From: Germany (ccie.gergonza@gmail.com)
Date: Mon Feb 11 2008 - 23:56:25 ARST
Hi... When there's a spanning tree loop, one of the sides must block and the
other one is going to be the designated port.
The designated port is chosen based on the lowest Root Path Cost of the 2
switches.
In your scenario, DS2 has a RPC of 4 and SW1 has a RPC of 3.
So in this case, SW1's port to DS2 becomes the DP for the segment. DS2
already has a root port, and any port that's neither Root Port nor
Designated Port goes into the blocking state.
I just read that from BCMSN.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Geert Nijs
Sent: Monday, February 11, 2008 7:36 AM
To: thomas.rader@freesurf.ch; ccielab@groupstudy.com
Subject: RE: STP cost
Spanning tree always only blocks one side of a connection. The interface
stays blocked by the receiving of BPDU's from the other side.
If i remember correctly (it has been a while, up for recertification shortly
:-), STP takes the cost from the received BPDU packet, add the cost of the
RECEIVING interface to the value and advertises this back out all other
interfaces (cost defined on those interfaces does not matter)
regards,
Geert
CCIE #13729
________________________________________
From: nobody@groupstudy.com [nobody@groupstudy.com] On Behalf Of
thomas.rader@freesurf.ch [thomas.rader@freesurf.ch]
Sent: 08 February 2008 17:13
To: ccielab@groupstudy.com
Subject: STP cost
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
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