64CHAPTER 4: VIRTUAL LANS (VLANS)

VLANs ease the change and movement of devices.

With traditional networks, network administrators spend much of their time dealing with moves and changes. If users move to a different subnetwork, the addresses of each endstation must be updated manually.

For example, with a VLAN, if an endstation in VLAN Marketing is moved to a port in another part of the network, and retains its original subnet membership; you must only specify that the new port is in VLAN Marketing.

IGMP Overview

IGMP is a protocol used by an IP host to register its IP multicast group

 

membership with a router. The messaging protocol can also be

 

“snooped” by a layer 2 switch, to provide for intelligent forwarding of

 

multicast data streams within a VLAN. Periodically, the router queries the

 

multicast group to see if the group is still in use. If the group is still active,

 

a single IP host responds to the query, and group registration is

 

maintained.

IGMP Snooping

IGMP snooping is a layer 2 function of the switch. The feature reduces

 

the flooding of IP multicast traffic, optimizes the usage of network

 

bandwidth, and prevents multicast traffic from being flooded to parts of

 

the network that do not need it. The switch does not reduce any IP

 

multicast traffic in the local multicast domain (224.0.0.x). An optional

 

optimization for IGMP snooping is the strict recognition of multicast

 

routers only if the remote devices have joined the DVMRP (224.0.0.4) or

 

PIM (244.0.0.13) multicast groups.

 

IGMP snooping is enabled by default on the switch. If IGMP snooping is

 

disabled, all IGMP and IP multicast traffic floods within a given VLAN. This

 

is standard 802.1d bridge behavior . IGMP snooping expects to see

 

periodic IGMP reports from interested hosts on each port. Without an

 

IGMP querier, the switch may stop forwarding IP multicast packets to all

 

ports.

 

To support IGMP snooping in environments that do not have an IGMP

 

querier, the switch can function as an IGMP querier, per the rules of

 

standard IGMP Version 2.0. If IGMP snooping is enabled, the switch

 

periodically queries for multicast group memberships. However, if either

 

IGMP snooping is disabled or IGMP functionality is disabled, the switch

 

does not generate IGMP query messages. IGMP should be enabled when

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3Com 9100 manual Igmp Overview, VLANs ease the change and movement of devices, Igmp Snooping