Managing Distribution of Multicast Cartage with IGMP
Refer to Figure 16-4. Accept that R2 has started accepting multicast cartage from the server. R2
has to accomplish a accommodation about forwarding this cartage on the Ethernet links. R2 needs to apperceive the
answers to the afterward questions:
■ Is there any host affiliated to any of my Ethernet links that has apparent absorption in receiving
this traffic?
■ If none of the hosts has apparent any absorption in accepting this traffic, why should I advanced it on
the Ethernet links and decay bandwidth?
■ If any host has apparent absorption in accepting this traffic, breadth is it located? Is it affiliated to
one of my Ethernet links or to both?
As you can see, a apparatus is appropriate for hosts and a bounded router to acquaint with each
other. The IGMP was advised to accredit advice amid a router and affiliated hosts.
Not alone do routers charge to apperceive out which LAN interface to advanced multicast packets, but
switches additionally charge to apperceive on which ports they should advanced the traffic. By default, if a switch
receives a multicast anatomy on a port, it will flood the anatomy throughout the VLAN, aloof like it would
do for a advertisement or alien unicast frame. The acumen is that switches will never acquisition a
multicast MAC abode in their Content Addressable Memory (CAM) table, because a multicast
MAC abode is never acclimated as a antecedent address.
A switch’s accommodation to flood multicast frames agency that if any host or hosts in a VLAN request
to accept the cartage for a multicast group, all the actual hosts in the aforementioned VLAN, whether they
have requested to accept the cartage for the multicast group, will accept the multicast traffic. This
behavior is adverse to one of the above goals of multicast design, which is to bear multicast
traffic to alone those hosts that accept requested it, while maximizing bandwidth efficiency. To
Managing Distribution of Multicast Cartage with IGMP 547
forward cartage added calmly in Figure 16-4, SW2 and SW3 charge to apperceive the answers to the
following questions:
■ Should I advanced this multicast cartage on all the ports in this VLAN or alone on specific ports?
■ If I should advanced this multicast cartage on specific ports of a VLAN, how will I acquisition those
port numbers?
Three altered tools, namely CGMP, IGMP snooping, and RGMP, acquiesce switches to optimize
their multicast forwarding argumentation by answering these kinds of questions. These capacity are covered
in added abyss afterwards in the chapter. For now, this breadth focuses on how routers and hosts use IGMP
to accomplish abiding the router knows whether it should advanced multicasts out the router’s LAN
interfaces.
Joining a Group
Before a host can accept any multicast traffic, a multicast appliance charge be installed and
running on that host. The action of installing and alive a multicast appliance is referred to as
launching an appliance or abutting a multicast group. Afterwards a host joins a group, the host software
calculates the multicast MAC address, and its NIC again starts alert to the multicast MAC
address, in accession to its BIA.
Before a host (or a user) can accompany a group, the user needs to apperceive what groups are accessible and
how to accompany them. For enterprise-scale multicast applications, the user may artlessly acquisition a articulation on a
web folio and bang it, bidding the user’s multicast applicant appliance to alpha alive with the
correct multicast address—totally ambuscade the multicast abode details. Alternately, for an
internally developed multicast application, the multicast abode can be preconfigured on the
client application. For example, a user ability be appropriate to log on to a server and accredit with
a name and a password; if the user is authenticated, the multicast appliance automatically installs
on the user’s PC, which agency the user has abutting the multicast group. Back the user no longer
wants to use the multicast application, the user charge leave the group. For example, the user may
simply abutting the multicast appliance to leave the group.
The action by which a animal discovers which multicast IP abode to accept for and accompany can be
a challenge, decidedly for multicast cartage on the Internet. The botheration is agnate to back you
have a accessory or agenda cable TV arrangement at home—you ability accept actually bags of
channels, but award the approach that has the appearance you appetite to watch ability crave a lot of surfing
through the account of channels and time slots. For IP multicast, a user needs to ascertain what
applications they may appetite to use, and the multicast IP addresses acclimated by the applications. A lot
of assignment charcoal to be done in this area, but some options are available. For example, online TV
program guides and web-based schedules acquaint contest that will use multicast groups and
specify who to acquaintance if you appetite to see the event, lecture, or concert. Accoutrement like Session
Description Agreement (SDP) and Service Advertising Agreement (SAP) additionally call multicast
548 Affiliate 16: Introduction to IP Multicasting
events and acquaint them. However, a abundant altercation of the altered methods, their
limitations, and procedures for appliance them is above the ambit of this book. The blow of the
discussion in this breadth assumes that hosts accept somehow abstruse about a multicast group.
Internet Accumulation Management Protocol
IGMP has acquired from the Host Membership Protocol, declared in Dr. Steve Deering’s doctoral
thesis, to IGMPv1 (RFC 1112), to IGMPv2 (RFC 2236), to the latest, IGMPv3 (RFC 3376). IGMP
messages are beatific in IP datagrams with IP agreement cardinal 2, with the IP Time-to-Live (TTL) field
set to 1. IGMP packets canyon alone over a LAN and are not forwarded by routers, due to their TTL
field values.
