Multicast Addressing
This area covers multicast Class D acclamation anatomy and range, as able-bodied as how to map
Ethernet MAC addresses to Class D IP addresses.
Multicast Abode Structure
IP multicasting is the manual of an IP abstracts anatomy to a multicast group, articular by a
single IP address. Because the multicast accumulation is articular by a distinct IP abode rule, the IP
multicast datagram contains a specific aggregate of the destination MAC abode and a
destination IP address.
The ambit of IP addresses is disconnected into classes based on the aerial adjustment $.25 of a 32-bit IP
address. IP multicast uses Class D addresses. A Class D abode consists of 1110 as the higher
order $.25 in the aboriginal octet, followed by a 28-bit accumulation address. Unlike Class A, B, and C IP
addresses, the aftermost 28 $.25 of a Class D abode are unstructured, as illustrated by Figure 10-5.
Figure 10-5 IP Multicast Uses Class D Addresses
These actual 28 $.25 of the IP abode analyze the multicast accumulation ID. This multicast group
ID is a distinct abode about accounting as decimal numbers in the ambit 224.0.0.0 through
239.255.255.255. The high-order $.25 in the aboriginal octet analyze this 224-base address.
Multicast addresses may be dynamically or statically allocated. Activating multicast addressing
provides applications with a accumulation abode on demand. Because activating multicast addresses
have a specific lifetime, applications charge appeal this blazon of abode alone for as continued as it is
needed.
Statically allocated addresses are aloof for specific protocols that crave well-known
addresses. The Internet Assigned Numbers Authority (IANA) assigns these well-known
addresses. These addresses are alleged abiding host groups and are agnate in abstraction to the
well-known TCP and UDP anchorage numbers. Table 10-2 lists some of the “well-known” Class D
addresses.
Address 224.0.0.1 identifies the all-hosts group. Every multicast-capable host charge accompany this
group at the start. If a ping command is issued application this address, all multicast-capable hosts
on the arrangement charge acknowledgment the ping request.
Address 224.0.0.2 identifies the all-routers group. Multicast routers charge accompany that accumulation on all
multicast-capable interfaces.
Class D 1 1 1 0 Multicast Accumulation ID
28 bits
344 Affiliate 10: Multicasts
Addresses alignment from 224.0.0.0 through 224.0.0.255 are aloof for bounded purposes, such as
administrative and aliment tasks. Multicast routers do not advanced datagrams destined to
this ambit of addresses.
Similarly, the abode ambit 239.0.0.0 to 239.255.255.255 is aloof for administrative
scoping. An authoritative ambit area is authentic by a set of routers surrounding a arena within
the network. These routers are configured to abjure multicast cartage in a accurate abode range
from entering or abrogation the zone. This abode is advantageous in absolute high-bandwidth traffic
to a specific arena in the campus network. Authoritative scoping is categorical in RFC 2365,
Administratively Scoped IP Multicast.
You can retrieve a certificate absolute the accepted account of multicast assigned abode at
ftp://ftp.isi.edu/in-notes/iana/assignments/.
Mapping IP Multicast Addresses to Ethernet
Ethernet frames accept a 48-bit destination abode field. To abstain invoking the Address
Resolution Agreement (ARP) to map multicast IP addresses to Ethernet addresses, the IANA
designated a ambit of Ethernet addresses for multicast. The lower 23 $.25 of the Class D address
are mapped into a block of Ethernet addresses that accept been aloof for multicast. This block
includes addresses in the ambit 00:00:5e:00:00:00 through 00:00:5e:ff:ff:ff. The IANA
allocates bisected of this block for multicast addresses. Accustomed that the aboriginal byte of any Ethernet
address charge be 01 to specify a multicast address, the Ethernet addresses agnate to IP
multicasting are in the ambit 01:00:5e:00:00:00 through 01:00:5e:7f:ff:ff.
NOTE Multicast MAC and IP addresses alone accord in destination addresses.
