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Title: Multicast in a Campus Network: CGMP and IGMP Snooping
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Multicast in a Campus Network: CGMP and
IGMP Snooping
Contents
Introduction
Before You Begin
Conventions
Prerequisites
Components Used
Background Information
Multicast Address
Internet Group Management Protocol
IGMPv1
IGMPv2
IGMPv3
Interoperability Between IGMPv1 and IGMPv2
Interoperability Between IGMPv1/IGMPv2 and IGMPv3
IGMP on a Router
Practical Example on a Router
Cisco Group Management Protocol
CGMP Frames and Message Types
Learning Router Ports
Joining a Group with CGMP
Leaving a Group With CGMP
CGMP and Source-Only Network
Configuring Cisco Routers and Switches to Enable CGMP
Practical Example of CGMP Use and Debug Command and Output
IGMP Snooping
IGMP Snooping Overview
Learning the Router Port
Joining a Group With IGMP Snooping
IGMP / CGMP Interaction
Multicast Source-Only Network
Limitations
Configuration of IGMP Snooping on Cisco Switches
Practical Example of IGMP Snooping
Related Information
Introduction
The purpose of Cisco Group Management Protocol (CGMP) and Internet Group Management
Protocol (IGMP) snooping is to restrain multicast traffic in a switched network. By default, a LAN
switch floods multicast traffic within the broadcast domain, and this can consume a lot of
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bandwidth if many multicast servers are sending streams to the segment.
Before You Begin
Conventions
For more information on document conventions, see the Cisco Technical Tips Conventions.
Prerequisites
There are no specific prerequisites for this document.
Components Used
This document is not restricted to specific software and hardware versions.
Background Information
Multicast traffic becomes flooded because a switch usually learns MAC addresses by looking into
the source address field of all the frames it receives. A multicast MAC address is never used as
source address for a packet. Such addresses do not appear in the MAC address table, and the
switch has no method for learning them.
The first solution to this issue is to configure static MAC addresses for each group and each client.
This solution works well, however, it is neither scalable nor dynamic. You use this solution on a
Catalyst 4000, 5000, or 6000 switch by issuing one of the following commands:
set cam static <multicast_mac> <mod/port>
G
set cam permanent <multicast_mac> <mod/port>
G
These two commands have the same effect, except that the static entries disappear at reboot, and
permanent entries do not.
The second solution is to use CGMP, which is a Cisco proprietary protocol that runs between the
multicast router and the switch. CGMP enables the Cisco multicast router to understand IGMP
messages sent by hosts, and informs the switch about the information contained in the IGMP
packet.
The last (and most efficient) solution is to use IGMP snooping. With IGMP snooping, the switch
intercepts IGMP messages from the host itself and updates its MAC table accordingly. Advanced
hardware is required to support IGMP snooping.
CGMP configurations given in this document are for Catalyst 4000 and 5000 switches running
CatOS (CGMP is not supported on Catalyst 6000 switches), and IGMP snooping configurations
are for Catalyst 5000 and 6000 switches running CatOS.
The following section briefly describes a multicast address, explains the functionality of IGMP, and
provides additional detail on CGMP and IGMP snooping.
Multicast Address
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Multicast IP addresses are Class D IP addresses. Therefore, all IP addresses from 224.0.0.0
to 239.255.255.255 are multicast IP addresses. They are also referred to as Group
Destination Addresses (GDA).
1.
For each GDA there is an associated MAC address. This MAC address is formed by 01-00-
5e, followed by the last 23 bits of the GDA translated into hex, as shown below.239.20.20.20
corresponds to MAC 01-00-5e-14-14-14.239.10.10.10 corresponds to MAC 01-00-5e-0a-0a-
0a.Consequently, this is not a one-to-one mapping, but a one-to-many mapping. From these
two addresses, you can see that the first octet (239) is not used in the MAC address. So the
multicast addresses with the same last three octet but different first octet have overlapping
MAC addresses.
2.
Some Multicast IP addresses are reserved for special use, as shown below.224.0.0.1 - All
multicast-capable hosts.224.0.0.2 - All multicast-capable routers.224.0.0.5 and 224.0.0.6 are
used by Open Shortest Path First (OSPF).
3.
In general, addresses from 224.0.0.1 to 224.0.0.255 are reserved and used by various protocols
(standard or proprietary, such as Hot Standby Router Protocol (HSRP)). Cisco recommends that
you not use these for GDA in a multicast network. CGMP and IGMP snooping do not work with
this reserved address range.