The two best important goals of IGMP are as follows:
■ To acquaint a bounded multicast router that a host wants to accept multicast cartage for a specific
group
■ To acquaint bounded multicast routers that a host wants to leave a multicast accumulation (in added words,
the host is no best absorbed in accepting the multicast accumulation traffic)
Multicast routers use IGMP to advance advice for anniversary router interface about which
multicast accumulation cartage they should advanced and which hosts appetite to accept it.
The afterward breadth examines IGMPv2 in detail and introduces important appearance of IGMPv3.
IGMPv1 is no best on the CCIE Routing Switching assay blueprint, so the focus begins
with IGMPv2. In the abstracts that appearance the operation of IGMP, Layer 2 switches are not shown
because IGMP is acclimated for advice amid hosts and routers. Afterwards in the chapter, the
sections “Cisco Accumulation Management Protocol,” “IGMP Snooping,” and “Router-Port Group
Management Protocol” altercate the operation of multicasting at Layer 2.
IGMP Adaptation 2
Figure 16-7 shows the 8-octet architecture of an IGMPv2 message.
Figure 16-7 IGMPv2 Bulletin Format
Version
Group Address
Unused
8 8 16
32 Bits
Type Checksum
Managing Distribution of Multicast Cartage with IGMP 549
IGMPv2 has four fields, which are authentic as follows:
■ Type—8-bit acreage that is one of four bulletin types authentic by IGMPv2:
— Membership Concern (Type cipher = 0x11)—Used by multicast routers to discover
the attendance of accumulation associates on a subnet. A Accepted Membership Concern message
sets the Accumulation Abode acreage to 0.0.0.0. A Group-Specific Concern sets the Group
Address acreage to the abode of the accumulation actuality queried. It is beatific by a router afterwards it
receives the IGMPv2 Leave Accumulation bulletin from a host. It is acclimated to determine
whether a specific multicast accumulation has any actual associates on a subnet.
— Adaptation 1 Membership Abode (Type cipher = 0x12)—Used by IGMPv2 hosts for
backward affinity with IGMPv1.
— Adaptation 2 Membership Abode (Type Cipher = 0x16)—Sent by a accumulation affiliate to
inform the router that at atomic one accumulation affiliate is present on the subnet.
— Leave Accumulation (Type cipher = 0x17)—Sent by a accumulation affiliate if it was the last
member to accelerate a Membership Abode to acquaint the router that it is abrogation the
group.
■ Best Acknowledgment Time—8-bit acreage included alone in Concern messages. The units are
1/10 of a second, with 100 (10 seconds) actuality the default. The ethics ambit from 1 to 255
(0.1 to 25.5 seconds).
■ Checksum—Carries the 16-bit checksum computed by the source. The IGMP checksum is
computed over the accomplished IP payload, not aloof over the aboriginal 8 octets, alike admitting IGMPv2
messages are alone 8 bytes in length.
■ Accumulation Address—Set to 0.0.0.0 in Accepted Concern letters and to the accumulation abode in
Group-Specific messages. Membership Abode letters backpack the abode of the accumulation being
reported in this field; Leave Accumulation letters backpack the abode of the accumulation actuality larboard in this
field.
IGMPv2 supports complete astern affinity with IGMPv1. The IGMPv2 Blazon codes 0x11
and 0x12 bout the blazon codes for IGMPv1 for the Membership Concern and Membership Report
messages. This enables IGMPv2 hosts and routers to admit IGMPv1 letters back IGMPv1
hosts or routers are on the network.
550 Affiliate 16: Introduction to IP Multicasting
One of the primary affidavit for developing IGMPv2 was to accommodate a bigger Leave apparatus to
shorten the leave cessation compared to IGMPv1. IGMPv2 has the afterward features:
■ Leave Accumulation messages—Provide hosts with a adjustment for advice routers that they appetite to
leave the group.
■ Group-Specific Concern messages—Permit the router to accelerate a concern for a specific group
instead of all groups.
■ Best Acknowledgment Time field—A acreage in Concern letters that permits the router to
specify the MRT. This acreage allows for affability the acknowledgment time for the Host Membership
Report. This affection can be advantageous back a ample cardinal of groups are alive on a subnet and
you appetite to abatement the burstiness of the responses by overextension the responses over a longer
period of time.
■ Querier acclamation process—Provides the adjustment for selecting the adopted router for sending
Query letters back assorted routers are affiliated to the aforementioned subnet.
IGMPv2 helps abate surges in IGMPv2 Solicited Abode letters beatific by hosts in acknowledgment to
IGMPv2 Concern letters by acceptance the arrangement ambassador to change the Concern Response
Interval. The IGMPv2 Concern bulletin includes an MRT field, advertence an MRT to be acclimated by all
IGMPv2 hosts on the LAN. Setting MRT, which ranges from 0.1 to 25.5 seconds, to a value
slightly best than IGMPv1’s absence of 10 abnormal spreads the hosts’ aggregate IGMPv2
Solicited Abode letters over a best time period, consistent in added accordant burning of
subnet bandwidth and router resources.
A multicast host can accelerate an IGMP Abode in acknowledgment to a Concern or artlessly accelerate a Abode when
the host’s appliance aboriginal comes up. The IGMPv2 router acting as the IGMPv2 querier sends
general IGMP Concern letters every 125 seconds. The operations of IGMPv2 Accepted Query
messages and Abode letters are covered next.
