xStack
®
DGS-3200 Series Layer 2 Gigabit Ethernet Managed Switch
80
The IEEE 802.1D-2004 Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D-1998 STP standard. RSTP was
developed in order to overcome some limitations of STP that impede the function of some recent switching innovations, in
particular, certain Layer 3 functions that are increasingly handled by Ethernet switches. The basic function and much of the
terminology is the same as STP. Most of the settings configured for STP are also used for RSTP. This section introduces some
new Spanning Tree concepts and illustrates the main differences between the two protocols.
Port Transition States
An essential difference between the three protocols is in the way ports transition to a forwarding state and in the way this
transition relates to the role of the port (forwarding or not forwarding) in the topology. MSTP and RSTP combine the transition
states disabled, blocking and listening used in 802.1D-1998 and creates a single state Discarding. In either case, ports do not
forward packets. In the STP port transition states disabled, blocking or listening or in the RSTP/MSTP port state discarding, there
is no functional difference, the port is not active in the network topology. Table 7-3 below compares how the three protocols differ
regarding the port state transition.
All three protocols calculate a stable topology in the same way. Every segment will have a single path to the root bridge. All
bridges listen for BPDU packets. However, BPDU packets are sent more frequently - with every Hello packet. BPDU packets are
sent even if a BPDU packet was not received. Therefore, each link between bridges is sensitive to the status of the link. Ultimately
this difference results in faster detection of failed links, and thus faster topology adjustment. A drawback of 802.1D-1998 is this
absence of immediate feedback from adjacent bridges.
802.1Q-2005 MSTP
802.1D-2004 RSTP
802.1D-1998 STP
Forwarding
Learning
Disabled Disabled Disabled No
No
Discarding Discarding Blocking
No No
Discarding Discarding Listening
No No
Learning Learning Learning
No
Yes
Forwarding Forwarding Forwarding Yes
Yes
Table 3 - 2. Comparing Port States
RSTP is capable of a more rapid transition to a forwarding state - it no longer relies on timer configurations - RSTP compliant
bridges are sensitive to feedback from other RSTP compliant bridge links. Ports do not need to wait for the topology to stabilize
before transitioning to a forwarding state. In order to allow this rapid transition, the protocol introduces two new variables: the
edge port and the point-to-point (P2P) port.
Edge Port
The edge port is a configurable designation used for a port that is directly connected to a segment where a loop cannot be created.
An example would be a port connected directly to a single workstation. Ports that are designated as edge ports transition to a
forwarding state immediately without going through the listening and learning states. An edge port loses its status if it receives a
BPDU packet, immediately becoming a normal spanning tree port.
P2P Port
A P2P port is also capable of rapid transition. P2P ports may be used to connect to other bridges. Under RSTP/MSTP, all ports
operating in full-duplex mode are considered to be P2P ports, unless manually overridden through configuration.
802.1D-1998/802.1D-2004/802.1Q-2005 Compatibility
MSTP or RSTP can interoperate with legacy equipment and is capable of automatically adjusting BPDU packets to 802.1D-1998
format when necessary. However, any segment using 802.1D-1998 STP will not benefit from the rapid transition and rapid
topology change detection of MSTP or RSTP. The protocol also provides for a variable used for migration in the event that legacy
equipment on a segment is updated to use RSTP or MSTP.
The Spanning Tree Protocol (STP) operates on two levels:
1.
On the switch level, the settings are globally implemented.
2.
On the port level, the settings are implemented on a per user-defined group of ports basis.
