Mstp Topology Change Guard; Mstp Supported Features - Cisco ASR 9000 Series Configuration Manuallines
L2vpn and ethernet services configuration guide
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Also See for ASR 9000 Series:
- Command reference manual (1138 pages) ,
- Routing configuration manual (702 pages) ,
- Reference manual (696 pages)
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MSTP Topology Change Guard
The root guard feature provides a mechanism that allows the administrator to enforce the location of the root
bridge. When root guard is configured on an interface, it prevents that interface from becoming a root port
(that is, a port via which the root can be reached). If superior information is received via BPDUs on the
interface that would normally cause it to become a root port, it instead becomes a backup or alternate port. In
this case, it is placed in the blocking state and no data traffic is forwarded.
The root bridge itself has no root ports. Thus, by configuring root guard on every interface on a device, the
administrator forces the device to become the root, and interfaces receiving conflicting information are blocked.
Note
Root Guard is implemented as a Cisco-proprietary extension in Cisco implementations of legacy STP and
RSTP. However, it is encompassed in the standard for MSTP, where it is known as Restricted Role.
MSTP Topology Change Guard
In certain situations, it may be desirable to prevent topology changes originating at or received at a given port
from being propagated to the rest of the network. This may be the case, for example, when the network is not
under a single administrative control and it is desirable to prevent devices external to the core of the network
from causing MAC address flushing in the core. This behavior can be enabled by configuring Topology
Change Guard on the port.
Note
Topology Change Guard is known as Restricted TCN in the MSTP standard.
MSTP Supported Features
Cisco ASR 9000 Series Routers support MSTP, as defined in IEEE 802.1Q-2005, on physical Ethernet
interfaces and Ethernet Bundle interfaces. Note that this includes the Port Fast, Backbone Fast, Uplink Fast
and Root Guard features found in Cisco implementations of legacy STP, RSTP and PVST, as these are
encompassed by the standard MSTP protocol. Cisco ASR 9000 Series Routers can operate in either standard
802.1Q mode, or in Provide Edge (802.1ad) mode. In provider edge mode, a different MAC address is used
for BPDUs, and any BPDUs received with the 802.1Q MAC address are forwarded transparently.
In addition, these additional Cisco features are supported:
• BPDU Guard—This Cisco feature protects against misconfiguration of edge ports.
• Flush Containment—This Cisco feature helps prevent unnecessary MAC flushes that would otherwise
• Bringup Delay—This Cisco feature prevents an interface from being added to the active topology before
Note
Interoperation with RSTP is supported, as described in the 802.1Q standard; however, interoperation with
legacy STP is not supported.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
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occur following a topology change.
it is ready to forward traffic.
Implementing Multiple Spanning Tree Protocol
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