Internet Group Management Protocol
IGMP is a standard defined in RFC1112 for IGMPv1, in RFC2236 for IGMPv2 and in RFC3376 for
IGMPv3. IGMP specifies how a host can register with a router in order to receive specific multicast
traffic. The next section gives a brief overview on IGMP.
IGMPv1
IGMP Version 1 (IGMPv1) messages are transmitted in IP datagrams and contain the following
fields:
Version: 1
G
Type: There are two types of IGMP messages, Membership Query and Membership Report.
G
Checksum
G
GDA
G
Membership reports are issued by hosts that want to receive a specific multicast group (GDA).
Membership queries are issued by routers at regular intervals to check whether there is still a host
interested in the GDA in that segment.
Host membership reports are issued either unsolicited (when the host wants to receive GDA traffic
first) or in response to a membership query. They are sent with the following fields:
L2 Information
Source MAC: Host MAC address
G
Destination MAC: Destination MAC for the GDA
G
L3 Information
Source IP: IP address of the host
G
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Destination IP: GDA
G
IGMP Packet
IGMP data contains, furthermore, the GDA and some other fields.
G
Host membership queries are sent by the router to the all-multicast address: 224.0.0.1. These
queries use 0.0.0.0 in the IGMP GDA field. A host for each group must respond to that query, or
the router stops forwarding the traffic for that GDA to that segment (after three attempts). The
router keeps a multicast routing entry for each source, and links it to a list of outgoing interfaces
(interface from where the IGMP report came). After three IGMP query attempts with no answer,
this interface is erased from the outgoing interface list for all entries linked to that GDA.
Note: IGMPv1 has no leave mechanism. If a host no longer wants to receive the traffic, it
simply quits. If it is the last host on the subnet, the router does not receive any answer to its
query, and deletes the GDA for that subnet.
IGMPv2
In IGMP Version 2 (IGMPv2), the version field has been removed, and the type field can now
accept different values. The types are shown below.
Membership Query
G
IGMPv1 Membership Report
G
Version 2 Membership Report
G
Leave Group
G
Descriptions of the most important new features added in IGMPv2 are listed below.
IGMP Leave Message: when a host wants to leave a group, it should send a Leave Group
IGMP message to destination 224.0.0.2 (instead of leaving silently like in IGMPv1).
G
A router can now send a group-specific query by sending a Membership Query to the group
GDA instead of sending it to 0.0.0.0.
G
IGMPv3
In IGMP Version 3 (ICMPv3), there is a type field that can have the following values:
Membership query
G
Version 3 Membership Report
G
An implementation of IGMPv3 must also support the following three message types, for
interoperation with previous versions of IGMP:
Version 1 Membership Report [RFC1112]
G
Version 2 Membership Report [RFC2236]
G
Version 2 Leave Group [RFC2236]
G
IGMPv3 adds support for source filtering, that is, the ability for a system to report interest in
receiving packets from specific source addresses, or from all but specific source addresses sent
to a specific multicast address. This feature is also called Source Specific Multicast (SSM).
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In order for a computer to support SSM, it must support IGMPv3. Relatively few OS, however,
support IGMPv3. Windows XP supports IGMPv3, and there are IGMPv3 support patches available
for FreeBSD and Linux.
Administrators must distinguish between IGMPv3 support at the router level and IGMPv3 snooping
at the switch level. They are two different features.
Support of IGMPv3 on Catalyst Switches (L2)
The Catalyst 6000 running hybrid mode software (CatOS on Supervisor and Cisco IOS®
Software on MSFC) officially supports IGMPv3 snooping starting in version 7.5(1).
G
In versions prior to 7.5(1), the Catalyst 6000 switch did not have official support for IGMPv3,
but it should normally be able to handle IGMPv3 packets.
G
The Catalyst 6000 running Integrated IOS Software supports IGMPv3 at the router level (L3
interface) starting in Version 12.1(8a)E.
G
The Catalyst 4000 only supports IGMPv3 at the router level on the Supervisor III and IV. It
does not support IGMPv3 snooping.
G
Support of IGMPv3 on Cisco Routers (L3)
IGMPv3 is supported on all platforms running Cisco IOS® software Release 12.1(5)T and later
releases.
Caveats
When a switch is running IGMP snooping, it intercepts the IGMP packets and populates the static
Layer 2 (L2) forwarding table based on the content of the intercepted packets. When there are
IGMPv1 or v2 hosts on the network, the switch reads the IGMP Joins and Leaves to determine
which hosts wants to receive which multicast stream, or stop receiving the multicast stream.
