Samsung Ubigate iBG3026, Configuration Manual

Page: 112 / 718

CHAPTER 10. Layer-2 Switching
74
© 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 Data
Unit(BPDU) messages that communicate information on ports, addresses,
priorities and costs. A switch is elected as the root bridge based on MAC
address and priority value. All ports on the root bridge are assigned as
designated ports. All other bridges determine the lowest cost path(the best
path) 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 is moved
into Forwarding State while all remaining switch ports are considered as
redundant and are placed in Blocking State. The spanning tree algorithm
reconfigures the spanning tree topology if STP costs change, or if one network
segment in the STP becomes unreachable.
IEEE 802.1s-Multiple Spanning Tree
In a network where multiple links are used to separate VLAN traffic, the use
of STP could cause certain data paths to be disabled. 802.1S Multiple
Spanning Tree Protocol(MSTP) resolves the problem by supporting multiple
spanning trees within a network. The standard lets administrators assign
VLAN traffic to unique paths. With this feature load balancing can be
achieved through assigning multiple VLANs to 2 distinct paths.
With MSTP each VLAN is mapped to an instance of a MSTP. Moving bridge
ports into blocking state will cause blocking of specific VLAN traffic and not
the entire link.
IGMP Snooping
The Internet Group Management Protocol(IGMP) is a layer-3 protocol used
by IP hosts to report multicast information to neighboring multicast routers.
IGMP snooping allows a Layer-2 router or switch to read(snoop) IGMP
packets transferred between IP Multicast Routers and IP Multicast hosts to
learn the IP Multicast group membership. IGMP snooping allows the Layer-2
device to forward only that multicast traffic that is addressed to ports in its
own multicast group. Without IGMP snooping, multicast traffic is forwarded
to all ports. This reduces multicast traffic through the Layer-2 device. IGMP
snooping drops all but one of the host join messages per multicast group and
only forwards this one join message to the multicast router.