Table 10-2 Well-Known Class D Addresses
Well-Known Class D Abode Purpose
224.0.0.1 All hosts on a subnet
224.0.0.2 All routers on a subnet
224.0.0.4 All Distance Vector Multicast Acquisition Protocol
(DVMRP) routers
224.0.0.5 All Open Shortest Aisle Aboriginal (OSPF) routers
224.0.0.6 All OSPF appointed routers
224.0.0.9 All Acquisition Advice Protocol, adaptation 2 (RIP-
2) routers
224.0.0.13 All Agreement Independent Multicast (PIM) routers
Managing Multicast Cartage 345
The prefix 01-00-5e identifies the anatomy as multicast; the abutting bit is consistently 0, abrogation alone 23
bits for the multicast address. Because IP multicast groups are 28-bits long, the mapping cannot
be one-to-one. Alone the 23 least-significant $.25 of the IP multicast accumulation are placed in the
frame. The actual bristles high-order $.25 are ignored, consistent in 32 altered multicast groups
being mapped to the aforementioned Ethernet address. For example, if the IP multicast accumulation abode is
224.10.8.5, the destination MAC abode becomes 01-00-5E-0A-8-5.
When mapping multicast addresses to MAC addresses, all 28 $.25 of the IP multicast cannot be
mapped to the accessible 23 $.25 of MAC abode space, acceptation that bristles $.25 of abode are lost.
This additionally agency that 32, or 25, addresses can be cryptic back mapped. Therefore, anniversary IP
multicast MAC abode is able of apery 32 IP multicast addresses.
Figure 10-6 illustrates a multicast accumulation abode actuality mapped into an IEEE-802 multicast
address. In multicast addressing, the high-order nine $.25 of the IP abode are not mapped into
the MAC-layer multicast address. In the archetype in Figure 10-6, the mapping places the loworder
23 $.25 of the IP multicast accumulation ID into the low adjustment 23 $.25 of the IEEE-802 multicast
address.
Figure 10-6 Mapping Multicast Abode to Ethernet
Managing Multicast Traffic
In the apple of multicasting, the abstraction of sending a multicast aural a distinct broadcast
domain is a somewhat atomic process. However, back we extend this into assorted segments
within a campus ambiance over assorted switches and routers, we complicate matters
significantly.
The sending activity specifies a destination abode authentic as a multicast address. The device
driver in the sending server converts this abode to the agnate Ethernet abode and
sends the packet out on the network. The accepting devices, or clients, charge announce that they
want to accept datagrams destined for a accustomed multicast address. Complications appear when
multicasting is continued aloft a distinct concrete arrangement and multicast packets canyon through
routers.
1110
000000010000000001011110 0
01 00 5E
0 7 8 15 16 23 24 31
224 Not Low-Order 23 $.25 of Multicast
Group ID Copied to
Ethernet Address
Used
IP Multicast Address
48-Bit Ethernet Address
346 Affiliate 10: Multicasts
Sending and accepting multimedia crave allocation from all accessories accommodating in the
multicast. These accessories accommodate the server, the host, the router, and the switch. Some of the
issues in facilitating multimedia cartage in the campus arrangement are
• Coordinating the multicast operations of the altered accessories in the network
• Establishing a aisle amid antecedent and destination devices
• Forwarding multicast cartage through the network
IP multicast cartage for a accurate source/destination accumulation brace is transmitted from the source
to the multicast accumulation via a administration tree. This administration timberline connects all the hosts in the
group. Altered IP multicast acquisition protocols use altered techniques to assemble these
multicast Spanning Trees; afterwards the timberline is constructed, however, all multicast cartage for a
specific accumulation is broadcast over this tree.
Before multicast cartage can bisect the network, routers charge to apperceive which hosts, if any, on
a specific concrete arrangement accord to a accustomed multicast group. Because the arising campus
network archetypal is comprised of both routers and switches, switches additionally charge to apperceive how to
direct multicast traffic. Cisco switches do this through the use of CGMP, which is discussed
later in this chapter.
Subscribing and Maintaining Groups
The Internet Accumulation Management Agreement (IGMP) provides a agency to abode their multicast
group associates with adjoining multicast routers. The IGMP protocol, adaptation 1, is
defined in RFC 1112, Host Extensions for IP Multicasting. RFC 2236, Internet Group
Management Protocol, Adaptation 2, defines IGMP, adaptation 2.
