Table of Contents
Advertisement
Configuring Spanning Tree Parameters
Note. Disabling BPDU switching on a Spanning Tree disabled VLAN must not cause network loops to go
undetected.
Configuring the Path Cost Mode
The path cost mode controls whether the switch uses a 16-bit port path cost (PPC) or a 32-bit PPC. When
a 32-bit PPC switch connects to a 16-bit PPC switch, the 32-bit switch has a higher PPC value that
advertises an inferior path cost to the 16-bit switch. In this case, it is desirable to set the 32-bit switch to
use STP or RSTP with a 16-bit PPC value.
The path cost mode is automatically set to use a 16-bit value for all ports that are associated with an STP
instance or an RSTP instance and a 32-bit value for all ports associated with an MSTP value. It is also
possible to set the path cost mode to always use a 32-bit regardless of which protocol is active.
To change the path cost mode, use the
PPC value based on protocol) or 32bit (always use a 32-bit PPC value). For example, the following
command changes the default path cost mode from auto to 32-bit:
-> spantree path-cost-mode 32bit
Note. Cisco supports two default path cost modes: long or short just like in OmniSwitch per vlan
implementation. If you have configured PVST+ mode in the OmniSwitch, it is recommended that the
same default path cost mode must be configured in the same way in all the switches, so that, the path costs
for similar interface types are consistent when connecting ports between OmniSwitch and Cisco Switches.
Using Automatic VLAN Containment
In a Multiple Spanning Tree (MST) configuration, it is possible for a port that belongs to a VLAN that is
not a member of an instance to become the root port for that instance. This can cause a topology change
that could lead to a loss of connectivity between VLANs/switches. Enabling Automatic VLAN
Containment (AVC) helps to prevent this from happening by making such a port an undesirable choice for
the root.
When AVC is enabled, it identifies undesirable ports and automatically configures them with an infinite
path cost value. For example, in the following diagram a link exists between VLAN 2 on two different
switches. The ports that provide this link belong to default VLAN 1 but are tagged with VLAN 2. In
addition, VLAN 2 is mapped to MSTI 1 on both switches.
MSTI-1
In the above diagram, port 4/2 is the Root port and port 5/1 is a Designated port for MSTI 1. AVC is not
enabled. If another link with the same speed and lower port numbers is added to default VLAN 1 on both
OmniSwitch AOS Release 7 Network Configuration Guide
spantree path-cost-mode
VLAN 1
4/2
802.1q tag
VLAN 2
Configuring STP Bridge Parameters
command and specify either auto (uses
VLAN 1
5/1
VLAN 2
March 2011
MSTI-1
page 5-31
Advertisement
Table of Contents
