2016-09-02
Today
I am continuing to research RSTP in an effort to produce a design guide at work. I started the guide and realized I don't know as much as I thought about RSTP.
STP, RSTP's predicessor was originally defined in IEEE 802.1D. RSTP was initially defined in IEEE 802.1w. However, IEEE 802.1w was incorporated into the 2004 version of IEEE 802.1D. STP was removed from IEEE 802.1D-2004.
I printed out the IEEE 802.1D-2004 standard and have begun reviewing it. This is one that is going to take a few readings to fully understand. I am trying to keep notes as I go through it.
IEEE 802.1D-2004
RSTP is defined in Clause 17 of IEEE 802.1D-2004. Item i) in 17.1 is interesting. It states "In normal operation, the time taken to configure the active topology of a network comprising point-to-point LANs is independent of the timer values of the protocol.". I am curious what "normal operation" referrs to.
Clause 17.2 referrs to Clause 7.12.3 which defines the Group MAC Address that identifies the Spanning Tree Protocol. This MAC address is called as the Bridge Group Address and is defined as 01-80-C2-00-00-00. This adress is a Multicast Ethernet Address because the least-significant bit of the first octet is a 1.
Port States
Clause 17.3 references Clause 7.4 to define the port states controlled by RSTP. The port states defined by IEEE 802.1D-2004 are:
- Discarding
- Any port that is not enabled or has been dynamically excluded from forwarding and learning from MAC frames.
- Learning
- Any port where Learning is enabled for Forwarding is disabled.
- Forwarding
- Any port that both learns and forwards frames.
Priority Vectors
Clause 17.3 references Clause 17.5 to define RSTP Priority Vectors. The Priority Vector is the information sent by each switch when selecting the Root Bridge and the shortest path to it. Priority Vectors contain the following information:
- Root Bridge Identifier
- The Bridge Identifier of the Bridge believed to be the Root by the transmitter.
- Root Path Cost
- Path cost from the transmitting Bridge what is believed to be the Root Bridge.
- Bridge Identifier
- Bridge Identifier of the transmitting Bridge
- Port Identifier
- Port through which the message was transmitted
- Port Identifier
- Port through which the message was received (where relevant)
Port Roles
Warning
This section will need more development as I get further into this. See the WTF? items.
Clause 17.3 references Clause 17.7 to define the port roles. The Port Roles defined by IEEE 802.1D-2004 are:
- Disabled Port
- Port is disabled if it is not operational or if it is excluded from the active topology by management. This can occur if the MAC_Operational status is False, if it is disabled by 802.1X, or if it is disabled by the administrator.
- Root Port
- The source of the root priority vector is the Root Port on every bridge except the Root Bridge. The Root Bridge does not have a Root Port. This is the one port on the Bridge that provides the lowest cost path to the Root Bridge.
- Designated Port
- Port whose port priority vector is its designated priority vector (WTF?). This is the one port attached to each LAN that provides the lowest cost path from that LAN to the Root Bridge.
- Alternate Port
- Port (not including the Root Port) whose priority vector has been received from another Bridge is an Alternate Port (WTF?). Ports that can provide connectivity if other network components fail.
- Backup Port
- Port that has a priority vector that has been received from another port on this Bridge (WTF?). Ports that can provide connectivity if other network components fail.
Edge Ports
Bridge ports attached to a LAN that has no other Bridges attached to it may be administratively configured as an Edge Port. Edge Ports transition directly to the Forwarding Port State since there is no possibility of it participating in a loop.
Conclusion
I made it Clause 17.3.2 which contains example topologies. I will be studying this over the weekend.