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Figure 19-4 Forwarding Tagged/Untagged Frames to 802.1Q VLAN Aware/Unaware Devices
19.2.7
IGMP Snooping
Traditionally, IP packets are transmitted in one of either two ways - Unicast (one sender to one recipient) or
Broadcast (one sender to everybody on the network). Multicast delivers IP packets to just a group of hosts on the
network.
IGMP (Internet Group Multicast Protocol) is a session-layer protocol used to establish membership in a multicast
group - it is not used to carry user data. Refer to RFC 1112 and RFC 2236 for information on IGMP versions 1
and 2 respectively.
A layer-2 switch can passively snoop on IGMP Query, Report and Leave (IGMP version 2) packets transferred
between IP multicast routers/switches and IP multicast hosts to learn the IP multicast group membership. It checks
IGMP packets passing through it, picks out the group registration information, and configures multicasting
accordingly.
Without IGMP snooping, multicast traffic is treated in the same manner as broadcast traffic, that is, it is forwarded
to all ports. With IGMP snooping, group multicast traffic is only forwarded to ports that are members of that
group. IGMP Snooping generates no additional network traffic, allowing you to significantly reduce multicast
traffic passing through your switch.
19.2.8
Quality of Service (QoS)
IEEE 802.1p defines up to 8 separate traffic classes by inserting a tag into a MAC-layer frame that contains bits to
define class of service.
Priority 7
Typically used for network control traffic such as router configuration messages.
Priority 6
Typically used for voice traffic that is especially sensitive to jitter (jitter is the variations in delay.
Priority 5
Typically used for video that consumes high bandwidth and is sensitive to jitter.
Priority 4
Typically used for controlled load, latency-sensitive traffic such as SNA transactions.
General, Switch and IP Setup
Table 19-6 QoS Priority Listing
IES-2000/3000 User's Guide
19-7
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Troubleshooting
