Restrictions for configuring MSTP
These restrictions apply when using MSTP:
• MSTP must only be enabled on interfaces where the interface itself (if it is in L2 mode) or all of the
subinterfaces have a simple encapsulation configured. These encapsulation matching criteria are considered
simple:
• Single-tagged 802.1Q frames
• Double-tagged Q-in-Q frames (only the outermost tag is examined)
• 802.1ad frames (if MSTP is operating in Provider Bridge mode)
• Ranges or lists of tags (any of the above)
• If an L2 interface or subinterface is configured with an encapsulation that matches multiple VLANs,
then all of those VLANs must be mapped to the same spanning tree instance. There is therefore a single
spanning tree instance associated with each L2 interface or subinterface.
• All the interfaces or subinterfaces in a given bridge domain must be associated with the same spanning
tree instance.
• Multiple subinterfaces on the same interface must not be associated with the same spanning tree instance,
unless those subinterfaces are in the same split horizon group. In other words, hair-pinning is not possible.
• Across the network, L2 interfaces or subinterfaces must be configured on all redundant paths for all the
VLANs mapped to each spanning tree instance. This is to avoid inadvertent loss of connectivity due to
STP blocking of a port.
A subinterface with a default or untagged encapsulation will lead to an MSTP state machine failure.
Caution
Access Gateway
One common deployment scenario for Cisco ASR 9000 Series Routers is as an nPE gateway device situated
between a network of uPE access devices and a core or aggregation network. Each gateway device may provide
connectivity for many access networks, as shown in following figure. The access networks (typically rings)
have redundant links to the core or aggregation network, and therefore must use some variant of STP or a
similar protocol to ensure the network remains loopfree.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
418
Implementing Multiple Spanning Tree Protocol
Restrictions for configuring MSTP
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