User guide

3
Choosing layer 2 switches
This section provides various general and specic recommendations for switches to
use with ALIF, however, there is no substitute for testing in real world situations. If
you are in doubt about which network switch to choose, the safest approach is to
select one from the recommended list of switches on the next page.
General recommendaons
There are certain minimum features that you need to ensure:
• Gigabit (1000Mbps) or faster Ethernet ports,
• Support for IGMP v2 (or v3) snooping,
• Support for Jumbo frames (packets) up to 9216-byte size,
• High bandwidth connections between switches, preferably Fibre Channel.
Additionally:
• Look specically for switches that perform their most onerous tasks (e.g. IGMP
snooping) using multiple dedicated processors - i.e. the tasks are carried out in
custom ASIC hardware rather than software routines on a general processor.
• Check the maximum number of concurrent ‘snoopable groups’ each switch can
handle and ensure that they meet or exceed the number of ALIF transmitters
that will be used to create multicast groups.
• Check the throughput speeds of the switch. Ensure that each port is full duplex
(i.e. bi-directional communication) and that the up and down stream data
speeds for each port are 1Gigabit per second.
• Wherever possible, use the same switch manufacturer throughout a
single subnet and, if possible, the same model of switch - this will simplify
conguration and lessen the chances of compatibility issues.
• When choosing Layer 3 switches for the network, at least one must be capable
of operating as an IGMP Querier.
IGMP and internal switch design
In recent years, the number of Layer 2 switches that support IGMP snooping has
proliferated; however, there is a wide variance in performance between the most
effective and the least.
In order to take a peek at (snoop) IGMP messages, Layer 2 switches are required
to do something they were not originally designed for: Deciphering every
data packet at Layer 3 in order to read the logical addressing and multicast
instructions. This requires considerably more processing horsepower than their
normal day-job of reading physical MAC addresses at Layer 2.
continued
The trouble with mulcasng
Where an ALIF transmitter is required to
stream video to two or more receivers,
multicasting is the method used.
Multicasting involves the delivery of
identical data to multiple receivers
simultaneously without the need to
maintain individual links. When multicast
data packets enter a subnet, the natural
reaction of the switches that bind all the
hosts together within the subnet, is to
spread the multicast data to all of their
ports. This is referred to as Multicast
flooding and means that the hosts (or at
least their network interfaces) are required
to process plenty of data that they didn’t
request. IGMP offers a partial solution.
IGMP (Internet Group Management
Protocol) was developed to help prevent
flooding by requiring individual hosts
to opt into multicasts. It also provided
a mechanism for routers to determine
whether any hosts located within their
subnet still wished to receive the
multicasts. However, this only has an effect
at the gateway to the subnet – so if one
host requests a multicast, all hosts within
the subnet would also receive it.
A soluon: IGMP snooping
Cue a development in the switches that
glue together all the hosts within a subnet:
IGMP snooping. IGMP snooping means
these layer 2 switches now have the ability
to take a peek at the instructions that
help routers to do their job. As a result,
the switches can then determine exactly
which of their own hosts have requested
to receive a multicast – and only pass on
multicast data to those hosts.