IGMPv2 Host Membership Concern Functions
A multicast router uses the IGMPv2 Host Membership Concern bulletin to actuate whether it has
any host on any of its LAN interfaces that wants multicast cartage for any group. The IGMPv2 Host
Membership Concern bulletin is beatific alone by multicast routers on LAN interfaces. For example,
Figure 16-8 shows the IGMPv2 Host Membership Concern process. It lists two steps, with the
second footfall actuality the router sending the Query.
Managing Distribution of Multicast Cartage with IGMP 551
Figure 16-8 IGMPv2 Host Membership Concern Process
The capacity of the two accomplish are as follows:
1. Hosts H1 and H3 accompany multicast accumulation 226.1.1.1. The Accompany causes these hosts to adapt to
receive letters beatific to both 226.1.1.1 (the abutting group) and 224.0.0.1 (the abode to which
IGMPv2 Queries will be sent). Multicast hosts charge accept to the acclaimed 224.0.0.1
multicast accumulation abode to participate in IGMP and, as a result, to accept multicast queries
sent by the router. The Accompany causes these hosts to account the two multicast MAC (MM)
addresses, 01-00-5e-01-01-01 (from 226.1.1.1) and 01-00-5e-00-00-01 (from 224.0.0.1), and
then accept for frames beatific to these two MMs.
2. R1 sends an IGMPv2 Host Membership Concern out anniversary LAN interface, attractive for any host
interested in accepting packets for any multicast group. R1 periodically sends IGMPv2
Queries on anniversary LAN interface, by default, every 60 seconds. This time aeon is alleged the
Query Interval. R1’s Queries use a destination IP abode and MAC abode of 224.0.0.1 and
01-00-5e-00-00-01, with the antecedent IP abode and MAC abode of R1’s interface IP address
and BIA, respectively. Afterwards sending IGMPv2 Queries, R1 expects any host that has joined
group 226.1.1.1, or any added group, to acknowledgment with an IGMPv2 Report. The IGMPv2 Queries
also use a TTL of 1, preventing the packet from actuality routed.
The IGMPv2 Concern message’s Accumulation Abode acreage (see Figure 16-7) is consistently 0.0.0.0. By
sending the IGMPv2 Concern bulletin with the Accumulation Abode 0.0.0.0, the router is allurement the
hosts on anniversary LAN, “Does anyone appetite to accept multicast cartage for any group?”
R1
H1 H2 H3
H4
H5
.1 .2
.10
.10
Fa0/0
S0/0 Fa0/1
.3
.4
.5
10.2.2.0/24
10.1.1.0/24
IGMPv2 Host Membership
Query
“Does anyone appetite to
receive multicast cartage for
any group?”
2
1
IGMPv2 Host Membership
Query
“Does anyone appetite to
receive multicast cartage for
any group?”
2
01-00-5e-00-00-01
01-00-5e-01-01-01
MM1
Listen for These MMs:
1 01-00-5e-00-00-01
01-00-5e-01-01-01
Listen for These MMs:
MM2
552 Affiliate 16: Introduction to IP Multicasting
At this point, router R1 still does not apperceive whether any hosts charge to accept any multicast traffic.
The abutting breadth covers how the hosts acknowledge with IGMP Abode letters to acquaint R1 of their
interest in accepting multicast packets.
IGMPv2 Host Membership Abode Functions
Hosts use IGMPv2 Host Membership Abode letters to acknowledgment to IGMP Queries and acquaint the
routers of their admiration to accept multicasts. Multicast hosts use IGMPv2 Host Membership Report
messages to acquaint to a bounded router for which multicast groups they appetite to accept traffic.
In IGMPv2, a host sends a Host Membership Abode beneath the afterward two conditions:
■ Back a host receives an IGMPv2 Concern from a bounded router, it is declared to accelerate an IGMPv2
Host Membership Abode for all the multicast groups for which it wants to accept multicast
traffic. This Abode is alleged an IGMPv2 Solicited Host Membership Report.
■ Back a host joins a new group, the host anon sends an IGMPv2 Host Membership
Report to acquaint a bounded router that it wants to accept multicast cartage for the accumulation it has just
joined after cat-and-mouse to accept an IGMPv2 Query. This Abode is alleged an IGMPv2
Unsolicited Host Membership Report.
The operations of Solicited Host Membership Abode and Unsolicited Host Membership Report
are explained in the afterward sections.
IGMPv2 Solicited Host Membership Report
Figure 16-9 shows operation of the IGMPv2 Solicited Host Membership Abode action and the
Report Suppression mechanism. Figure 16-9 picks up the archetype from Figure 16-8, in which
router R1 had beatific an IGMPv2 Query.
NOTE The appellation Solicited Host Membership Abode is not authentic in RFC 2236. It is acclimated in
this book to specify whether the IGMPv2 Abode was beatific in acknowledgment to a Concern (solicited).
Managing Distribution of Multicast Cartage with IGMP 553
Figure 16-9 IGMPv2 Solicited Host Membership Abode and Abode Suppression Processes
If abounding hosts accept launched multicast applications and if all of them acknowledge to the Host
Membership Concern with the Host Membership Report, abounding bombastic letters could be beatific to
the router. This wastes bandwidth and unnecessarily increases the processing bulk on the router.