IGMPv3 is more complicated, because it uses not only the group address (multicast address), but
also the sources from which traffic is expected. Apart from the Catalyst 6000 switch running
CatOS 7.5 or later and Native IOS Version 12.1(8a)E or later, no other switches are currently able
to effectively snoop those packets and build a forwarding table based on this information.
Therefore, IGMP snooping should be turned off when there is an IGMPv3 host on the switch.
When IGMP snooping is turned off, the switch cannot dynamically build a L2 forwarding table for
the multicast streams. In other words, the switch floods the multicast streams.
When IGMP snooping is disabled, one solution is to manually configure multicast dynamic
Content-Addressable Memory (CAM) entries in order to avoid flooding the subnet with multicast
traffic. This is an administrative burden, however, and is not a dynamic solution. When a client no
longer wants to receive the traffic, the CAM entry is not removed from the switch (unless by
manual intervention), so network traffic is still addressed to the host.
Also, when using IGMPv3 in the network, switches using CGMP work normally apart from the fact
that CGMP Fastleave does not work. If CGMP Fastleave is needed, it is best to revert to IGMPv2.
The outstanding platform-specific caveats can be found in the release notes for the respective
switches.
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Interoperability Between IGMPv1 and IGMPv2
With IGMPv1 and IGMPv2, only one router per IP subnet sends queries. This router is called the
query router. In IGMPv1, the query router is chosen with the help of multicast routing protocol. In
IGMPv2, it is chosen by the lowest IP address among the routers. Below are several possibilities:
Scenario 1: IGMPv1 Router with a Mix of IGMPv1 and IGMPv2 Hosts
The router does not understand the IGMPv2 report, and thus, all hosts must only use the IGMPv1
report.
Scenario 2: IGMPv2 Router with a Mix of IGMPv2 and IGMPv3 Hosts
IGMPv1 hosts do not understand the IGMPv2 query or the IGMPv2 group membership query. The
router must only use IGMPv1, and suspend the leave operation.
Scenario 3: IGMPv1 Router and IGMPv2 Router Located on the Same Segment
The IGMPv1 router has no way to detect the IGMPv2 router. Therefore, the IGMPv2 router must
be configured by the administrator as a IGMPv1 router. In any case, it possible that they do not
agree on the query router.
Interoperability Between IGMPv1/IGMPv2 and IGMPv3
With all versions of IGMP, only one router per IP subnet sends queries. This router is called the
query router. In IGMPv1, the query router is chosen with the help of the multicast routing protocol.
In IGMPv2 and IGMPv3, it is chosen by the lowest IP address among the routers. Below are
several interoperability options.
Scenario 1: IGMPv1/IGMPv2 Router with a Mix of IGMPv1/IGMPv2 and IGMPv3 Hosts
Because the router does not understand the IGMPv3 reports, all hosts use the IGMPv1/IGMPv2
reports.
Scenario 2: IGMPv3 Router with a Mix of IGMPv1/IGMPv2 and IGMPv3 Hosts
The IGMPv1/IGMPv2 hosts do not understand the IGMPv3 query or IGMPv3 membership query.
The router must only use the IGMP version that corresponds to the lowest IGMP client version
present. If there are IGMPv3 and IGMPv2 clients, the router uses IGMPv2. If there are IGMPv1,
IGMPv2, and IGMPv3 clients, the router uses IGMPv1.
Scenario 3: Different Version Routers on the Same Segment
When routers of different versions are present on the same segment, the lower-version routers
have no means to detect the higher-version routers. Therefore, the different routers must be
configured by the administrator as the same version. This version has to match the lowest version
on any querying router present.
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IGMP on a Router
If, by default, there is no user registered to a specific group in a subnet, the router does not
forward multicast traffic for that group into that subnet. That means that a router needs to receive
an IGMP report for a GDA in order to add it to the multicast routing table and to start forwarding
traffic for that group.
On a router, you need to perform the following actions:
Enable multicast routing in the global mode, as shown below.
ip multicast-routing
1.
Configure a multicast routing protocol on the involved interface, as shown below.
ip pim dense-mode
2.
Monitor IGMP, as shown below.
show ip igmp interface
show ip igmp group
show ip mroute
3.
Configure a router to send the IGMP report (on the interface), as shown below.
ip igmp join-group [GDA_ip_address]
ip igmp version [1 | 2 | 3]
4.
