From: thomas.rader@freesurf.ch
Date: Fri Feb 08 2008 - 14:13:37 ARST
hi,
I thought I understood the STP cost concept until I labed a simple scenario and couldn't work out why ports were being blocked.
Here is what I did:
My scenario has an access switch (SW1) connected to two distribution switches (DS1 & DS2) via 2xgig etherchannel (stp cost 3)
The distributibution switches (DS1 & DS2) are also connected via gig trunk (stp cost 4).
DS1 is STP root.
I used the "spanning-tree cost" under ther interface or in the case of etherchannel on the port-channel.
If I change the STP cost on SW1 etherchannel connected to DS2 to 1000, I would expect that STP blocks SW1 port to DS2.
However in fact it blocks DS2 connected to SW1. Why ?
Here is how I see it:
SW1 to DS1 = cost 3
SW1 to DS1 (via DS2) = cost 1004 (1000 + 4 crossing the DS2-DS1 link)
DS2 to DS1 = cost 4
DS2 to DS1 (via SW1) = cost 6 (3 + 3)
This assumes that only the outgoing cost per hop is added to the cost. Is this correct ?
I have seen some confilicting information regarding per VLAN ("spanning-tree vlan 1-1024 cost"). The way I see it you can simply control this better per per VLAN. Right ?
Also why does STP only block one side of the connection ? I would have expected both sides of a connection to be blocked ?
thanks, thomas
This archive was generated by hypermail 2.1.4 : Sat Mar 01 2008 - 16:54:48 ARST