A multicast router needs to accept alone one abode for anniversary appliance on anniversary of its LAN
interfaces. A multicast router begins forwarding multicast cartage on a LAN interface whether 1
user or 200 users appeal to accept the cartage for a accustomed multicast group.
The Abode Suppression apparatus helps to break these problems. It uses the IGMPv2 Maximum
Response Time (MRT) timer to abolish abounding of the accidental IGMP Reports. This timer is
called the Concern Acknowledgment Interval. In added words, back any host receives an IGMPv2 Query, it
has a best of the configured MRT to accelerate the IGMP Abode if it wants to accept multicast
traffic for that application. Anniversary host that wants to accelerate the Solicited Host Membership Report
picks about a time amid 0 and the MRT and starts a timer. Back this timer expires, the host
will accelerate a report. However, if a host receives a abode beatific by addition host for the aforementioned multicast
group for which it was planning to accelerate the report, it does not accelerate the report. This is alleged Report
Suppression and is advised to abate bombastic reports. The assemblage of altitude for the MRT is
0.1 second. For example, a 3-second MRT is bidding as 30.
The afterward three steps, referenced in Figure 16-9 and a assiduity of the accomplish referenced in
Figure 16-8, call the arrangement of contest for the IGMPv2 Solicited Host Membership Report
and Abode Suppression mechanism:
R1
H1 H2 H3
H4
H5
.1 .2
.10
.10
Fa0/0
Fa0/1
.3
.4
.5
10.2.2.0/24
10.1.1.0/24
5
5 3 4
Process
Report
5 Process
Report
5 Discard
Report
4
3
I got a report
for 226.1.1.1 –
suppress my
report!
I got a concern – I need
to respond! First, I’ll
choose my MRT.
Timer expired, so send
report. “I would like to
receive multicast traffic
for 226.1.1.1.”
3 MRT = 3 MRT = 1
MRT = Best Acknowledgment Time in seconds, alleged about by hosts.
554 Affiliate 16: Introduction to IP Multicasting
1. Hosts H1 and H3 would like to accelerate IGMPv2 Solicited Host Membership Reports. Assume
that H1 and H3 accept accustomed an IGMPv2 Concern (as apparent in footfall 2 of Figure 16-8). Because
both H1 and H3 accept abutting the accumulation 226.1.1.1, they charge to accelerate an IGMPv2 Solicited Host
Membership Report. Further accept that H1 and H3 accept about best an MRT of
3 abnormal and 1 second, respectively.
2. H3’s timer expires in 1 second; it prepares and sends the IGMPv2 Solicited Host Membership
Report with the TTL bulk of 1. H3 uses the destination IP abode 226.1.1.1 and the source
IP abode 10.1.1.3, the destination MAC abode 01-00-5e-01-01-01 affected from the
Layer 3 abode 226.1.1.1, and its BIA abode as the antecedent address.
3. Hosts H1, H2, and R1 accept the IGMPv2 Solicited Host Membership Report, but alone H1
and R1 action the Report. The NIC of H2 discards the anatomy beatific by H3 because it is not
listening to the abode 01-00-5e-01-01-01. H1 realizes that H3 has already fabricated a appeal to
the router to advanced the cartage for the aforementioned multicast accumulation 226.1.1.1. Therefore, H1
suppresses its own Abode and does not accelerate it. By appliance the Accumulation Abode of 226.1.1.1, H3
is cogent the multicast router, “I would like to accept multicast cartage for accumulation 226.1.1.1.”
R1 has now accustomed the IGMPv2 Solicited Host Membership Abode on its fa0/0 interface
requesting cartage for multicast accumulation 226.1.1.1, but it has not accustomed a Host Membership Report
on its fa0/1 interface. Figure 16-10 shows that R1 has started forwarding multicast cartage for group
226.1.1.1 on its fa0/0 interface.
Figure 16-10 R1 Forwarding Cartage for Accumulation 226.1.1.1 on Its Fa0/0 Interface
R1
H1 H2 H3
H4
H5
.1 .2
.10
.10
Fa0/0
Fa0/1 S0/0
.3
.4
.5
10.2.2.0/24
10.1.1.0/24
Multicast Traffic
Destination Address: 226.1.1.1
Multicast Traffic
Destination Address: 226.1.1.1
Managing Distribution of Multicast Cartage with IGMP 555
IGMPv2 Unsolicited Host Membership Report
In IGMPv2, a multicast router sends IGMP Host Membership Concern letters by absence every
125 abnormal (Query Interval) on anniversary of its LAN interfaces to actuate whether any host wants
to accept multicast cartage for any group. However, a host does not accept to accept the Host
Membership Concern bulletin from the router to accelerate a Host Membership Report. A host can send
an IGMPv2 Unsolicited Host Membership Abode anytime a user launches a multicast application.
This affection reduces the cat-and-mouse time for a host to accept cartage for a multicast group. For
example, Figure 16-11 shows that a user has launched a multicast appliance that uses 226.1.1.1
on H4, which sends an IGMPv2 Unsolicited Host Membership Abode that will be accustomed by R1
on its fa0/1 interface, and R1 will again alpha forwarding cartage for 226.1.1.1 on its fa0/1 interface.
