Introduction to IP Multicasting
LAN Multicast Optimizations
This final above area of this affiliate introduces the basics of three accoutrement that optimize the flow
of multicast over a LAN. Specifically, this area covers the afterward topics:
■ Cisco Accumulation Management Agreement (CGMP)
■ IGMP snooping
■ Router-Port Accumulation Management Agreement (RGMP)
Cisco Accumulation Management Protocol
IGMP helps routers to actuate how to administer multicast traffic. However, IGMP works at
Layer 3, and switches do not accept IGMP messages. Switches, by default, flood multicast
traffic to all the hosts in a advertisement domain, which wastes bandwidth. Figure 16-15 illustrates the
problem.
Figure 16-15 Switches Flood Multicast Traffic
Hosts H1, H2, H3, H4, and R1 are all in the aforementioned advertisement area of VLAN 5. The following
three steps, referenced in Figure 16-15, call the arrangement of contest back H3 sends an IGMP
Join message:
1. H3 sends an IGMP Accompany bulletin for accumulation 226.6.6.6.
SW1
SW2
R1
H1
Why am I getting
the accumulation traffic?
Group
226.6.6.6
H3
H2
H4
3
Why am I getting
the accumulation traffic?
3
Why am I getting
the accumulation traffic?
3
1
2
2
2
2
2
IGMP Join
226.6.6.6.
Trunk VLAN 5
VLAN 5 VLAN 5
VLAN 5 VLAN 5
IGMP Accompany 226.6.6.6
Multicast to 226.6.6.6
LAN Multicast Optimizations 565
2. R1 assiduously the accumulation cartage to SW1. The destination MAC abode on the anatomy is
0x0100.5e06.0606. SW1 cannot acquisition this abode in its CAM table because it is never acclimated by
any accessory as a antecedent address. Therefore, SW1 starts forwarding the accumulation cartage to H1, H2,
and SW2 because the accumulation cartage is for VLAN 5. Similarly, SW2 starts forwarding the group
traffic to H3 and H4.
3. All the hosts, H1 to H4, accept the accumulation traffic, but alone H3 requested it. H3 requested the
group cartage and has started accepting it. However, H1, H2, and H4 did not ask for the group
traffic, and they are abounding by switches with the accumulation traffic.
In this illustration, alone four hosts are apparent in the advertisement area of VLAN 5. What happens
if a advertisement area is collapsed and has hundreds of users? If a distinct host joins a multicast group, all
the hosts would be abounding with the accumulation cartage whether they accept requested the accumulation traffic.
The ambition of multicasting is to bear the accumulation cartage to alone those hosts that accept requested it
and aerate the use of bandwidth.
There are two accepted methods for allowance Layer 2 switches actuate how to administer the
multicast cartage to hosts:
■ CGMP, which is Cisco proprietary and discussed throughout the blow of this section.
■ IGMP snooping, discussed in the abutting section.
CGMP, a Layer 2 protocol, is configured on both a Cisco router and switches and permits the
router to acquaint Layer 2 advice it has abstruse from IGMP to switches. A multicast
router knows the MAC addresses of the multicast hosts, and the groups to which they listen, based
on IGMP advice with hosts. The ambition of CGMP is to accredit the router to communicate
this advice through CGMP letters to switches so that switches can dynamically modify
their CAM table entries. Alone the routers aftermath CGMP messages, while switches alone accept to
the CGMP messages. To do this, CGMP charge be enabled at both ends of the router-switch
connection over which CGMP is operating, because both accessories charge apperceive to use CGMP.
The destination abode on the CGMP letters is consistently the acclaimed CGMP multicast MAC
address 0x0100.0cdd.dddd. The use of the multicast destination MAC abode on the CGMP
messages armament switches to flood the bulletin through all the ports so that all the switches in a
network accept the CGMP messages. The important advice in the CGMP letters is one
or added pairs of MAC addresses:
■ Accumulation Destination Abode (GDA)
■ Unicast Antecedent Abode (USA)
The afterward bristles accomplish call the accepted action of CGMP. Later, these accomplish are explained
using a abundant example.
566 Affiliate 16: Introduction to IP Multicasting
1. Back a CGMP-capable router gets affiliated to the switch, it sends a CGMP Accompany message
with the GDA set to aught and the USA set to its own MAC address. The CGMP-capable
switch now knows that a multicast router is affiliated to the anchorage on which it accustomed the
router’s CGMP message. The router repeats the bulletin every 60 seconds. A router can also
tell the about-face that it no best participates in CGMP by sending a CGMP Leave bulletin with
the GDA set to aught and the USA set to its own MAC address.
2. Back a host joins a group, it sends an IGMP Accompany message. Normally, a multicast router
examines alone Layer 3 advice in the IGMP Accompany message, and the router does not have
to action any Layer 2 information. However, back CGMP is configured on a router, the
router additionally examines the Layer 2 destination and antecedent MAC addresses of the IGMP Join
message. The antecedent abode is the unicast MAC abode of the host that beatific the IGMP Join
message. The router afresh generates a CGMP Accompany bulletin that includes the multicast MAC
address associated with the multicast IP abode (to the GDA acreage of the CGMP join) and the
unicast MAC abode of the host (to the USA acreage of the CGMP message). The router sends
the CGMP Accompany bulletin application the acclaimed CGMP multicast MAC address
0x0100.0cdd.dddd as the destination address.
