Cisco ASR 9000 Series

Figure 63: Core connectivity Failure
The device that loses core connectivity starts sending start-up BPDUs. The following diagram illustrates how
traffic flows in the access network when an nPE device loses core connectivity.
Figure 64: Active Topology Change
Start-up BPDUs are sent from the disconnected router to ensure that it is not used as a path to the core from
the access ring. This changes the active topology in the access rings depending on the STP configuration.
Benefits
The MSTAG Uplink Tracking feature has these benefits:
• Reduction in the frequency of traffic outages
• Reduced need for redundancy in connectivity from the gateway to the core network
Prerequisites
• Configure MSTAG
Restrictions
Only physical and bundle interfaces, and their sub interfaces can be tracked for core connectivity. Psuedowires
themselves cannot be tracked, only the underlying interface carrying the psuedowire traffic can be tracked.
Configure MSTAG Uplink Tracking
Only physical and bundle interfaces, and their sub-interfaces can be tracked for core connectivity. Pseudowires
themselves cannot be tracked, only the underlying interface carrying the pseudowire traffic can be tracked.
Perform this task to configure MSTAG Uplink Tracking feature.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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Implementing Multiple Spanning Tree Protocol
Benefits