Practical Example on a Router
A router is configured to route between two subinterfaces, Fast-Ethernet 0.2 and Fast-Ethernet
0.3. Both interfaces are also configured to run IGMP. In the output below, you can see the IGMP
version, the group joined, and so on.
Configuration
ip multicast-routing
interface FastEthernet0
no ip address
no ip directed-broadcast
!
interface FastEthernet0.2
encapsulation isl 2
ip address 10.2.2.1 255.255.255.0
no ip redirects
no ip directed-broadcast
ip pim dense-mode
!
interface FastEthernet0.3
encapsulation isl 3
ip address 10.3.3.1 255.255.255.0
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no ip redirects
no ip directed-broadcast
ip pim dense-mode
!
show ip igmp interface
Fa0.2 is up, line protocol is up
Internet address is 10.2.2.1/24
IGMP is enabled on interface
Current IGMP version is 2
CGMP is disabled on interface
IGMP query interval is 60 seconds
IGMP querier timeout is 120 seconds
IGMP max query response time is 10 seconds
Inbound IGMP access group is not set
IGMP activity: 3 joins, 2 leaves
Multicast routing is enabled on interface
Multicast TTL threshold is 0
Multicast designated router (DR) is 10.2.2.1 (this system)
IGMP querying router is 10.2.2.1 (this system)
Multicast groups joined: 224.0.1.40
Fa0.3 is up, line protocol is up
Internet address is 10.3.3.1/24
IGMP is enabled on interface
Current IGMP version is 2
CGMP is disabled on interface
IGMP query interval is 60 seconds
IGMP querier timeout is 120 seconds
IGMP max query response time is 10 seconds
Inbound IGMP access group is not set
IGMP activity: 1 joins, 1 leaves
Multicast routing is enabled on interface
Multicast TTL threshold is 0
Multicast designated router (DR) is 10.3.3.1 (this system)
IGMP querying router is 10.3.3.1 (this system)
No multicast groups joined
show ip mroute and show ip igmp group
Router_A#show ip mroute
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, C - Connected, L - Local, P - Pruned
R - RP-bit set, F - Register flag, T - SPT-bit set, J - Join SPT
Timers: Uptime/Expires
Interface state: Interface, Next-Hop or VCD, State/Mode
(*, 239.10.10.10), 00:01:15/00:02:59, RP 0.0.0.0, flags: DJC
Incoming interface: Null, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0.3, Forward/Dense, 00:01:16/00:00:00
(10.2.2.2, 239.10.10.10), 00:00:39/00:02:20, flags: CT
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Incoming interface: FastEthernet0.2, RPF nbr 0.0.0.0
Outgoing interface list:
FastEthernet0.3, Forward/Dense, 00:00:39/00:00:00
Router_A#show ip igmp groups
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter
239.10.10.10 Fa0.3 00:02:48 00:02:04 10.3.3.2
Router_A#
Cisco Group Management Protocol
For CGMP support on Catalyst switches, refer to Multicast Catalyst Switches Support Matrix.
CGMP Frames and Message Types
CGMP was first implemented by Cisco to restrain multicast traffic in a L2 network. Because a
switch is, by essence, not capable of looking at L3 packets, it cannot distinguish an IGMP packet.
With CGMP, the router provides the interface between the hosts. The routers "talk" IGMP, and the
switches "talk" CGMP.
CGMP frames are Ethernet frames with the destination MAC address 01-00-0c-dd-dd-dd, and with
a Subnetwork Access Protocol (SNAP) header with the value 0x2001. The CGMP frames contain
the following fields:
Version: 1 or 2.
G
Message Type: Join or Leave.
G
Count: The number of multicast/unicast address pairs in the message.
G
GDA: The 48-bit MAC address of the multicast group.
G
Unicast Source Address (USA): The 48-bit MAC unicast address of the devices that wants to
join the GDA.
G
Note: The value of the count field determines how many times the last two fields display.
By default, the processors of a switch (called NMP in Catalyst) only listen to multicast addresses
when the show cam system command is issued. When you enable CGMP on a switch, the address
01-00-0c-dd-dd-dd is added to the show cam system command output.
The table below lists all of the possible CGMP messages.
GDA
USA
Join/Leav
e
Meanin
g
Mcast MAC
Client MAC
Join
Add port
to
group.
Mcast MAC
Client MAC
Leave
Delete
port
from
group.
00-00-00-00-00-
00
Router MAC
Join
Assign
router
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port.
00-00-00-00-00-
00
Router MAC
Leave
Unassig
n router
port.
