Scheduling
services the queues based on their configured priorities.
Queues can be configured in one of two ways:
•
Strict priority (SP):
SP helps ensure that the highest-priority queues
are serviced first and that critical, time-sensitive traffic is prioritized
over less critical or less time-sensitive traffic.
•
Weighted round robin (WRR):
WRR helps ensure that a single high-
priority application does not take over all bandwidth by allocating
queues proportionally to their assigned weights.
Figure 3 shows the QoS Global Queue Settings screen in the Dell
OpenManage Switch Administrator GUI for the PowerConnect 5316M. The
SP and WRR settings apply to all ports for egress traffic.
Snooping with Internet Group Management Protocol
IGMP is the standard protocol for IP multicasting in a broadcast domain.
Multicast groups are segregated host groups that share packets among
themselves; multicast datagrams are transmitted only to multicast group
members. The router maintains group membership lists through IGMP:
for example, a host joining a group sends an IGMP join message to the
router, or, alternatively, the switch forwards IGMP queries from the router
to all the ports connected to it. The advantage of the latter scheme is that
it allows a single copy to be sent to the switch, which then creates copies
and transmits them. By default, a Layer 2 switch handles IP multicast
traffic in the same manner as broadcast traffic, by forwarding it to all
interfaces. Thus, IGMP traffic can potentially flood the interfaces and
cause network congestion.
Layer 2 switches can use a feature called IGMP snooping to help pre-
vent this flooding. With this feature enabled, the switching application-
specific integrated circuit (ASIC) forwards all IGMP packets to the switch
processor, which then analyzes the incoming packets, maps the ports to
multicast groups, and determines which ports will send out the IGMP
queries and which routing protocols to use. This method uses a multicast
filtering database; the switch checks this database and only forwards
multicast datagrams to member interfaces, helping optimize interface
and bandwidth usage.
To use this feature on the PowerConnect 5316M, administrators must
enable both the bridge multicast filtering feature and IGMP snooping.
They can do this in the Dell OpenManage Switch Administrator GUI from
the Multicast Global Parameters screen and the IGMP Snooping screen
(see Figure 4), or by using the following commands:
console(config)# bridge multicast filtering
console(config)# ip igmp snooping
Removing hosts from groups requires updating the filtering database,
which can be done immediately upon receiving an IGMP leave command
or after a set leave time-out period. Administrators can configure this
setting from the IGMP Snooping screen or with the following command
(where
setting
is either
time-out
or
immediate-leave
):
console(config-if)# ip igmp snooping
leave-time-out
setting
“
as network complexity increases,
administrators require advanced
management options to help ensure
quality and efficiency.
”
network and communications
DELL
POWER
SOLUTIONS |
May 2007
3
Figure 3.
QoS Global Queue Settings screen for the Dell PowerConnect
5316M switch
Figure 4.
IGMP Snooping screen for the Dell PowerConnect 5316M switch
reprinted from
Dell Power Solutions,
May 2007. Copyright © 2007 dell inc. all rights reserved.
