PVST+ and RSTP


Cisco developed PVST+ so that a network can run an STP instance for each VLAN in the network. With PVST+, more than one trunk can block for a VLAN and load sharing can be implemented. However, implementing PVST+ means that all switches in the network are engaged in converging the network, and the switch ports have to accommodate the additional bandwidth used for each PVST+ instance to send its own BPDUs.
PVST+ Bridge ID
As you recall, in the original 802.1D standard, an 8-byte BID is composed of a 2-byte bridge priority and a 6-byte MAC address of the switch. There was no need to identify a VLAN because there was only one spanning tree in a network. PVST+ requires that a separate instance of spanning tree run for each VLAN. To support PVST+, the 8-byte BID field is modified to carry a VLAN ID (VID). In the figure, the bridge priority field is reduced to 4 bits and a new 12-bit field, the extended system ID field, contains the VID. The 6-byte MAC address remains unchanged.
The following provides more details on the PVST+ fields:
Bridge priority - A 4-bit field carries the bridge priority. Because of the limited bit count, the priority is conveyed in discrete values in increments of 4096 rather than discreet values in increments of 1, as they would be if the full 16-bit field was available. The default priority, in accordance with IEEE 802.1D, is 32,768, which is the midrange value.
Extended system ID - A 12-bit field carrying the VID for PVST+.
MAC address - A 6-byte field with the MAC address of a single switch.
The MAC address is what makes a BID unique. When the priority and extended system ID are prepended to the switch MAC address, each VLAN on the switch can be represented by a unique BID.
Caution: If no priority has been configured, every switch has the same default priority, and the election of the root bridge for each VLAN is based on the MAC address. Therefore, to ensure that you get the root bridge you want, it is advisable to assign a lower priority value to the switch that should serve as the root bridge.
What is RSTP?
RSTP (IEEE 802.1w) is an evolution of the 802.1D standard. The 802.1w STP terminology remains primarily the same as the IEEE 802.1D STP terminology. Most parameters have been left unchanged, so users familiar with STP can rapidly configure the new protocol.
RSTP Characteristics
RSTP speeds the recalculation of the spanning tree when the Layer 2 network topology changes. RSTP can achieve much faster convergence in a properly configured network, sometimes in as little as a few hundred milliseconds. RSTP redefines the type of ports and their state. If a port is configured to be an alternate or a backup port it can immediately change to a forwarding state without waiting for the network to converge. The following briefly describes RSTP characteristics:
RSTP is the preferred protocol for preventing Layer 2 loops in a switched network environment. Many of the differences were informed by Cisco-proprietary enhancements to 802.1D. These enhancements, such as BPDUs carrying and sending information about port roles only to neighboring switches, require no additional configuration and generally perform better than the earlier Cisco-proprietary versions. They are now transparent and integrated in the protocol's operation.
Cisco-proprietary enhancements to 802.1D, such as UplinkFast and BackboneFast, are not compatible with RSTP.
RSTP (802.1w) supersedes STP (802.1D) while retaining backward compatibility. Much of the STP terminology remains, and most parameters are unchanged. In addition, 802.1w is capable of reverting back to 802.1D to interoperate with legacy switches on a per-port basis. For example, the RSTP spanning-tree algorithm elects a root bridge in exactly the same way as 802.1D.
RSTP keeps the same BPDU format as IEEE 802.1D, except that the version field is set to 2 to indicate RSTP, and the flags field uses all 8 bits. The RSTP BPDU is discussed later.
RSTP is able to actively confirm that a port can safely transition to the forwarding state without having to rely on any timer configuration.
RTSP BPDU
RSTP (802.1w) uses type 2, version 2 BPDUs, so an RSTP bridge can communicate 802.1D on any shared link or with any switch running 802.1D. RSTP sends BPDUs and populates the flag byte in a slightly different manner than in 802.1D:
Protocol information can be immediately aged on a port if hellos are not received for three consecutive hello times, 6 seconds by default, or if the max age timer expires.
Because BPDUs are used as a keepalive mechanism, three consecutively missed BPDUs indicate lost connectivity between a bridge and its neighboring root or designated bridge. The fast aging of the information allows failures to be detected quickly.
Note: Like STP, an RSTP bridge sends a BPDU with its current information every hello time period (2 seconds by default), even if the RSTP bridge does not receive any BPDUs from the root bridge.

2 comments:

CISCO NETWORKING on October 19, 2008 at 9:44 PM said...

nice post.

Anonymous said...

cool post Man

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