Mcast MAC
00-00-00-00-00-
00
Leave
Delete
group.
00-00-00-00-00-
00
00-00-00-00-00-
00
Leave
Delete
all
groups.
Learning Router Ports
The switch needs to be aware of all router ports so that they are automatically added to any newly
created multicast entries. The switch learns router ports when it receives a CGMP Join to GDA 00-
00-00-00-00-00 with Router MAC USA (third type of message in the table). These messages are
generated by the router on all interfaces configured to run CGMP. There is also a static method,
however, for configuring router ports on the switch.
Joining a Group with CGMP
A new client requests to receive traffic for a GDA, so the client sends an IGMP Membership
report message.
G
The router receives the IGMP report, processes it, and sends a CGMP message to the switch.
The router copies the destination MAC address into the GDA field of the CGMP Join, and
copies the source MAC address into the USA of the CGMP join. It then sends it back to the
switch.
G
A switch with CGMP enabled needs to listen to the CGMP 01-00-0c-dd-dd-dd addresses. The
processor of the switch looks into the CAM table for the USA. Once the USA is seen in the
CAM table, the switch knows which port the USA is located on, and does one of the
following:Creates a new static entry for the GDA and links the USA port to it along with all
router ports.Adds the USA port to the list of the ports for this GDA (if the static entry already
exists).
G
Leaving a Group With CGMP
Static entries learned with CGMP are permanent, unless a spanning tree topology change occurs
in the VLAN, or the router sends one of the last CGMP Leave messages in the previous table.
When IGMPv1 is the host, do not send IGMP Leave messages. The router only sends Leave
messages if it does not receive a reply to three consecutive IGMP queries. This means that no
port is deleted from a group if any users are still interested in that group.
With the introduction of IGMPv2 and the presence of IGMP Leave, Cisco added to the original
CGMP specification (CGMPv2). This addition is called CGMP Fast-Leave.
CGMP Fast-Leave processing allows the switch to detect IGMPv2 Leave messages sent to the all-
router multicast address (224.0.0.2) by hosts on any of the supervisor engine module ports. When
the supervisor engine module receives a Leave message, it starts a query-response timer and
sends a message on the port on which that leave was received to determine if there is still a host
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willing to receive this multicast group on that port. If this timer expires before a CGMP Join
message is received, the port is pruned from the multicast tree for the multicast group specified in
the original leave message. If it is the last port in the multicast group, it forwards the IGMP Leave
message to all router ports. The router then starts the normal deletion process by sending a
group-specific query. Because no responses are received, the router removes this group from the
multicast routing table for that interface. It also sends a CGMP Leave message to the switch that
erases the group from the static table. Fast-Leave processing ensures optimal bandwidth
management for all hosts on a switched network, even when multiple multicast groups are in use
simultaneously.
When CGMP Leave is enabled, two entries are added to the show cam system command output, as
shown below.
01-00-5e-00-00-01
01-00-5e-00-00-02
IGMP Leave uses 224.0.0.2 and IGMP Query uses 224.0.0.1.
Use the following steps to troubleshoot CGMP:
Due to a conflict with the HSRP, CGMP Leave processing is disabled by default. HSRP uses
MAC address 01-00-5e-00-00-02, which is the same as IGMP Leave with IGMP Version 2.
With CGMP Fast-Leave, all HSRP packets go to the switch CPU. Because an HSRP
message is not an IGMP packet, the switch regenerates all such messages and sends them
to all router ports. Routers receiving hsrp hello or hsrp peers lose connectivity. Therefore, in
debugging HSRP problems, try disabling CGMP Fast-Leave.To enable CGMP Leave
processing, issue the set cgmp leave enable command.
1.
When CGMP Leave processing is enabled, the Catalyst 5000 family switch learns router
ports through PIM-v1, HSRP, and CGMP Self-Join messages. When CGMP Leave
processing is disabled, the Catalyst 5000 family switch learns router ports through CGMP
Self-Join messages only.
2.
CGMP does not prune multicast traffic for any IP multicast address that maps into the MAC
address range of 01-00-5E-00-00-00 to 01-00-5E-00-00-FF. The reserved IP multicast
addresses, in the range 224.0.0.0 to 224.0.0.255, are used to forward local IP multicast
traffic in a single L3 hop.
3.