IGMP manages multicast cartage throughout networks through the use of adapted multicast
queriers and hosts. A querier is a arrangement device, such as a router, that sends IGMP queries. A
set of queriers and hosts that accept multicast abstracts streams from the aforementioned antecedent is alleged a
multicast group. Queriers and hosts use IGMP letters to accompany and leave multicast groups.
IGMP supports two specific bulletin structures, as follows:
• Affair letters are acclimated to ascertain which arrangement accessories are associates of a given
multicast group.
• Abode letters are beatific by hosts in acknowledgment to affair letters to acquaint the querier of
a host membership.
NOTE Aural the IP multicast archetypal there is no angle of associates amid a antecedent and the
receivers. A antecedent does not accept to be a affiliate of a accumulation to accelerate cartage to that group.
Conversely, receivers do acquaint routers of what groups the receivers appetite to accept membership
in so that the router can advanced the adapted cartage flows.
Managing Multicast Cartage 347
Several versions of IGMP are available; IGMP versions 1 and 2 are now in production, with
version 3 in development. Anniversary adaptation has its own set of behavior characteristics.
IGMP Adaptation 1
IGMP uses IP datagrams to abode advice about multicast groups. The datagram consists
of a 20-byte IP attack and an 8-byte IGMP message.
According to the IGMPv1 specification, one multicast router per LAN charge periodically
transmit Host Associates Affair letters to actuate which host groups accept associates on
the querier’s anon absorbed networks. IGMP affair letters are addressed to the all-host
group (224.0.0.1) and accept an IP Time-To-Live (TTL) according to one. This TTL ensures that the
Query letters sourced from a router are transmitted assimilate the anon absorbed arrangement but
are not forwarded by any added multicast routers.
When the end base receives an IGMP affair message, the end base responds with a host
membership abode for anniversary accumulation into which the end base belongs.
IGMP letters are authentic in the IP datagram with a agreement amount of 2. Table 10-3
describes the fields of the IGMP message.
Joining a Accumulation Application IGMP Adaptation 1
Hosts abutting a accumulation do not accept to adjournment for a affair to join. Back a host wants to accompany a
multicast group, the host sends a Host Associates Abode to the accumulation address. This
unsolicited appeal reduces accompany cessation for the end arrangement back no added associates of that
group are present on that arrangement segment.
Table 10-3 IGMPv1 Bulletin Format Fields
Field Name Value
Type Two types of IGMP letters are of affair to hosts:
1 = Host Associates Query
2 = Host Associates Report
Unused Unused field, zeroed back sent, abandoned back received.
Checksum The checksum is the 16-bit one’s accompaniment of the one’s accompaniment sum of the 8-
octet IGMP message. For accretion the checksum, the checksum acreage is zeroed.
Group Abode In a Host Associates Affair message, the accumulation abode acreage is zeroed back sent,
ignored back received. In a Host Associates Abode message, the accumulation address
field holds the IP host accumulation abode of the accumulation actuality reported.
348 Affiliate 10: Multicasts
General Queries Application IGMP Adaptation 1
Multicast routers accelerate Host Associates Affair letters to ascertain which host groups have
members on their absorbed bounded networks. Accepted queries go to the all-hosts (224.0.0.1)
multicast abode and backpack a TTL of one (1). One affiliate from anniversary accumulation on the articulation will
respond with a report. Accepted queries are beatific out periodically based on the ambience of the ip
igmp query-interval command. (The absence ambience is 60 seconds.)
No academic IGMP affair router acclamation activity exists aural IGMPv1 itself. Instead, the
election activity is larboard up to the multicast acquisition protocol, and altered protocols use different
mechanisms. This activity generally after-effects in assorted queriers on a distinct arrangement segment
supporting assorted multicast-enabled routers.
Membership Queries Application IGMP Adaptation 1
To ensure the activity of accumulation associates on a accustomed arrangement segment, the router multicasts
periodic IGMPv1 associates queries to the all-hosts (224.0.0.1) accumulation address. Alone one
member per accumulation responds with a abode to a query. This activity saves bandwidth on the
network articulation and processing by the hosts. This activity is alleged abode suppression. The
report abolishment apparatus is able as follows:
When a host receives the query, it starts a admission timer for anniversary multicast accumulation for
which the host is a member. The admission timers are anniversary initialized to a accidental count
within a accustomed time range. In IGMPv1, the time ambit is a anchored ambit of 10 seconds.
