- What is STP?
- The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links.
- What is Portfast?
- The time Spanning Tree Protocol (STP) takes to transition ports over to the Forwarding state can cause problems. PortFast is a Cisco network function which can be configured to resolve this problem. This factor of time is not an issue for many people, but it can cause problems for some. (i.e. Fog imaging) You may see this issue is with Pre-Boot Execution (PXE) devices, such as Windows Deployment Services. PortFast is the solution to this problem of delays when client computers are connecting to switches. PortFast is not enabled by default. With PortFast enabled on a port, you effectively take the port and tell spanning tree not to implement STP on that port.
- What is Rapid STP(RSTP)?
- The 802.1D Spanning Tree Protocol (STP) standard was designed at a time when the recovery of connectivity after an outage within a minute or so was considered adequate performance. With the advent of Layer 3 switching in LAN environments, bridging now competes with routed solutions where protocols, such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP), are able to provide an alternate path in less time. Cisco enhanced the original 802.1D specification with features such as Uplink Fast, Backbone Fast, and Port Fast to speed up the convergence time of a bridged network. The drawback is that these mechanisms are proprietary and need additional configuration. Rapid Spanning Tree Protocol (RSTP; IEEE 802.1w) can be seen as an evolution of the 802.1D standard more than a revolution. The 802.1D terminology remains primarily the same. Most parameters have been left unchanged so users familiar with 802.1D can rapidly configure the new protocol comfortably. In most cases, RSTP performs better than proprietary extensions of Cisco without any additional configuration. 802.1w can also revert back to 802.1D in order to interoperate with legacy bridges on a per-port basis. This drops the benefits it introduces.
- What is Multiple STP (MSTP)?
- The Multiple Spanning Tree Protocol (MSTP), originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2005, defines an extension to RSTP to further develop the usefulness of virtual LANs (VLANs). This Multiple Spanning Tree Protocol configures a separate Spanning Tree for each VLAN group and blocks all but one of the possible alternate paths within each Spanning Tree. If there is only one Virtual LAN (VLAN) in the network, single (traditional) STP works appropriately. If the network contains more than one VLAN, the logical network configured by single STP would work, but it is possible to make better use of the alternate paths available by using an alternate spanning tree for different VLANs or groups of VLANs.
What do I enable and disable?
- If you don't need STP all these options should be disabled already and nothing should need to be done. (DISABLE ALL)
- If you have to use STP, to get (ipxe/dhcp) Fog (v1.x.x) working correctly you will need to ENABLE PORTFAST OR ENABLE RSTP.
- Currently MSTP is untested with Fog but may be useful for networks with multiple VLANS.
More information on Spanning Tree Protocol