CGMP and Source-Only Network
A source-only network is a segment with only a source multicast and no real client. Therefore,
there is a chance that no IGMP reports are generated in that segment. CGMP still needs to restrict
the flooding of this source (for router use only)however. If a router detects multicast traffic on one
interface with no IGMP report, it is identified as a multicast source-only network. The router
generates a CGMP Join message for itself, and the switch simply adds this group (with only the
router port).
Configuring Cisco Routers and Switches to Enable CGMP
The commands below are only valid for Catalyst 4000 and 5000 series (plus 2901, 2902, 2926,
2948G, and 4912).
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Multicast RouterEnable IP multicasting (global command):ip multicast-routingEnable each
interface running CGMP (interface mode) with the following commands:ip pim <sparse-
mode/dense-mode>ip igmpip cgmpDebug the L2 multicast problem with the following
commands:debug ip igmpdebug ip cgmp
G
Catalyst 4000 or 5000 seriesEnable/disable CGMP with the following commands:set cgmp
<enable/disable>Enable/disable CGMP Fast-Leave with the following commands:set cgmp leave
<enable/disable>Configure the multicast router (static) with the following commands:set multicast
router <slot /port>Clear the multicast router with the following commands:clear multicast router
<slot/port>Listed below are various commands to verify the CGMP operation.show cam staticshow
cgmp statistic <VLAN_id>show cgmp leaveshow multicast routershow multicast groupshow multicast group
cgmpshow multicast group count
G
Practical Example of CGMP Use and Debug Command and Output
This is a practical configuration example for a Cisco router and Catalyst switches.
This configuration shows the operations involved as a host joins a group. This configuration alos
shows the operations as a host leaves a group with Fast-Leave enabled. Sniffer traces and the
configuration of the switch and the router are also provided.
Joining a Group with CGMP
Refer to these steps when joining a group with CGMP.
Enable CGMP on the switch, as shown below.
Switch_B (enable) set cgmp en
MCAST-CGMP: Set CGMP Sys Entrie
MCAST-CGMP: Set CGMP Sys Entrie
MCAST-CGMP: Set CGMP Sys Entrie
CGMP support for IP multicast enabled.
Switch_B (enable)
As you can see below, the entry 01-00-0c-dd-dd-dd is included for all VLANs in the show cam
system command output. In addition, as the network is running CGMP Fast-Leave, you can
see the entries for 01-00-5e-00-00-01 and 01-00-5e-00-00-02.
Switch_B (enable) show cgmp leave
CGMP: enabled
CGMP leave: enabled
Switch_B (enable) show cam system
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
1 00-10-2f-00-14-00 # 7/1
1 00-e0-fe-4b-f3-ff # 1/9
1 01-00-0c-cc-cc-cc # 1/9
1 01-00-0c-cc-cc-cd # 1/9
1 01-00-0c-dd-dd-dd # 1/9
1 01-00-0c-ee-ee-ee # 1/9
1 01-80-c2-00-00-00 # 1/9
1 01-80-c2-00-00-01 # 1/9
1.
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2 00-10-2f-00-14-00 # 7/1
2 01-00-0c-cc-cc-cc # 1/9
2 01-00-0c-cc-cc-cd # 1/9
2 01-00-0c-dd-dd-dd # 1/9
2 01-80-c2-00-00-00 # 1/9
2 01-80-c2-00-00-01 # 1/9
3 01-00-0c-cc-cc-cc # 1/9
3 01-00-0c-cc-cc-cd # 1/9
3 01-00-0c-dd-dd-dd # 1/9
3 01-80-c2-00-00-00 # 1/9
3 01-80-c2-00-00-01 # 1/9
Total Matching CAM Entries Displayed = 19
The router sends a CGMP Join message to GDA 00-00-00-00-00-00 with the USA MAC of
the router. Therefore, the router port is added to the router port list (see the first example
below).On the router
6d01h: CGMP: Sending self Join on Fa0.3
6d01h: GDA 0000.0000.0000, USA 0060.70cd.83c9
On the switch
MCAST-CGMP-JOIN: recvd CGMP JOIN msg on port 3/1 vlanNo 2
MCAST-CGMP-JOIN: join GDA 00-00-00-00-00-00 MCAST-CGMP-JOIN:USA
00-60-70-cd-83-c9
MCAST-ROUTER: Adding QUERIER port 3/1, vlanNo 2
MCAST-ROUTER: Creating RouterPortTimer for port 3/1, vlanNo 2
Switch_B (enable) show multi router
CGMP enabled
IGMP disabled
Port Vlan
3/1 2-3
Total Number of Entries = 1
'*' - Configured
2.