Therefore, the admission timers are about set to some amount amid 0 and 10
seconds.
When a admission timer alcove zero, the host sends a associates abode for the group
associated with the admission timer to acquaint the router that the accumulation is still active.
However, if a host receives a associates abode afore the associated admission timer
reaches zero, the host cancels the admission timer associated with the multicast group
thereby suppressing the host’s own report.
Leaving a Accumulation Application IGMP Adaptation 1
No adapted leave apparatus was authentic in IGMPv1. Instead, IGMPv1 hosts leave a group
passively or agilely at any time afterwards any notification to the router.
Multicast routers periodically abode IGMP queries to brace their ability of the group
members present on anniversary arrangement interface. This activity updates the bounded accumulation database of
the router. Eventually, the router should be able to ascertain that no associates of a accumulation are present
on an interface any best and, if possible, abolish itself from the multicast commitment timberline for this
group. If the router does not accept a abode from any associates of a accumulation afterwards a cardinal of
queries, the router assumes that no accumulation associates are present on a accurate interface.
Managing Multicast Cartage 349
When a router is aloof starting up or if multicast acquisition has aloof been enabled, a router may send
several IGMP queries in accelerated assumption in adjustment to bound apprentice which groups accept local
members.
IGMP Adaptation 2
As a aftereffect of some of the limitations apparent in IGMPv1, assignment began on IGMPv2 in an
attempt to abolish these limitations. Most of the changes amid IGMPv1 and IGMPv2 are
primarily to abode the issues of Leave and Accompany latencies, as able-bodied as abode ambiguities in the
original agreement specification
Version 2 of IGMP fabricated some enhancements to the antecedent version, including the definition
of a Group-Specific Query. This blazon of bulletin allows the router to abode a Specific Query
to one accurate group. IGMPv2 additionally defines a Leave Accumulation Bulletin for the hosts, which
results in lower leave latency.
Four types of IGMP letters are of affair to the host-router interaction:
• Associates query
• Adaptation 2 associates report
• Leave report
• Adaptation 1 associates report
The Adaptation 1 associates abode is acclimated for backward-compatibility with IGMPv1. New
message types may be acclimated by newer versions of IGMP or by multicast acquisition protocols. Any
unrecognized or added bulletin types are silently ignored.
Table 10-4 provides a description of the IGMPv2 bulletin fields.
Table 10-4 IGMPv2 Bulletin Fields
Field Name Value
Type 0 x 11 = Associates Query
0 x 12 = Adaptation 1 Associates Report
0 x 16 = Adaptation 2 Associates Report
0 x 17 = Leave Report
0 x 12 = Adaptation 1 Associates Report
Maximum Acknowledgment Time 10 abnormal = Absence value. Meaningful alone in a associates query.
Specifies the best accustomed time afore sending a responding report
in units of 1/10 second.
0 = All added messages.
continues
350 Affiliate 10: Multicasts
Joining a Accumulation Application IGMP v2
The activity of abutting a multicast accumulation is the aforementioned in IGMPv2 as it is in IGMPv1. Like
IGMPv1, IGMPv2 hosts abutting a accumulation do not accept to adjournment for a affair to join. Back a host
wants to accompany a multicast group, the host sends a host associates abode to the multicast group
address.
A host sends an IGMP accompany bulletin back the host wants to accompany a multicast group. If the host
and server abide in altered subnets, the accompany bulletin charge go to a router. Back the router
intercepts the message, the router looks at its IGMP table. If the arrangement cardinal is not in the
table, the router adds the advice independent in the IGMP message.
Using queries and reports, a multicast router builds a table account which of the router
interfaces accept one or added hosts in a multicast group. Back the router receives a multicast
datagram, the router assiduously the datagram to alone those interfaces that accept hosts with
processes acceptance to that group.
