46
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Hello Kim,

yes default settings of STP timers takes in account an STP diameter of 7 switch hops.


default settings are:

hello frequency : 2 seconds

time to move from listening to learning = 15s = time to move from learning to forwarding.


this 15 seconds are:

2*7 + 1


STP wants to prevent bridging loops so classic STP uses timed transitions to move a port when information has been propagated every where.


for example you can build a ring of 7 switches but if one link fails it becomes a chain of 7 switches.


use of hierarchy with distribution is a great help for scalabiltiy of networks and also for STP.

with distribution each access layer switch has two uplinks one to distrib1 and one to distrib2.

only care is that one of the distrib nodes has to be elected root bridge.

and the other secondary root bridge.


This can be on a per vlan basis with Cisco Per vlan STP PVST+.



Hope to help

Giuseppe


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