As there is no block for the L3 MSTI in the access network, the topology change notification could loop
forever. To avoid that situation, MSTAG Edge mode completely disables handling of topology change
notifications in the gateway devices.
PVSTAG on Bundle Interfaces
Per-VLAN Spanning Tree Access Gateway (PVSTAG) support has been extended on bundle interfaces, along
with physical interfaces, to cater to an increasing number of customers that support PVST access networks.
For physical interfaces, bridge protocol data units (BDPUs) are sent from the line card that hosts the interfaces.
However, for bundle interfaces BPDUs are sent from the route processor (RP). When an RP failover occurs,
the data traffic flowing over the bundle interface is not affected; as a result, no BPDUs are sent until the
failover is complete and the newly active RP takes over. If there is a delay, the peer device times out the BPDU
information. This leads to a forwarding loop, which results in disruption in Ethernet networks. It is therefore
important to ensure that, upon RP failover, the peer device does not time out the BPDU information.
Per-VLAN Rapid Spanning Tree
Per-VLAN Rapid Spanning Tree (PVRST) or Rapid PVST or PVST+ is the IEEE 802.1w (RSTP) standard
implemented per VLAN. A single instance of STP runs on each configured VLAN (if you do not manually
disable STP). Each Rapid PVST+ instance on a VLAN has a single root switch. You can enable and disable
STP on a per-VLAN basis when you are running Rapid PVST+.
PVRST uses point-to-point wiring to provide rapid convergence of the spanning tree. The spanning tree
reconfiguration can occur in less than 1 second with PVRST (in contrast to 50 seconds with the default settings
in the 802.1D STP).
PVRST supports one STP instance for each VLAN.
Note
Using PVRST, STP convergence occurs rapidly. Each designated or root port in the STP sends out a BPDU
every 2 seconds by default. On a designated or root port in the topology, if hello messages are missed three
consecutive times, or if the maximum age expires, the port immediately flushes all protocol information in
the table. A port considers that it loses connectivity to its direct neighbor root or designated port if it misses
three BPDUs or if the maximum age expires. This rapid aging of the protocol information allows quick failure
detection.
PVRST achieves rapid transition to the forwarding state only on edge ports and point-to-point links. Although
the link type is configurable, the system automatically derives the link type information from the duplex setting
of the port. Full-duplex ports are assumed to be point-to-point ports, while half-duplex ports are assumed to
be shared ports.
Disadvantages
• Increased load in terms of increased packet rate.
• Greater CPU and memory utilization due to one STP instance for every LAN.
Implementation of PVRST in IOS-XR
The implementation of PVRST in IOS-XR has the following characteristics:
• Configuration of the Forward Delay and Max Age timers is only supported globally and not per VLAN.
• Configuration of the Hello timer is supported per port and not per VLAN. The Hello timer configured
on a port applies to all VLANs on that specific port.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
424
Implementing Multiple Spanning Tree Protocol
PVSTAG on Bundle Interfaces
To Next Page
Loading...
Need help?
General
