IGMP Snooping
What happens if your arrangement has non-Cisco switches? You cannot use CGMP because it is Cisco
proprietary. IGMP concern can be acclimated for a multivendor switched arrangement to control
distribution of multicast cartage at Layer 2. IGMP concern requires the about-face software to
eavesdrop on the IGMP chat amid multicast hosts and the router. The about-face examines
IGMP letters and learns the area of multicast routers and accumulation members.
NOTE Abounding Cisco switches abutment IGMP snooping, including the 3550 and 3560 switches
used in the CCIE Acquisition and Switching lab exam.
LAN Multicast Optimizations 571
The afterward three accomplish call the accepted action of IGMP snooping. Later, these accomplish are
explained in detail.
1. To ascertain whether assorted routers are affiliated to the aforementioned subnet, Cisco switches accept to
the afterward acquisition agreement letters to actuate on which ports routers are connected:
— IGMP Accepted Query bulletin with GDA 01-00-5e-00-00-01
— OSPF letters with GDA 01-00-5e-00-00-05 or 01-00-5e-00-00-06
— Agreement Independent Multicast (PIM) adaptation 1 and Hot Standby Acquisition Protocol
(HSRP) Hello letters with GDA 01-00-5e-00-00-02
— PIMv2 Hello letters with GDA 01-00-5e-00-00-od
— Distance Vector Multicast Acquisition Agreement (DVMRP) Probe letters with GDA
01-00-5e-00-00-04
As anon as the about-face detects router ports in a VLAN, they are added to the anchorage account of all
GDAs in that VLAN.
2. Back the about-face receives an IGMP Report on a port, its CPU looks at the GDA, creates an
entry in the CAM table for the GDA, and adds the anchorage to the entry. The router anchorage is also
added to the entry. The accumulation cartage is now forwarded on this anchorage and the router port. If other
hosts accelerate their IGMP Reports, the about-face adds their ports to the accumulation access in the CAM table
and assiduously the accumulation cartage on these ports.
3. Similarly, back the about-face receives an IGMP Leave bulletin on a port, its CPU looks at
the GDA, removes the anchorage from the accumulation access in the CAM table, and does not advanced the
group cartage on the port. The about-face checks whether this is the aftermost nonrouter anchorage for
the GDA. If it is not the aftermost nonrouter anchorage for the GDA, which agency there is at atomic one
host in the VLAN that wants the accumulation traffic, the about-face discards the Leave message;
otherwise, it sends the Leave bulletin to the router.
Thus, IGMP concern helps switches accelerate accumulation cartage to alone those hosts that appetite it and helps
to abstain ashen bandwidth.
For able operations, IGMP concern requires accouterments clarification abutment in a about-face so that it
can differentiate amid IGMP Reports and absolute multicast traffic. The about-face CPU needs to see
IGMP Report letters (and Multicast Acquisition Agreement messages) because the IGMP snooping
process requires the CPU. However, the forwarding of multicast frames does not crave the CPU,
instead acute alone a switch’s forwarding ASICs. Older switches, decidedly those that have
no Layer 3 awareness, could not analyze a packet as IGMP; these switches would have
overburdened their CPUs by accepting to accelerate all multicasts to the CPU. Most of today’s more
572 Chapter 16: Introduction to IP Multicasting
modern switches abutment abundant Layer 3 acquaintance to admit IGMP so that IGMP snooping
does not dissipate the CPU.
Figure 16-19 shows an archetype of the IGMP concern process.
Figure 16-19 Joining a Accumulation Application IGMP Concern and CAM Table Entries
The afterward three steps, referenced in Figure 16-19, call the arrangement of contest back H1
and H2 accelerate IGMP Join messages:
1. H1 sends an IGMP Join bulletin for 226.6.6.6. At Layer 2, H1 uses the multicast MAC
address 0x0100.5e06.0606 (the MAC for accumulation 226.6.6.6) as the destination abode and uses
its own BIA 0x0006.7c11.1101 as the antecedent address. SW1 receives the packet on its fa0/1
port and, acquainted that it is an IGMP packet, assiduously the packet to the about-face CPU. The CPU
NOTE CGMP was a accepted Cisco about-face affection in years accomplished because IGMP
implementations on some switches would accept appropriate too abundant work. Today, abounding of the
Cisco accepted about-face artefact offerings do not alike abutment CGMP, in acquiescence to IGMP
snooping.
SW1
CAM Table
R1
H1
Group
226.6.6.6
H2
1
1
2
IGMP Join
226.6.6.6
2 IGMP Join
226.6.6.6
3 Multicast Traffic
GDA: 0x0100.5e06.0606
VLAN 5
VLAN 5 VLAN 5
Fa0/8
Fa0/1 Fa0/2
Layer 3-Aware Switch
Destination Abode Type of Packet Ports
01-00-5e-06-06-06 L3-IGMP 0
01-00-5e-06-06-06 Non-IGMP 1, 8
01-00-5e-06-06-06 L3-IGMP 0
01-00-5e-06-06-06 Non-IGMP 1, 2, 8
LAN Multicast Optimizations 573
uses the advice to set up a multicast forwarding table entry, as apparent in the CAM table
that includes the anchorage numbers 0 for CPU, 1 for H1, and 8 for R1. Notice that the CAM table
lists two entries for the aforementioned destination MAC abode 0x0100.5e06.0606—one for the IGMP
frames for anchorage 0 and the added for the non-IGMP frames for ports 1 and 8. The CPU of the
switch instructs the switching agent to not advanced any non-IGMP frames to anchorage 0, which
is affiliated to the CPU.