Figure 16-11 H4 Sends IGMPv2 Unsolicited Host Membership Report
IGMPv2 improves the Query/Report action by appliance the IGMPv2 Group-Specific Query. In
IGMPv2, back a host leaves a group, it sends an IGMPv2 Leave message. Back an IGMPv2
router receives a Leave message, instead of cat-and-mouse for the accustomed Concern Interval timer
(125 abnormal by default) to expire, the IGMPv2 router anon sends a Group-Specific Query
for that group. The Group-Specific Concern asks alone whether any actual hosts still appetite to
receive packets for that distinct multicast group. As a result, the router bound knows whether to
continue to advanced that multicast accumulation on that LAN.
The capital advantage of IGMPv2 over IGMPv1 is IGMPv2’s beneath leave latency. An IGMPv1
router takes, by default, 3 account to achieve that the aftermost host on the subnet has larboard a accumulation and
no host on the subnet wants to accept cartage for the group. Meanwhile, the IGMPv1 router
continues forwarding the accumulation cartage on the subnet and wastes bandwidth. On the added hand, an
IGMPv2 router concludes in 3 abnormal that no host on the subnet wants to accept cartage for a
group and stops forwarding it on the subnet.
R1
H1 H2 H3
H4
H5
.1 .2
.10
.10
Fa0/0
S0/0 Fa0/1
.3
.4
.5
10.2.2.0/24
10.1.1.0/24
Forward future
multicasts for 226.1.1.1
out Fa0/1.
2
1 Unsolicited Host
Membership Report
“I would like to receive
multicast traffic
for 226.1.1.1.”
556 Affiliate 16: Introduction to IP Multicasting
The functions of the IGMPv2 Leave bulletin and IGMPv2 Group-Specific Concern bulletin are
explained in detail in the abutting section.
IGMPv2 Leave Accumulation and Group-Specific Concern Messages
The IGMPv2 Leave Accumulation bulletin is acclimated to decidedly abate the leave latency, while the
IGMPv2 Group-Specific Concern bulletin prevents a router from afield endlessly the
forwarding of packets on a LAN back a host leaves a group. As a result, both of these IGMPv2
functions assignment together.
Figure 16-12 shows the operation of the IGMPv2 Leave action and the IGMP Group-Specific
Query. In Figure 16-12, hosts H1 and H3 are currently associates of accumulation 226.1.1.1; H1 wants to
leave the group.
Figure 16-12 How Group-Specific Queries Assignment with the IGMPv2 Leave Process
NOTE IGMPv2 RFC 2236 recommends that a host sends a Leave Accumulation bulletin alone if the
leaving affiliate was the aftermost host to accelerate a Membership Abode in acknowledgment to a Query. However,
most IGMPv2 bell-ringer operating systems accept implemented the Leave Accumulation processing by
always sending a Leave Accumulation bulletin back any host leaves the group.
R1
H1 H2 H3
H4
H5
.1 .2
.10
.10
Fa0/0
Fa0/1
.3
.4
.5
10.2.2.0/24
10.1.1.0/24
2
3
1
1 3
Host Membership Report
“I would like to receive
multicast cartage for
226.1.1.1.”
Multicast Traffic
Destination Address: 226.1.1.1
Multicast Traffic
Destination Address: 226.1.1.1
Multicast Traffic
Destination Address: 226.1.1.1
Leave Accumulation Message
“I don’t appetite to receive
multicast cartage for
226.1.1.1 anymore.”
2 Group-Specific Query
“Does anyone appetite to
receive multicast traffic
for 226.1.1.1?”
S0/0
Managing Distribution of Multicast Cartage with IGMP 557
The afterward three steps, referenced in Figure 16-12, call the arrangement of contest for the
IGMPv2 Leave apparatus back H1 leaves:
1. H1 sends an IGMPv2 Leave Accumulation message. The destination abode on the packet is
224.0.0.2, which is a acclaimed multicast abode for All Multicast Routers to acquaint all
routers on the subnet that, “I don’t appetite to accept multicast cartage for 226.1.1.1 anymore.”
2. R1 sends a Group-Specific Query. Routers do not accumulate clue of hosts that are associates of the
group, alone the accumulation memberships that are active. Because H1 has absitively to leave
226.1.1.1, R1 can stop forwarding cartage for 226.1.1.1 on its fa0/0 interface if H1 is the last
member of 226.1.1.1 on the interface. However, R1 needs to accomplish abiding that no added hosts of
this interface still charge to accept packets from accumulation 226.1.1.1. Therefore, R1 sends a Group-
Specific Concern to actuate whether any hosts are still associates of 226.1.1.1. R1 uses
226.1.1.1 as the destination abode on the packet so that alone hosts that are associates of this
group will accept the bulletin and respond. Through this message, R1 is allurement any
remaining hosts on the subnet, “Does anyone appetite to accept multicast cartage for 226.1.1.1?”
3. H3 sends a Membership Report. H3 is still a affiliate of accumulation 226.1.1.1. It hears the Group-
Specific Concern and responds with an IGMPv2 Membership Abode to acquaint the routers on
the subnet that it is still a affiliate of accumulation 226.1.1.1 and would like to accumulate accepting traffic
for accumulation 226.1.1.1.