3. Back switches accept a CGMP Accompany message, they chase in their CAM tables for the port
number associated with the host MAC abode listed in the USA field. Switches actualize a new
CAM table access (or use an absolute access if it was already created before) for the multicast
MAC abode listed in the GDA acreage of the CGMP Accompany message, add the anchorage number
associated with the host MAC abode listed in the USA acreage to the entry, and advanced the
group cartage on the port.
4. Back a host leaves a group, it sends an IGMP Leave message. The router learns the host’s
unicast MAC abode (USA) and the IP multicast accumulation it has aloof left. Because the Leave
messages are beatific to the All Multicast Routers MAC abode 0x0100.5e00.0002 and not to the
multicast accumulation abode the host has aloof left, the router calculates the multicast MAC address
(GDA) from the IP multicast accumulation the host has aloof left. The router afresh generates a CGMP
Leave message, copies the multicast MAC abode it has aloof affected in the GDA acreage and
unicast MAC abode in the USA acreage of the CGMP Leave message, and sends it to the wellknown
CGMP multicast MAC address.
5. Back switches accept a CGMP Leave message, they afresh chase for the anchorage number
associated with the host MAC abode listed in the USA field. Switches abolish this anchorage from
the CAM table access for the multicast MAC abode listed in the GDA acreage of the CGMP
Leave bulletin and stop forwarding the accumulation cartage on the port.
Thus, CGMP helps switches accelerate accumulation cartage to alone those hosts that appetite it, which helps to
avoid ashen bandwidth.
Figure 16-16, 16-17, and 16-18 appearance a complete archetype of how routers and switches use CGMP
in acknowledgment to a host abutting and afresh abrogation a group. Figure 16-16 begins the archetype by
LAN Multicast Optimizations 567
showing a router’s acknowledgment to an IGMP Report, which is to accelerate a CGMP Accompany to the switches on
a LAN. The afterward two steps, referenced in Figure 16-16, call the arrangement of contest when
H3 sends an IGMP Accompany message:
1. H3 sends an IGMP Accompany bulletin for 226.6.6.6. At Layer 2, H3 uses 0x0100.5e06.0606 (the
multicast MAC abode associated with 226.6.6.6) as the destination abode of a anatomy and
its own BIA 0x0006.7c11.1103 as the antecedent MAC address.
2. R1 generates a CGMP Accompany message. Back a CGMP-capable router receives an IGMP Join
message, it generates a Layer 2 CGMP Accompany message. The destination abode on the anatomy is
the acclaimed multicast MAC abode 0x0100.0cdd.dddd, which is accepted alone by
Cisco switches but is forwarded by all switches. R1 sets the GDA to the accumulation MAC address
0x0100.5e06.0606 and sets the USA to H3’s MAC abode 0x0006.7c11.1103, which
communicates to switches that, “A host with the USA 0x0006.7c11.1103 has requested
multicast cartage for the GDA 0x0100.5e06.0606, so map your CAM tables accordingly.” This
message is accustomed by both switches.
Figure 16-16 CGMP Accompany Bulletin Process
SW1 and SW2 chase their CAM table entries and acquisition that a host with the USA
0x0006.7c11.1103 is amid on their anchorage cardinal fa0/20 and fa0/3, respectively. Figure 16-17
shows that SW1 and SW2 accept mapped the GDA 0x0100.5e06.0606 to their anchorage numbers fa0/20
and fa0/3, respectively.
SW1
SW2
R1
H1
H3
H2
H4
1
2
IGMP Accompany 266.6.6.6
DMAC: 0100.5e06.0606
SMAC: 0006.7c11.1103
VLAN 5
Trunk
VLAN 5
Fa0/8
Fa0/3 Fa0/4
Fa0/1 Fa0/2
Fa0/20
Fa0/20
VLAN 5
VLAN 5 VLAN 5
2
2 CGMP Accompany Message:
“Map GDA 0100.5e06.0606
to USA 0006.7c11.1103.”
568 Affiliate 16: Introduction to IP Multicasting
Figure 16-17 Switches Map GDA to Anchorage Numbers and Don’t Flood All the Hosts in a Advertisement Domain
When R1 assiduously multicast cartage with GDA 0x0100.5e06.0606 to SW1, as apparent in Figure 16-17,
SW1 searches its CAM table and notices that this cartage should be forwarded alone on anchorage fa0/20.
Therefore, alone SW2 receives the accumulation traffic. Similarly, SW2 searches its CAM table and
forwards the accumulation cartage alone on its anchorage fa0/3, and alone H3 receives the accumulation traffic.
CGMP optimizes the forwarding of IGMP cartage as well. Although not apparent in the figures,
assume that H1 sends an IGMP Accompany bulletin for 226.6.6.6. R1 will accelerate addition CGMP Join
message, and SW1 will add the GDA 0x0100.5e06.0606 to its anchorage fa0/1 also. Back a router sends
IGMP Accepted Queries, switches advanced them to host associates who accept abutting any group, for
example, H1 and H3. Back hosts accelerate IGMP Reports, switches advanced them to the associates of
the accumulation and the router.
