CHAPTER 1. Layer-2 Switching74 © SAMSUNG Electronics Co., Ltd.STP defines a tree that spans all switches in an extended network.All participating switches in an extended LAN sends Bridge Protocol DataUnit(BPDU) messages that communicate information on ports, addresses,priorities and costs. A switch is elected as the root bridge based on MACaddress and priority value. All ports on the root bridge are assigned asdesignated ports. All other bridges determine the lowest cost path(the bestpath) to get to the root bridge, and assign that port as designated port.On non-root bridges only one port can be designated. The designated port ismoved into Forwarding State while all remaining switch ports are consideredas redundant and are placed in Blocking State. The spanning tree algorithmreconfigures the spanning tree topology if STP costs change, or if one networksegment in the STP becomes unreachable.IEEE 802.1s-Multiple Spanning TreeIn a network where multiple links are used to separate VLAN traffic, the useof STP could cause certain data paths to be disabled. 802.1S MultipleSpanning Tree Protocol(MSTP) resolves the problem by supporting multiplespanning trees within a network. The standard lets administrators assignVLAN traffic to unique paths. With this feature load balancing can beachieved through assigning multiple VLANs to 2 distinct paths.With MSTP each VLAN is mapped to an instance of a MSTP. Moving bridgeports into blocking state will cause blocking of specific VLAN traffic and notthe entire link.IGMP SnoopingThe Internet Group Management Protocol(IGMP) is a layer-3 protocol usedby IP hosts to report multicast information to neighboring multicast routers.IGMP snooping allows a Layer-2 router or switch to read(snoop) IGMPpackets transferred between IP Multicast Routers and IP Multicast hosts tolearn the IP Multicast group membership. IGMP snooping allows the Layer-2device to forward only that multicast traffic that is addressed to ports in itsown multicast group. Without IGMP snooping, multicast traffic is forwardedto all ports. This reduces multicast traffic through the Layer-2 device. IGMPsnooping drops all but one of the host join messages per multicast group andonly forwards this one join message to the multicast router.