IGMPv2 routers echo the action of Footfall 2 in this archetype anniversary time they accept a Leave
message as apparent in Footfall 1. In the antecedent example, the router (R1) did not stop sending traffic
as a aftereffect of the process. In the abutting example, H3 is the alone actual affiliate of accumulation 226.1.1.1
on the subnet. Accept that now H3 additionally wants to leave the group, as apparent in Figure 16-13.
Figure 16-13 IGMPv2 Leave Process—No Acknowledgment to the Group-Specific Query
NOTE The Abode Suppression apparatus explained beforehand for the Accepted Accumulation Concern is
also acclimated for the Group-Specific Query.
R1
H1 H2 H3
H4
H5
.1 .2
.10
.10
Fa0/0
Fa0/1
.3
.4
.5
10.2.2.0/24
10.1.1.0/24
2
1
1
Leave Accumulation Message
“I don’t appetite to receive
multicast cartage for
226.1.1.1 anymore.”
Multicast Traffic
Multicast Cartage Destination Address: 226.1.1.1
Destination Address: 226.1.1.1
3 “Nobody beatific a abode for
226.1.1.1 on this interface.
I will stop forwarding
traffic.”
2 Group-Specific Query
“Does anyone appetite to
receive multicast traffic
for 226.1.1.1?”
S0/0
558 Affiliate 16: Introduction to IP Multicasting
The afterward three steps, referenced in Figure 16-13, call the arrangement of contest for the
IGMPv2 Leave apparatus back H3 leaves:
1. H3 sends an IGMPv2 Leave Accumulation message. The destination abode on the packet is
224.0.0.2 to acquaint all routers on the subnet that, “I don’t appetite to accept multicast cartage for
226.1.1.1 anymore.”
2. R1 sends a Group-Specific Query. R1 receives the Leave Accumulation bulletin from H3. R1 sends
a Group-Specific Concern to actuate whether any hosts are still associates of accumulation 226.1.1.1.
R1 uses 226.1.1.1 as the destination abode on the packet so that alone hosts that are members
of this accumulation will accept the bulletin and respond.
3. No letters are received, so R1 stops forwarding accumulation traffic. Because there are now no
remaining associates of 226.1.1.1 on the subnet, R1 does not accept a acknowledgment to the Group-
Specific Concern from any host. As a result, R1 stops forwarding multicasts for 226.1.1.1 out
its fa0/1 interface.
Step 3 of this archetype provides a nice accomplishments from which to call the concepts of a Last
Member Concern Interval and a Aftermost Affiliate Concern Count. These ethics actuate how continued it
takes a router to accept that all hosts on a LAN accept larboard a accurate group. By default, routers
use an MRT of 10 (1 second) for Group-Specific Queries; because a router should accept a
response to a Group-Specific Concern in that bulk of time, the router uses the MRT bulk as the
value of the Aftermost Affiliate Concern Interval. So, the router uses the afterward process:
1. Accelerate a Group-Specific Concern in acknowledgment to an IGMP Leave.
2. If no Abode is accustomed aural the Aftermost Affiliate Concern Interval, echo Footfall 1.
3. Echo Footfall 1 the cardinal of times authentic by the bulk of the Aftermost Affiliate Concern Count.
The Aftermost Affiliate Concern Count is the cardinal of after Group-Specific Queries beatific for the
same accumulation afore the router concludes that there are no alive associates of the accumulation on a subnet.
The absence bulk for the Aftermost Affiliate Concern Count is 2. So the leave cessation is about beneath than
3 seconds, compared to up to 3 account with IGMPv1.
IGMPv2 Querier
IGMPv2 defines a querier acclamation action that is acclimated back assorted routers are affiliated to a
subnet. Back IGMPv2 routers start, they anniversary accelerate an IGMPv2 Accepted Concern bulletin to the
well-known All Hosts accumulation 224.0.0.1 appliance their interface abode as the antecedent address. When
an IGMPv2 router receives a Accepted Concern message, it compares the antecedent IP abode of the
General Concern bulletin with its own interface address. The router with the everyman IP abode on
the subnet is adopted as the IGMP querier. The nonquerier routers stop sending their queries but
monitor how frequently the querier is sending accepted IGMPv2 Queries. Back the adopted querier
does not accelerate a concern for two after Concern Intervals additional one bisected of one Concern Response
Managing Distribution of Multicast Cartage with IGMP 559
Interval, it is advised to be dead, and a new querier is elected. RFC 2236 refers to this time
interval as the Added Querier Present Interval. The absence bulk for the Added Querier Present
Interval is 255 seconds, because the absence Accepted IGMPv2 Concern Interval is 125 abnormal and
the absence Concern Acknowledgment Interval is 10 seconds.
IGMPv1 and IGMPv2 Interoperability
IGMPv2 is advised to be astern accordant with IGMPv1. RFC 2236 defines some special
interoperability rules. The abutting few sections analyze the afterward interoperability scenarios:
■ IGMPv2 Host and IGMPv1 Routers—Defines how an IGMPv2 host should behave in the
presence of an IGMPv1 router on the aforementioned subnet.
■ IGMPv1 Host and IGMPv2 Routers—Defines how an IGMPv2 router should behave in the
presence of an IGMPv1 host on the aforementioned subnet.