The final footfall of the example, apparent in Figure 16-18, demonstrates what happens back H3 leaves
the group. Note that for this example, H1 has additionally abutting the aforementioned multicast group.
SW1
SW2
R1
H1
H3
H2
H4
M
M
VLAN 5
Trunk
VLAN 5
Fa0/8
Fa0/3 Fa0/4
USA: 0006.7c11.1103
GDA: 0100.5e06.0606
USA: 0006.7c11.1103
GDA: 0100.5e06.0606
Fa0/1 Fa0/2
Fa0/20
Fa0/20
VLAN 5
VLAN 5 VLAN 5
M
Group
226.6.6.6
M = Multicast Cartage for GDA 0x0100.5e06.0606
LAN Multicast Optimizations 569
Figure 16-18 CGMP Leave Bulletin Process
The afterward three steps, referenced in Figure 16-18, call the arrangement of contest back H3
sends an IGMP Leave message:
1. H3 sends an IGMP Leave bulletin for 226.6.6.6. At Layer 2, H3 uses the All Multicast
Routers MAC abode 0x0100.5e00.0002 as the destination abode and its own BIA
0x0006.7c11.1103 as the antecedent address.
2. R1 generates a CGMP Leave message. Back a CGMP-capable router receives an IGMP
Leave message, it generates a Layer 2 CGMP Leave message. The destination abode on the
frame is the acclaimed multicast MAC abode 0x0100.0cdd.dddd. R1 calculates the group
MAC abode 0x0100.5e06.0606 from the Layer 3 abode 226.6.6.6 and sets the GDA to that
value. It sets the USA to H3’s MAC unicast MAC abode of 0x0006.7c11.1103. This Leave
message communicates to switches that, “A host with the USA 0x0006.7c11.1103 does not
want to accept multicast cartage for GDA 0x0100.5e06.0606, so amend your CAM tables
accordingly.” This bulletin is accustomed by both switches.
3. Switches amend their CAM table entries. SW1 and SW2 chase their CAM table entries and
find that a host with the USA 0x0006.7c11.1103 is amid on their anchorage numbers fa0/20 and
fa0/3, respectively. Figure 16-19 shows that SW1 and SW2 accept removed the GDA
0x0100.5e06.0606 from their anchorage numbers fa0/20 and fa0/3, respectively.
H1 is still a affiliate of the accumulation 266.6.6.6, so R1 keeps forwarding the cartage with GDA
0x0100.5e06.0606 to SW1, as apparent in Figure 16-18. SW1 searches its CAM table and finds that
this cartage should be forwarded alone on anchorage fa0/1. Therefore, alone H1 receives the accumulation traffic.
SW1
SW2
R1
H1
H3
H2
H4
1
3 3
2
IGMP Leave 226.6.6.6
DMAC: 0100.5e00.0002
SMAC: 0006.7c11.1103
Trunk VLAN 5
VLAN 5
Fa0/8
Fa0/3 Fa0/4
Fa0/1 Fa0/2
Fa0/20
Fa0/20
VLAN 5
VLAN 5 VLAN 5
2
2 CGMP Leave Message:
“Delete GDA 0100.5e06.0606
for USA 0006.7c11.1103.”
USA: 0006.7c11.1103
3 USA: 0006.7c11.1103
USA: 0006.7c11.1101
GDA: 0100.5e06.0606
Group
226.6.6.6
Continuing the archetype further, now accept that H1 sends an IGMP Leave bulletin for 226.6.6.6.
R1 will accelerate a Group-Specific Query for 226.6.6.6. Because no host is currently a affiliate of this
group, R1 does not accept any IGMP Membership Reports for the group. R1 sends the CGMP
Leave bulletin with the GDA set to the accumulation MAC abode and the USA set to 0. This message
communicates to switches that, “No hosts are absorbed in accepting the multicast accumulation cartage for
the MAC abode 0x0100.5e06.0606, so abolish all the CAM table entries for this group.”
Table 16-6 summarizes the accessible combinations of the GDA and the USA in CGMP messages
and the meanings of each. The aboriginal bristles letters accept been discussed.
CGMP Messages
Type
Group Destination
Address
Unicast Source
Address Meaning
Join Accumulation MAC Host MAC Add USA anchorage to group
Leave Accumulation MAC Host MAC Annul USA anchorage from group
Join Aught Router MAC Learn which anchorage connects to the
CGMP router
Leave Aught Router MAC Release CGMP router port
Leave Accumulation MAC Aught Annul the accumulation from the CAM
Leave Aught Aught Annul all groups from the CAM
The aftermost Leave bulletin in Table 16-6, Annul All Groups, is acclimated by the router for special
maintenance functions. For example, back the bright ip cgmp command is entered at the router
for allowance all the CGMP entries on the switches, the router sends the CGMP Leave bulletin with
GDA set to aught and USA set to zero. Back switches accept this message, they annul all group
entries from the CAM tables.