Black Box ACR1000A Switch User Manual


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ServSwitch Agility and Agility Dual
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Appendix C. Glossary
C.1 Internet Group Management Protocol
Where an ServSwitch Agility 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 solu-
tion.
The Internet Group Management Protocol (IGMP) is designed to prevent multicast flooding by allowing Layer 3 switches to check
whether host computers within their care are interested in receiving particular multicast transmissions. They can then direct multi-
cast data only to those points that require it and can shut off a multicast stream if the subnet has no recipients.
There are currently three IGMP versions: 1, 2 and 3, with each version building upon the capabilities of the previous one:
• IGMPv1allowshostcomputerstooptintoamulticasttransmissionusingaJoinGroupmessage,itisthenincumbentonthe
routertodiscoverwhentheynolongerwishtoreceive;thisisachievedbypollingthem(seeIGMPQuerierbelow)untiltheyno
longer respond.
• IGMPv2includesthemeansforhoststooptoutaswellasin,usingaLeaveGroupmessage.
• IGMPv3encompassestheabilitiesofversions1and2butalsoaddstheabilityforhoststospecifyparticularsourcesofmulti-
cast data.
ServSwitch Agility units make use of IGMPv2 when performing multicasts to ensure that no unnecessary congestion is caused.
C.1.2 IGMP Snooping
The IGMP messages are effective but only operate at layer 2 - intended for routers to determine whether multicast data should
enter a subnet. A relatively recent development has taken place within the switches that glue together all of the hosts within each
subnet: IGMP Snooping. IGMP snooping means these layer 2 devices now have the ability to take a peek at the IGMP messages.
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.
C.1.3 IGMP Querier
When IGMP is used, each subnet requires one Layer 3switchtoactasaQuerier.Inthisleadrole,theswitchperiodicallysends
outIGMPQuerymessagesandinresponseallhostsreportwhichmulticaststreamstheywishtoreceive.TheQuerierdeviceand
allsnoopingLayer2switches,thenupdatetheirlistsaccordingly(thelistsarealsoupdatedwhenJoinGroupandLeaveGroup
(IGMPv2) messages are received).
C.1.4 IGMP Fast-Leave (aka Immediate Leave)
When a device/host no longer wishes to receive a multicast transmission, it can issue an IGMP Leave Group message as men-
tionedabove.ThiscausestheswitchtoissueanIGMPGroup-SpecificQuerymessageontheport(thattheLeaveGroupwas
received on) to check no other receivers exist on that connection that wish to remain a part of the multicast. This process has a
cost in terms of switch processor activity and time.
Where ServSwitch Agility units are connected directly to the switch (with no other devices on the same port) then enabling IGMP
Fast-Leave mode means that switches can immediately remove receivers without going through a full checking procedure. Where
multiple units are regularly joining and leaving multicasts, this can speed up performance considerably.
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