| um to large scale redundant LAN environments, | | | | In complex multi-switch environments a STP root |
| the Spanning Tree Protocol must be enabled. The | | | | switch should be manually selected. This is |
| Spanning Tree Protocol (STP) is a service that | | | | normally the most reliable and powerful switch at |
| allows LAN switches and bridges to be | | | | the centre of the LAN. This device functions as |
| redundantly interconnected by providing a | | | | the reference point and controls much of the STP |
| mechanism to prevent bridging loops. In complex | | | | communications. |
| multi-switch LAN networks or environments that | | | | The minimum and the recommended |
| required high convergence, STP protocol must be | | | | requirements when you deploy the redundant links |
| manually tuned. | | | | with STP protocol enabled are: |
| Without the existence of STP Protocol, frames | | | | 1. The IEEE 802.1d version of STP protocol must |
| would loop for an indefinite period of time in | | | | be enabled on all switches. Even though STP is |
| networks with physically redundant links. To | | | | normally turned on by default, you should assure |
| prevent looping frames, STP blocks some ports | | | | it is enabled and should never be disabled. |
| from forwarding frames so that only one active | | | | 2. Network documentation must exist that |
| path exists between any pair of LAN segments | | | | identifies the LAN topology connections including all |
| (collision domains). The result of STP is that | | | | redundant links that may be involved in a STP |
| frames do not loop infinitely, which makes the | | | | bridging loop. |
| LAN usable. One thing to note is that the | | | | 3. Port-fast (also sometimes known as fast-start |
| redundant links are not for load balance, because | | | | or start-forwarding) on all Switch port connections |
| only one link is active. | | | | to desktop PC’s, server, printers and |
| Bridging loops can be avoided to happen when the | | | | routers (but not switches, bridges or hubs) must |
| STP protocol is used in all bridges and switches. | | | | be enabled. |
| With STP protocol, each interface on a bridging | | | | 4. In complex multiple switch environments, the |
| device is settled into a blocking state or a | | | | definition of a STP root is required. If using |
| forwarding state. The interface that is in | | | | multiple VLANs, this will also need to be done on a |
| Blocking state’ cannot forward or | | | | per VLAN basis. |
| receive data frames, while the interface in | | | | 5. In case of large data pipes are required, Ether |
| Forwarding state’ can send and receive | | | | Channel technologies (FEC\GEC) should be used in |
| data frames. By having a correct subset of the | | | | excess of standard Fast or Gigabit Ethernet |
| interfaces blocked, a single currently active logical | | | | speeds. |
| path will exist between each pair of LANs. | | | | The fact that the bridging loops do occur naturally |
| STP protocol behaves identically for a transparent | | | | at some time in most network environments, |
| bridge and a switch. So, speaking about STP, the | | | | STP protocol should never be disabled on |
| terms bridge, switch, and bridging device all are | | | | switches. If the redundant connections were to |
| used interchangeably. Remember that the | | | | exist, and the STP protocols were disabled, then |
| switches forward frames sent to both unknown | | | | a single broadcast event would endless loop and |
| unicast MAC addresses and the broadcast | | | | cripple the switched LAN that leads to a slow |
| address, out all interfaces (except the incoming | | | | network phenomena. |
| interface). | | | | STP port-fast (also known as fast-start or |
| The Spanning Tree Protocol (STP) was developed | | | | start-forwarding) can be used to quickly transition |
| to prevent the occurrence of bridging loops within | | | | host ports without the need to transition slowly |
| redundant layer-2 networks. A dominant issue | | | | via the various STP states. Without STP port-fast |
| facing switched LANs is the use of redundant links | | | | enabled, many connections simply timeout when |
| and/or switches which are used to provide fault | | | | trying to establish a connection for the first time. |
| tolerance in the network. STP protocol is a | | | | Some Novell IPX and DHCP connections have |
| protocol that should be configured correctly to | | | | been known to fail without STP port-fast support |
| suit the specific requirements of any complex | | | | enabled. |
| (multiple switches with redundant connections) | | | | Do not enable STP port-fast on any connection to |
| switched or bridged LAN. | | | | another switch as this may introduce spurious |
| STP protocol analyses the layer-2 LAN topology | | | | bridging loops into the network. STP port-fast is a |
| and from a reference point (root bridge), locates | | | | feature that is available on most modern intelligent |
| any redundant pathways (bridge loops) within the | | | | switches and is not enabled by default. |
| network. Bridging loops can cause a number of | | | | Without correct LAN switch design and STP |
| network problems including broadcast storms, | | | | tuning, convergence and LAN stability problems |
| duplicate frame copies and LAN Switch | | | | may occur in any complex redundant LAN. |
| MAC-database instability problems. | | | | |