From: jfaure@sztele.com
Date: Mon Jun 30 2003 - 16:09:30 GMT-3
Hi all:
Only a question, apparently very basic about STP but that i don't totally
understand:
Imagine you connect with a 2 crossover cables two group of ports of the
SAME L2 switch, each group bellonging to a different vlan, such as:
Port 1 vlan 1----------(crossover)-----Port 10 vlan 2
Port 2 vlan 1----------(crossover)-----Port 11 vlan 2
All the 4 four ports are in access mode (not trunking) and you have
configured for the two vlans PVSTP (STP per vlan).
Before trying this "stupid connection", i thought: the 4 ports will be in
forwarding state because i don't see how spanning tree can work fine if you
have different vlan in each end of the two links, and you are using PVST,
this is you have a tree for each vlan. Then i tried and i've seen that this
doesn't happen at this manner and that the switch is able to block one of
the 4 ports, disabling then the fisical loop created.
Anyone can explain why the switch works so? Is there any special feature
that permits a switch to recognize itself and avoid loops like this? Am i
missing anything basic about STP?
I did the test with a 6500 Catalyst with Catos 7.4.3.
Regards
Juan Faure Ferrer
email: jfaure@sztele.com
Lmnea de Negocio de Telematica y CC
Ingeniero de Integracisn de Redes y Sistemas
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