The PC on 3/1 sends IGMP a report containing the GDA: 239.10.10.10 (see frame 2 below).
Shown below is the show ip igmp group command output on the router Router_A.This shows
that the router now forwards traffic for 224.10.10.10 to fa0.3 . This is a consequence of the
reception of the IGMP report from 10.3.3.2, which is the client PC.
Router_A#show ip igmp groups
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter
239.10.10.10 Fa0.3 00:02:48 00:02:04 10.3.3.2
Router_A#
3.
The router receives the report and sends a CGMP Join message along with the following
information:Source MAC: MAC address of routerDest MAC: 01-00-cc-dd-dd-ddContents:
MAC address of the client PC (USA): 00-00-0c-07-ac-00 MAC address of the multicast
group: 01-00-5e-0a-0a-0a (see frame 3 below)On the router
6d01h: IGMP: Received v2 Report from 10.3.3.2 (Fa0.3) for 239.10.10.10
6d01h: CGMP: Received IGMP Report on Fa0.3
6d01h: from 10.3.3.2 for 239.10.10.10
6d01h: CGMP: Sending Join on Fa0.3
4.
The switch with 01-00-cc-dd-dd-dd in the show cam system command output has CGMP
enabled. The switch is able to process the packet.The switch makes a lookup in the dynamic
CAM table to determine on which port the MAC address of the client PC is located. The
5.
Page 14
address is located on port 3/2, and the switch makes a static entry in the CAM table for 01-
00-5e-0a-0a-0a bounded to port 3/2. The switch also adds the router port 3/1 to the static
entry for that GDA.On the switch
MCAST-CGMP-JOIN: recvd CGMP JOIN msg on port 3/1 vlanNo 3
MCAST-CGMP-JOIN: join GDA 01-00-5e-0a-0a-0a MCAST-CGMP-JOIN:USA 00-60-5c-f4-bd-e2
MCAST-CGMP-JOIN: 3/2/3: index 81
MCAST-CGMP-JOIN: recvd CGMP JOIN msg on port 3/1 vlanNo 2
MCAST-CGMP-JOIN: join GDA 01-00-5e-00-01-28 MCAST-CGMP-JOIN:USA 00-60-70-cd-83-c9
MCAST-CGMP-JOIN: 3/1/2: index 80
All subsequent traffic for multicast group 239.10.10.10 are forwarded only to this port in this
VLAN. Below is the static entry in the Catalyst switch where 3/1 is the router port and 3/2 is
the client port.
Switch_B (enable) show cam static
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
3 01-00-5e-0a-0a-0a 3/1-2
Total Matching CAM Entries Displayed = 3
Switch_B (enable)
6.
Leaving a Group With CGMP Fast-Leave Enabled
The example below requires that the client is an IGMP Version 2 client and that Fast-Leave is
enabled on the switch.
The following procedure enables CGMP Fast-Leave. Look at the show cgmp leave command
output to determine if it is enabled. Also, look at the show cam system command output to
determine if the switch is listening to 01-00-5e-00-00-01 and 01-00-5e-00-00-02 (addresses
used for the leave).
Switch_B (enable) show cgmp leave
CGMP: enabled
CGMP leave: enabled
Switch_B (enable) show cam sys
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
1 00-10-2f-00-14-00 # 7/1
1 00-e0-fe-4b-f3-ff # 1/9
1 01-00-0c-cc-cc-cc # 1/9
1 01-00-0c-cc-cc-cd # 1/9
1 01-00-0c-dd-dd-dd # 1/9
1 01-00-0c-ee-ee-ee # 1/9
1 01-80-c2-00-00-00 # 1/9
1 01-80-c2-00-00-01 # 1/9
2 00-10-2f-00-14-00 # 7/1
2 01-00-0c-cc-cc-cc # 1/9
2 01-00-0c-cc-cc-cd # 1/9
2 01-00-0c-dd-dd-dd # 1/9
2 01-00-5e-00-00-01 # 1/9
1.