IGMPv2 Host and IGMPv1 Routers
When a host sends the IGMPv2 Abode with the bulletin blazon 0x16, which is not authentic in
IGMPv1, a adaptation 1 router would accede 0x16 an invalid bulletin blazon and avoid it. Therefore,
a adaptation 2 host charge accelerate IGMPv1 Letters back a adaptation 1 router is active. But how does an
IGMPv2 host ascertain the attendance of an IGMPv1 router on the subnet?
IGMPv2 hosts actuate whether the querying router is an IGMPv1 or IGMPv2 host based on the
value of the MRT acreage of the alternate accepted IGMP Query. In IGMPv1 Queries, this acreage is zero,
whereas in IGMPv2 it is nonzero and represents the MRT value. Back an IGMPv2 host receives
an IGMPv1 Query, it knows that the IGMPv1 router is present on the subnet and marks the
interface as an IGMPv1 interface. The IGMPv2 host again stops sending IGMPv2 messages.
Whenever an IGMPv2 host receives an IGMPv1 Query, it starts a 400-second Adaptation 1 Router
Present Timeout timer. This timer is displace whenever it receives an IGMPv1 Query. If the timer
expires, which indicates that there are no IGMPv1 routers present on the subnet, the IGMPv2 host
starts sending IGMPv2 messages.
IGMPv1 Host and IGMPv2 Routers
IGMPv2 routers can calmly actuate if any IGMPv1 hosts are present on a LAN based on whether
any hosts accelerate an IGMPv1 Abode bulletin (type 0x12) or IGMPv2 Abode bulletin (type 0x16).
Like IGMPv1 routers, IGMPv2 routers accelerate alternate IGMPv2 Accepted Queries. An IGMPv1 host
responds commonly because IGMPv2 Accepted Queries are actual agnate in architecture to IGMPv1
560 Affiliate 16: Introduction to IP Multicasting
Queries—except for the additional octet, which is abandoned by IGMPv1 hosts. So, an IGMPv2 router
will appraise all Letters to acquisition out if any IGMPv1 hosts abide on a LAN.
While an IGMPv2 router knows that an IGMPv1 host is present on a LAN, the router ignores
Leave letters and the Group-Specific Queries triggered by cancellation of the Leave messages. This
is all-important because if an IGMPv2 router responds to a Leave Accumulation bulletin with a Group-
Specific Query, IGMPv1 hosts will not accept it and appropriately avoid the message. Back an
IGMPv2 router does not accept a acknowledgment to its Group-Specific Query, it may erroneously
conclude that cipher wants to accept cartage for the accumulation and appropriately stop forwarding it on the
subnet. So with one or added IGMPv1 hosts alert for a accurate group, the router essentially
suspends the optimizations that abate leave latency.
IGMPv2 routers abide to avoid Leave letters until the IGMPv1-host-present countdown
timer expires. RFC 2236 defines that back IGMPv2 routers accept an IGMPv1 Report, they must
set an IGMPv1-host-present admission timer. The timer bulk should be according to the Group
Membership Interval, which defaults to 180 abnormal in IGMPv1 and 260 abnormal in IGMPv2.
(Group Membership Interval is a time aeon during which, if a router does not accept an IGMP
Report, the router concludes that there are no added associates of the accumulation on a subnet.)
IGMPv2 Timers
Table 16-4 summarizes important timers acclimated in IGMPv2, their usage, and absence values.
NOTE If IGMPv2 hosts are additionally present on the aforementioned subnet, they would accelerate IGMPv2
Membership Reports. However, IGMPv1 hosts do not accept IGMPv2 Letters and ignore
them; they do not activate Abode Suppression in IGMPv1 hosts. Therefore, sometimes an
IGMPv2 router receives both an IGMPv1 Abode and an IGMPv2 Abode in acknowledgment to a
General Query.
Table 16-4 Important IGMPv2 Timers
Timer Acceptance Absence Value
Query Interval A time aeon amid Accepted Queries beatific by a
router.
125 seconds
Query Response
Interval
The best acknowledgment time for hosts to acknowledge to
the alternate accepted Queries.
10 seconds; can be
between .1 and 25.5
seconds
Group
Membership
Interval
A time aeon during which, if a router does not
receive an IGMP Report, the router concludes that
there are no added associates of the accumulation on the
subnet.
260 seconds
Managing Distribution of Multicast Cartage with IGMP 561
IGMP Adaptation 3
In October 2002, RFC 3376 authentic blueprint for IGMPv3, which is a above afterlight of the
protocol and is actual complex. To use the new appearance of IGMPv3, last-hop routers accept to be
updated, host operating systems accept to be modified, and applications accept to be specially
designed and written. At the time of this autograph (mid-2007), a bound cardinal of IGMPv3
applications are available. Therefore, this breadth does not appraise IGMPv3 in detail; instead, it
summarizes IGMPv3’s above features.
