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