Rapid Spanning Tree Protocol
IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) enhances the 802.1D standard with one goal in mind: improving STP convergence. To do so, RSTP defines new variations on BPDUs between switches, new port states, and new port roles, all with the capability to operate backwardly compatible with 802.1D switches. The key components of speeding convergence with 802.1w are as follows:
■ Waiting for only three missed Hellos on an RP before reacting (versus ten missed Hellos via the maxage timer with 802.1D)
■ New processes that allow transition from the disabled state (replaces the blocking state in 802.1D) to learning state, bypassing the concept of an 802.1D listening state
■ Standardization of features like Cisco PortFast, UplinkFast, and BackboneFast
■ An additional feature to allow a backup DP when a switch has multiple ports connected to the same shared LAN segment
To support these new processes, RSTP uses the same familiar Hello BPDUs, using some previously undefined bits to create the new features. For instance, RSTP defines a Hello message option for the same purpose as the Cisco proprietary RLQ used by the Cisco BackboneFast feature.
RSTP takes advantage of a switched network topology by categorizing ports, using a different link type to describe each. RSTP takes advantage of the fact that STP logic can be simplified in some cases, based on what is attached to each port, thereby allowing faster convergence. Table 3-8 lists the three RSTP link types.
|
Link Type |
Description |
|
Point-to-point |
Connects a switch to one other switch; Cisco switches treat FDX links in which Hellos are received as point-to-point links. |
|
Shared |
Connects a switch to a hub; the important factor is that switches are reachable off that port. |
|
Edge |
Connects a switch to a single end-user device. |
In most modern LAN designs with no shared hubs, all links would be either the point-to-point (a link between two switches) or edge link type. RSTP knows that link-type edge means the port is cabled to one device, and the device is not a switch. So, RSTP treats edge links with the same logic as Cisco PortFast—in fact, the same spanning-tree portfast command defines a port as link-type edge to RSTP. In other words, RSTP puts edge links into forwarding state immediately.
RSTP takes advantage of point-to-point links (which by definition connect a switch to another switch) by asking the other switch about its status. For instance, if one switch fails to receive its periodic Hello on a point-to-point link, it will query the neighbor. The neighbor will reply, stating whether it also lost its path to the root. It is the same logic as BackboneFast, but using IEEE standard messages to achieve the same goal.
RSTP also redefines the port states used with 802.1D, in part because the listening state is no longer needed. Table 3-9 compares the port states defined by each protocol.
|
Administrative State |
STP State (802.1D) |
RSTP State (802.1w) |
|
Disabled |
Disabled |
Discarding |
|
Enabled |
Blocking |
Discarding |
|
Enabled |
Listening |
Discarding |
|
Enabled |
Learning |
Learning |
|
Enabled |
Forwarding |
Forwarding |
In RSTP, a discarding state means that the port does not forward frames, receive frames, or learn source MAC addresses, regardless of whether the port was shut down, failed, or simply does not have a reason to forward. Once RSTP decides to transition from discarding to forwarding state (for example, a newly selected RP), it goes immediately to the learning state. From that point on, the process continues just as it does with 802.1D. RSTP no longer needs the listening state because of its active querying to neighbors, which guarantees no loops during convergence.
RSTP uses the term port role to refer to whether a port acts as an RP or a DP. RSTP uses the RP and DP port roles just as 802.1D does; however, RSTP adds several other roles, as listed in Table 3-10.
|
RSTP Role |
Definition |
|
Root Port |
Same as 802.1D Root Port. |
|
Designated Port |
Same as 802.1D Designated Port. |
|
Alternate Port |
Same as the Alternate Port concept in UplinkFast; an alternate Root Port. |
|
Backup Port |
A port that is attached to the same link-type shared link as another port on the same switch, but the other port is the DP for that segment. The Backup Port is ready to take over if the DP fails. |
The Alternate Port concept is like the UplinkFast concept—it offers protection against the loss of a switch's RP by keeping track of the Alternate Ports with a path to the root. The concept and general operation is identical to UplinkFast, although RSTP might converge more quickly via its active messaging between switches.
The Backup Port role has no equivalent with Cisco-proprietary features; it simply provides protection against losing the DP attached to a shared link when the switch has another physical port attached to the same shared LAN.
You can enable RSTP in a Cisco switch by using the spanning-tree mode rapid-pvst global command. Alternatively, you can simply enable 802.1s MST, which by definition uses 802.1w RSTP.
Continue reading here: Multiple Spanning Trees IEEE 8021s
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