From: ccie ccie (cciefun@gmail.com)
Date: Thu Nov 29 2007 - 03:35:53 ART
Hi
Real scnario need explanation :
CISCO TAC Replay :-->
6509-------Po256-------6509
Core1 Core2
\ #/
\ /# <<---- Alt Blocking / Root Inconsistent if root
guard enable.
Po1 Po2
\ /
\---3560---/
c3560-f1.oc2
- Data vlans using MST instance 1.
- Core1 switch is root(Primary RB). Core2 switch is backup root (secondary
RB). This is manually configured.
- Po256 has 2 Gig links
- Po1 and Po2 each have 4 Gig links
The Core switches are configured with spanning tree root guard on the
Portchannel interfaces going to the connected 3560 switches. On Core2 you
saw the Po2 interface going to "root inconsistent" state. Other interfaces
on Core2 (connecting to different 3560 switches) are not seeing this.
Having root guard configured on an interface means that we expect the port
to be designated forwarding. If we receive a better BPDU then the interface
becomes inconsistent.
In the normal 3560 setup you have just one uplink interface to each Core
switch. This means the 3560 can reach the root with a cost of 20000 (1 Gig
intf). The Core2 switch has a cost to the root of 10000 (Po256: 2 Gig intf).
As Core2 has a better path to the root than the 3560, we block on the 3560
side and the Core2 side becomes designated forwarding.
This does not violate the root guard config.
The c3560-f1.oc2 3560 switch however has 4 links in its uplink Portchannels
to the core switches. This means it can reach the root
(Core1) with a cost of 5000. This is better than the path Core2 has to the
root (10000) so the Portchannel 2 interface on 3560 will be forwarding and
blocking on Core2. This explain the root guard violation.
Need better explanation on above STP stuff?
Q. How the configuration BPDU flow in STP?
Q. Which debug command help me to see details of BPDU (sender BID, RBD,
max age ) or only tcpdump / sniffer help me on this?
Q. How the *cumulative root path cost *will calculate in above topology?
Q Why I can see better BPDU on core2 po2?
Regards
Mike
This archive was generated by hypermail 2.1.4 : Sat Dec 01 2007 - 06:37:31 ART