3-10
Multiple Instance Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
MSTP Operation with 802.1Q VLANs
As indicated in the preceding sections, within a given MST instance, a single
spanning tree is configured for all VLANs included in that instance. This means
that if redundant physical links exist in separate VLANs within the same
instance, MSTP blocks all but one of those links. However, you can prevent
the bandwidth loss caused by blocked redundant links for different VLANs in
an instance by using a port trunk. The following example shows how you can
use a port trunk with 802.1Q (tagged) VLANs and MSTP without unnecessarily
blocking any links or losing any bandwidth.
Figure 3-4. Example of Using a Trunked Link To Support Multiple VLAN
Connectivity within the Same MST Instance
Note All switches in a region should be configured with the VLANs used in that
region, and all ports linking MSTP switches together should be members of
all VLANs in the region. Otherwise, the path to the root for a given VLAN will
be broken if MSTP selects a spanning tree through a link that does not include
that VLAN.
Problem:
An MST instance with two
separate (non-trunked)
links blocks a VLAN link.
Solution:
Configure one trunked
link for the two VLAN
memberships.
Nodes 1 and 2 cannot
communicate because
MSTP is blocking the link.
Nodes 1 and 2 can communicate because the
MST instance sees the trunk as a single link and
802.1Q (tagged) VLANs enable the use of one
(trunked) link for both VLANs.
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