2. H2 sends an IGMP Join bulletin for 226.6.6.6. At Layer 2, H2 uses the multicast MAC
address 0x0100.5e06.0606 as the destination abode and uses its own BIA
0x0006.7c11.1102 as the antecedent address. SW1 receives the packet on its fa0/2 port, and its
switching agent examines the packet. The action of allegory the packet, as declared in
Step 1, is again and the CAM table entries are adapted as shown.
3. Router R1 assiduously the accumulation traffic. R1 is accepting multicast cartage for accumulation 226.6.6.6 and
starts forwarding the cartage to SW1. SW1 starts accepting the multicast cartage on its port
fa0/8. The switching agent would appraise the packet and actuate that this is a non-IGMP
packet, chase its CAM table, and actuate that it should advanced the packet on ports fa0/1
and fa0/2.
Compared to CGMP, IGMP concern is beneath able in advancement accumulation information. In
Figure 16-20, back R1 periodically sends IGMP Accepted Queries to the All Hosts group,
224.0.0.1 (GDA 0x0100.5e00.0001), SW1 intercepts the Accepted Queries and assiduously them
through all ports in VLAN 5. In CGMP, due to advice from the router through CGMP
messages, the about-face knows absolutely on which ports multicast hosts are affiliated and, therefore,
forwards IGMP Accepted Queries alone on those ports. Also, in IGMP snooping, back hosts send
IGMP Reports, the about-face charge ambush them to advance GDA advice in the CAM table.
As a result, the hosts do not accept anniversary other’s IGMP Report, which break the Report
Suppression apparatus and armament anniversary host to accelerate an IGMP Report. However, the about-face sends
only one IGMP Report per accumulation to the router. In CGMP, the about-face does not accept to intercept
IGMP Reports, because advancement accumulation advice in the about-face is not abased on
examining IGMP packets from hosts; instead, the about-face uses CGMP letters from the router.
Figure 16-20 shows the Leave action for IGMP snooping.
574 Chapter 16: Introduction to IP Multicasting
Figure 16-20 Leaving a Accumulation Application IGMP Concern and CAM Table Entries
The afterward three steps, referenced in Figure 16-20, call the arrangement of contest back H1
and H2 accelerate IGMP Leave messages:
1. H1 sends an IGMP Leave bulletin for 226.6.6.6, but SW1 does not advanced it to router R1 in
this case. At Layer 2, H1 uses the All Multicast Routers MAC abode 0x0100.5e00.0002 as
the destination abode and uses its own BIA 0x0006.7c11.1101 as the antecedent address. SW1
captures the IGMP Leave bulletin on its fa0/1 port, and its switching agent examines the
packet. The about-face sends an IGMP Accepted Query on anchorage fa0/1 to actuate whether there
are any added hosts that are associates of this accumulation on the port. (This affection was advised to
protect added hosts if they are affiliated to the aforementioned about-face anchorage application a hub.) If an IGMP
Report is accustomed on anchorage fa0/1, the about-face discards the Leave bulletin accustomed from H1.
Because, in this example, there is alone one host affiliated to anchorage fa0/1, the about-face does not
receive any IGMP Report and deletes the anchorage fa0/1 from the CAM table entry, as apparent in
Figure 16-20. H2 affiliated with anchorage fa0/2 is still a affiliate of the group, and its anchorage number
is in the CAM table entry. Hence, SW1 does not advanced the IGMP Leave bulletin to the
router.
2. Router R1 continues forwarding the accumulation traffic. R1 continues forwarding multicast traffic
for accumulation 226.6.6.6 to SW1 because R1 did not alike apperceive that H1 larboard the group. Based on
the adapted CAM table access for the accumulation apparent in Figure 16-20, SW1 now assiduously this
traffic alone on anchorage fa0/2.
SW1
CAM Table
R1
H1
Group
226.6.6.6
H2
1
1
3
IGMP Leave
226.6.6.6
3
2 Multicast Traffic
GDA: 0x0100.5e06.0606
IGMP Leave
226.6.6.6
VLAN 5
VLAN 5 VLAN 5
Fa0/8
Fa0/1 Fa0/2
Layer 3-Aware Switch
Destination Abode Type of Packet Ports
01-00-5e-06-06-06 L3-IGMP 0
01-00-5e-06-06-06 Non-IGMP 2, 8
01-00-5e-06-06-06 L3-IGMP 0
3. H2 sends an IGMP Leave bulletin for 226.6.6.6, and SW1 does advanced it to router R1 in this
case. At Layer 2, H2 uses the All Multicast Routers MAC abode 0x0100.5e00.0002 as the
destination abode and uses its own BIA 0x0006.7c11.1102 as the antecedent address. Again,
SW1 captures the IGMP Leave bulletin on its fa0/2 anchorage and its switching agent examines
the packet. The about-face sends an IGMP Accepted Query on anchorage fa0/2 to actuate whether
there are any added hosts that are associates of this accumulation on the port. Because, in this example,
there is alone one host affiliated to anchorage fa0/2, the about-face does not accept any IGMP Report
and deletes the anchorage fa0/2 from the CAM table entry. After SW1 deletes the port, it realizes
that this was the aftermost nonrouter anchorage for the CAM table access for Ox0100.5e06.0606.
Therefore, SW1 deletes the CAM table access for this group, as apparent in Figure 16-20, and
forwards the IGMP Leave bulletin to R1, which sends an IGMP Group-Specific Query and,
when no hosts respond, stops forwarding cartage for 226.6.6.6 against SW1.
IGMP concern becomes added complicated back assorted multicast routers are acclimated and many
LAN switches are commutual via accelerated trunks. Also, CGMP and IGMP concern control
distribution of multicast cartage alone on ports area hosts are connected. They do not accommodate any
control apparatus for ports area routers are connected. The abutting area briefly examines how
Router-Port Accumulation Management Agreement (RGMP) helps switches ascendancy administration of
multicast cartage on ports area routers are connected.