Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring MSTP (Rev. 03)
Overview
© 2008 Foundry Networks, Inc
Page 19 of 70
1.
The difference between Cisco-compliant mode and IEEE 802.1s-compliant mode concerns the
conditions under which the Agreement flag is set in a BPDU being sent:
•
In Cisco-compliant mode, MSTP sends a BPDU with the Agreement flag set when
o
the port is a Root port
AND
o
the port is synced, i.e. when the port is guaranteed to participate in a loop-free
topology.
•
In IEEE 802.1s-compliant mode, the Agreement flag is set when
o
the port is EITHER a Designated or a Root port
AND
o
when all ports of the device are synced, i.e. when ALL PORTS are guaranteed to
participate only in loop-free topologies.
2.
In both Cisco-compliant and IEEE 802.1s-compliant modes, an Agreement is sent in response
to a Proposal when the port is going to Root role. In Cisco-compliant mode, however, an
Agreement is sent also when the port is going to Alternate role. This is done to prevent a
known misbehavior issue in Cisco devices.
NOTE
The problem has been reported on Cisco Catalyst 2950 versions 12.1(13) and
12.1(20). If a designated port on a Cisco device is connected to an Alternate port, it
recognizes this port as Boundary only if it receives a Root Agreement on it in
response to the Proposal it has made. Consequently, if the Alternate port becomes
Root, the Cisco device does not recognize it as belonging to another region,
although it has received a Root Agreement packet. In this case, if a topology change
occurs within the region, a 30-second delay of traffic will result because the
Designated port of the Cisco device will be blocked (sending proposals) for the
affected instance expecting the other device to send an Agreement for this instance.
Note that the instance in which the topology change has occurred may even not
exist on the other device (it is in another instance). So, for this instance and the
VLANs mapped to it, the port will make a slow transition, synchronizing the state
of the port for this instance with the state for instance 0 (CIST), valid for every
region, which will cause the aforementioned 30-second traffic delay.
3.
If the corresponding port in the connected Cisco device is reset after the Agreement has been
received, the misbehavior mentioned in item 2 will occur.
Benefits
•
MSTP enables load balancing, over a large number of VLANs.
•
MSTP reduces the number of spanning-tree instances required to support a large number of
VLANs by using VLAN grouping.
•
MSTP provides rapid convergence, which can reduce link convergence time to less than two
seconds.
•
MSTP continues operating without loops in any physical connection topology including shared
spanning tree switches, 802.1Q mono spanning tree switches, and others.