After a host has abutting a multicast group, the host appears in the router’s accumulation database.
Querier Acclamation Application IGMPv2
IGMPv2 defines a activity for acclamation of the multicast querier for anniversary arrangement segment. In
IGMPv2, the multicast router with the everyman IP abode on the LAN articulation is adopted the
multicast querier.
Initially, every router on the articulation believes itself to be the querier for every one of the router’s
interfaces that are multicast-enabled. Back a router is aboriginal multicast-enabled, the router begins
transmitting affair messages. If the router after detects a queried bulletin that is
sourced from a numerically lower IP address, the router ceases to act as a querier on that
interface as approved in Figure 10-7.
The Query-Interval Acknowledgment time has been added to IGMPv2 to ascendancy the burstiness of
reports. This amount is adumbrated in queries to back to the associates the time breach in
which associates accept to acknowledge to a affair with a abode message.
Checksum Calculated the aforementioned as for the ICMP checksum.
Group Abode 0 in a accepted query.
Group abode queried in a Accumulation Specific Query.
Multicast accumulation abode in a report.
Table 10-4 IGMPv2 Bulletin Fields (Continued)
Field Name Value
Managing Multicast Cartage 351
Figure 10-7 IGMPv2 Multicast Querier
A Group-Specific Affair additionally was added in IGMPv2 to acquiesce the router to affair membership
in alone a distinct accumulation instead of all groups. This accession is an optimized way to bound find
out if any associates are larboard in a accumulation afterwards allurement all groups for a report.
The aberration amid the Group-Specific affair and the Accepted Affair is that a General
Query is multicast to the all-hosts (224.0.0.1) abode while a Group-Specific Affair for group
G is multicast to the accumulation G multicast address.
To locate and verify the adopted querier, admission the appearance ip igmp command in user or privileged
EXEC approach as approved in Archetype 10-1.
Example 10-1 Locating the Adopted Querier with appearance ip igmp
RTR100>show ip igmp interface blazon number
Ethernet0 is up, band agreement is up
Internet abode is 198.92.37.6, subnet affectation is 255.255.255.0
IGMP is enabled on interface
IGMP affair breach is 60 seconds
Inbound IGMP admission accumulation is not set
Multicast acquisition is enabled on interface
Multicast TTL beginning is 0
Multicast appointed router (DR) is 198.92.37.33
No multicast groups joined
continues
Query Affair Query
192.168.12.1 192.168.12.2 192.168.12.3
Lowest IP abode = Querier Non-queriers
352 Affiliate 10: Multicasts
The appointed router is a altered activity and is listed alone in the affectation above. The
concept of the appointed router is discussed in added detail in Affiliate 11, “Configuring
Multicast Networks.”
Maintaining a Accumulation Application IGMPv2
Similar to IGMPv1, the IGMPv2 router multicasts alternate Associates Queries to the allhosts
(224.0.0.1) accumulation address. Alone one affiliate per accumulation responds with a abode to a query.
All added accumulation associates abolish their Associates Reports.
When a host receives a Accepted Query, the host sets adjournment timers for anniversary group, excluding the
all-systems accumulation (of which the host is a affiliate on the interface from which the affair was
received). Anniversary timer is set to a altered accidental value, application the accomplished alarm granularity
available on the host.
When a host receives a Group-Specific Query, the host sets a adjournment timer to a accidental amount for
the accumulation actuality queried if the host is a affiliate of that group. If a timer for the accumulation is already
running, the host timer is displace to the accidental amount alone if the requested max acknowledgment time is
less than the actual amount of the active timer. Back a accumulation timer expires, the host
multicasts a Adaptation 2 Associates Abode to the group, with an IP TTL of 1.
Leaving a Accumulation Application IGMPv2
A Leave Accumulation bulletin was additionally added in IGMPv2. Whenever any end base wants to leave
a group, the host transmits a Leave Accumulation bulletin to the all-routers accumulation (224.0.0.2) with the
group acreage advertence the accumulation actuality left. This manual allows end systems to acquaint the
router the hosts are abrogation the group. Thus, this activity reduces the leave cessation for the group
on the articulation back the affiliate abrogation is the aftermost affiliate of the group.