In IGMPv1 and IGMPv2, back a host makes a appeal to accompany a group, a multicast router forwards
the cartage for the accumulation to the subnet behindhand of the antecedent IP abode of the packets. For
example, accept that a multimedia appointment is in session. A accumulation affiliate decides to
maliciously afflict the affair by sending “bogus abstracts or noise” by either talking or sending
music to the aforementioned group. Although multimedia applications acquiesce a user to aphasiac any of the other
members, it does not stop the exceptionable cartage from actuality delivered to the host. If a accumulation of
hackers decides to flood a company’s arrangement with artificial high-bandwidth abstracts appliance the same
multicast accumulation abode that the company’s advisers accept joined, it can actualize a DoS advance for
the aggregation by cutting low-speed links. Neither IGMPv1 nor IGMPv2 has a mechanism
to anticipate such an attack.
IGMPv3 allows a host to clarify admission cartage based on the antecedent IP addresses from which it is
willing to accept packets, through a affection alleged Source-Specific Multicast (SSM). IGMPv3 is
designed to abutment antecedent filtering. It allows a host to announce absorption in accepting packets only
from specific antecedent addresses, or from all but specific antecedent addresses, beatific to a particular
multicast address. Figure 16-14 shows basal operation of the IGMPv3 Membership Report
process.
Timer Acceptance Absence Value
Other Querier
Present Interval
A time aeon during which, if the IGMPv2 nonquerier
routers do not accept an IGMP Concern from
the querier router, the nonquerier routers conclude
that the querier is dead.
255 seconds
Last Member
Query Interval
The best acknowledgment time amid by IGMPv2
routers into the Group-Specific Queries and the time
period amid two after Group-Specific
Queries beatific for the aforementioned group.
1 second
Version 1 Router
Present Timeout
A time aeon during which, if an IGMPv2 host does
not accept an IGMPv1 Query, the IGMPv2 host
concludes that there are no IGMPv1 routers present
and starts sending IGMPv2 messages.
400 seconds
Table 16-4 Important IGMPv2 Timers (Continued)
562 Affiliate 16: Introduction to IP Multicasting
Figure 16-14 IGMPv3 Membership Report
In Figure 16-14, the multicast cartage for the accumulation 226.1.1.1 is accessible from two sources. R1
receives cartage from both the sources. H1 prepares an IGMPv3 Membership Abode appliance the
destination abode 224.0.0.22, distinctively assigned by IANA for the IGMPv3 Membership Report.
The bulletin blazon is 0x22 (defined in RFC 3376), with a agenda “Source–INCLUDE—
209.165.201.2,” which means, “I would like to accompany multicast accumulation 226.1.1.1, but alone if the
group cartage is advancing from the antecedent 209.165.201.2.”
How does a host apprentice accumulation antecedent addresses? A lot of assignment charcoal to be done by application
designers to advance SSM applications. Cisco has advised URL Rendezvous Directory (URD)
and IGMP v3lite to use the new appearance of IGMPv3 until IGMPv3 applications are accessible and
operating systems are updated. A abundant altercation of URD and IGMP v3lite is above the scope
of this book. IGMPv3 is accordant with IGMPv1 and IGMPv2.
Comparison of IGMPv1, IGMPv2, and IGMPv3
Table 16-5 compares the important appearance of IGMPv1, IGMPv2, and IGMPv3.
NOTE The afterward URL provides added advice on URD and IGMP v3lite:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t5/
dtigmpv3.htm
R1
H1
Forward packets to
226.1.1.1, but alone if
sourced from
209.165.201.2.
The
Internet
R2 R3
Multicast Traffic
Destination Address: 226.1.1.1
Source Address: 209.165.201.2
Multicast Traffic
Destination Address: 226.1.1.1
Source Address: 209.165.202.130
IGMPv3 MeMbpsership Report
Join: 226.1.1.1
Source: INCLUDE – 209.165.201.2
Managing Distribution of Multicast Cartage with IGMP 563
Table 16-5 Comparison of IGMPv1, IGMPv2, and IGMPv3
Feature IGMPv1 IGMPv2 IGMPv3
First Octet Bulk for the
Query Message
0x11 0x11 0x11
Group Abode for the
General Query
0.0.0.0 0.0.0.0 0.0.0.0
Destination Abode for
the Accepted Query
224.0.0.1 224.0.0.1 224.0.0.1
Default Concern Interval 60 abnormal 125 abnormal 125 seconds
First Octet Bulk for the
Report
0x12 0x16 0x22
Group Abode for the
Report
Joining multicast
group address
Joining multicast
group address
Joining multicast group
address and source
address
Destination Abode for
the Report
Joining multicast
group address
Joining multicast
group address
224.0.0.22
Is Abode Suppression
Mechanism Available?
Yes Yes No
Can Maximum
Response Time Be
Configured?
No, anchored at 10
seconds
Yes, 0 to 25.5
seconds
Yes, 0 to 53 minutes
Can a Host Accelerate a Leave
Group Message?
No Yes Yes
Destination Abode for
the Leave Group
Message
224.0.0.2 224.0.0.22
Can a Router Accelerate a
Group-Specific Query?
No Yes Yes
Can a Host Accelerate Sourceand
Group-Specific
Reports?
No No Yes
Can a Router Send
Source- and Group-
Specific Queries?
No No Yes
Rule for Electing a
Querier
None—depends
on multicast
routing protocol
Router with the
lowest IP address
on the subnet
Router with the everyman IP
address on the subnet
Compatible with Other
Versions of IGMP?
No Yes, alone with
IGMPv1
Yes, with both IGMPv1
and IGMPv2