Topology Changes; Variants Of Stp - Cisco ASR 9000 Series Configuration Manuallines

Implementing Multiple Spanning Tree Protocol
Once the active paths have been selected, any ports that do not form part of the active topology are moved to
the blocking state.

Topology Changes

Network devices in a switched LAN perform MAC learning; that is, they use received data traffic to associate
unicast MAC addresses with the interface out of which frames destined for that MAC address should be sent.
If STP is used, then a recalculation of the spanning tree (for example, following a failure in the network) can
invalidate this learned information. The protocol therefore includes a mechanism to notify topology changes
around the network, so that the stale information can be removed (flushed) and new information can be learned
based on the new topology.
A Topology Change notification is sent whenever STP moves a port from the blocking state to the forwarding
state. When it is received, the receiving device flushes the MAC learning entries for all ports that are not
blocked other than the one where the notification was received, and also sends its own topology change
notification out of those ports. In this way, it is guaranteed that stale information is removed from all the
devices in the network.

Variants of STP

There are many variants of the Spanning Tree Protocol:
• Legacy STP (STP)—The original STP protocol was defined in IEEE 802.1D-1998. This creates a single
• Rapid STP (RSTP)—This is an enhancement defined in IEEE 802.1D-2004 to provide more event-based,
• Multiple STP (MSTP)—A further enhancement was defined in IEEE 802.1Q-2005. This allows multiple
• Per-Vlan STP (PVST)—This is an alternative mechanism for creating multiple spanning trees; it was
• Per-Vlan Rapid Spanning Tree (PVRST)— This feature is the IEEE 802.1w (RSTP) standard implemented
• PVRST uses point-to-point wiring to provide rapid convergence of the spanning tree. The spanning tree
• REP (Cisco-proprietary ring-redundancy protocol)— This is a Cisco-proprietary protocol for providing
spanning tree which is used for all VLANs and most of the convergence is timer-based.
and hence faster, convergence. However, it still creates a single spanning tree for all VLANs.
spanning tree instances to be created over the same physical topology. By assigning different VLANs
to the different spanning tree instances, data traffic can be load-balanced over different physical links.
The number of different spanning tree instances that can be created is restricted to a much smaller number
than the number of possible VLANs; however, multiple VLANs can be assigned to the same spanning
tree instance. The BPDUs used to exchange MSTP information are always sent untagged; the VLAN
and spanning tree instance data is encoded inside the BPDU.
developed by Cisco before the standardization of MSTP. Using PVST, a separate spanning tree is created
for each VLAN. There are two variants: PVST+ (based on legacy STP), and PVRST (based on RSTP).
At a packet level, the separation of the spanning trees is achieved by sending standard STP or RSTP
BPDUs, tagged with the appropriate VLAN tag.
per VLAN, and is also known as Rapid PVST or PVST+. 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+.
reconfiguration can occur in less than one second with PVRST (in contrast to 50 seconds with the default
settings in the 802.1D STP).
resiliency in rings. It is included for completeness, as it provides MSTP compatibility mode, using which,
it interoperates with an MSTP peer.
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
Topology Changes
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