Cisco Catalyst 4500 Series Software Configuration Manual page 743

Chapter 23 Configuring STP and MST
It may result in loss of connectivity. For example, in
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port it elected as a root port. As a result of this situation, there is loss of connectivity (r1 and r2 are
designated, a1 is root and a2 is alternate. There is only a one way connectivity between A and R).
Figure 23-4
It may cause permanent bridging loops on shared segments. For example, in
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that bridge R has the best priority, and that port b1 cannot receive any traffic from the shared
segment 1 and sends inferior designated information on segment 1. Both r1 and a1 can detect this
inconsistency. However, with the current dispute mechanism, only r1 will revert to discarding while
the root port a1 opens a permanent loop. However, this problem does not occur in Layer 2 switched
networks that are connected by point-to-point links.
Figure 23-5
R
This example shows the spanning tree status when port Gi3/14 has been configured to disable PVST+
simulation and the port is currently in the peer type inconsistent state:
Switch# show spanning-tree
VLAN0010
Spanning tree enabled protocol rstp
Root ID
Bridge ID
Interface
---------------- ---- --- --------- -------- -------------------------
Gi3/14
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Loss of Connectivity
r1
R
r1
Bridging Loops on Shared Segments
r1 a1
a3
R
Segment 3
r1 a2
Priority 32778
Address 0002.172c.f400
This bridge is the root
Hello Time 2 sec
Priority 32778
Address 0002.172c.f400
Hello Time 2 sec
Aging Time 300
Role Sts Cost
Desg BKN 4
Figure
Segment 1
a1
A
a2
Segment 2
Segment 1
b1
B
b2
Segment 2
Max Age 20 sec Forward Delay 15 sec
(priority 32768 sys-id-ext 10)
Max Age 20 sec Forward Delay 15 sec
Prio.Nbr Type
128.270 P2p Dispute
About Detecting Unidirectional Link Failure
23-4, Bridge A cannot transmit on the
Alternate port
Unidirectional link
failure
Root port
Figure 23-5, suppose
Alternate port
Unidirectional link
failure
Root port
23-41