Page 15
2 01-00-5e-00-00-02 # 1/9
2 01-80-c2-00-00-00 # 1/9
2 01-80-c2-00-00-01 # 1/9
3 01-00-0c-cc-cc-cc # 1/9
3 01-00-0c-cc-cc-cd # 1/9
3 01-00-0c-dd-dd-dd # 1/9
3 01-00-5e-00-00-01 # 1/9
3 01-00-5e-00-00-02 # 1/9
3 01-80-c2-00-00-00 # 1/9
Do you wish to continue y/n [n]? y
Total Matching CAM Entries Displayed = 22
The client sends a IMPG Leave message to 224.0.0.2. The switch intercepts it and sends an
IGMP Query on the port on which he receives the leave. The following is debug output on the
switch:
MCAST-IGMP-LEAVE:Recvd leave on port 3/2 vlanNo 3
MCAST-IGMP-LEAVE:router_port_tbl[vlanNo].QueryTime = 0
MCAST-IGMP-LEAVE:deletion_timer = 1
MCAST-SEND:Transmitting IGMP Mac Based GS Query msg on port 3/2 vlanNo 3
MCAST-SEND: Transmit Succeeded for IGMP Group Specific Query msg on port 3/2 vlanNo 3
2.
Because no response was received, the Catalyst forwards the IGMP Leave message to the
router, as shown below.
MCAST-TIMER:IGMPLeaveTimer expired on port 3/2 vlanNo 3 GDA 01-00-5e-0a-0a-0a
MCAST-TIMER:IGMPLeaveTimer expiry: Transmit IGMP Leave on port 3/1 vlanNo 3
MCAST-SEND:Transmitting IGMP Leave msg on port 3/1 vlanNo 3
MCAST-SEND: Inband Transmit Succeeded for IGMP Leave Message on port 3/1 vlanNo 3
3.
The router receives an IGMP Leave message, so it sends a CGMP Leave message to the
switch and also deletes the group from its IGMP group list. Below is the debug command
output on the router.On the router
IGMP: Received Leave from 10.200.8.108 (Fa0.3) for 239.10.10.10
IGMP: Send v2 Query on Fa0.3 to 239.10.10.10
IGMP: Send v2 Query on Fa0.3 to 239.10.10.10
CGMP: Sending Leave on Fa0.3
GDA 0100.5e0a.0a0a, USA 0000.0000.0000
IGMP: Deleting 239.10.10.10 on Fa0.3
4.
CGMP Traces and Configuration
Frame 1
Frame 1 is a CGMP Join frame to GDA 00-00-00-00-00-00. It is used to add the router port to the
router port list.
ISL: ----- ISL Protocol Packet -----
ISL:
ISL: Destination Address = 01000C0000
ISL: Type = 0 (Ethernet)
ISL: User = 0 (Normal)
ISL: Source Address = 8C958B7B1000
Page 16
ISL: Length = 76
ISL: Constant value = 0xAAAA03
ISL: Vendor ID = 0x8C958B
ISL: Virtual LAN ID (VLAN) = 2
ISL: Bridge Protocol Data Unit (BPDU) = 0
ISL: Port Index = 193
ISL: Reserved
ISL:
ETHER: ----- Ethernet Header -----
ETHER:
ETHER: Destination = Multicast 01000CDDDDDD
!--- Send to the CGMP !--- macaddress present in show cam sys
!--- command output.
ETHER: Source = Station Cisco11411E1
ETHER: 802.3 length = 24
ETHER:
LLC: ----- LLC Header -----
LLC:
LLC: DSAP Address = AA, DSAP IG Bit = 00 (Individual Address)
LLC: SSAP Address = AA, SSAP CR Bit = 00 (Command)
LLC: Unnumbered frame: UI
LLC:
SNAP: ----- SNAP Header -----
SNAP:
SNAP: Vendor ID = Cisco1
SNAP: Type = 2001 (CGMP)
SNAP:
CGMP: ----- CGMP -----
CGMP:
CGMP: Version = 16
CGMP: Type = 0 (Join)
CGMP: Reserved
CGMP: Count = 1
CGMP:
CGMP: Group Destination Address and Unicast Source Address
CGMP:
CGMP: GDA =0000.0000.0000
CGMP: USA =0000.0C14.11E1
!--- MAC address of the router. CGMP:
The frame 1 result is on the switch, with 3/1 being the port that is connected to the router:
Frame 2
Frame 2 is an IGMP membership report sent by the host to request (or confirm) that users want to
receive traffic for group 239.10.10.10.
ISL: ----- ISL Protocol Packet -----
ISL:
ISL: Destination Address = 01000C0000
ISL: Type = 0 (Ethernet)
ISL: User = 0 (Normal)
ISL: Source Address = 8C958B7B1000
ISL: Length = 76
ISL: Constant value = 0xAAAA03
ISL: Vendor ID = 0x8C958B
ISL: Virtual LAN ID (VLAN) = 2
ISL: Bridge Protocol Data Unit (BPDU) = 0
ISL: Port Index = 195
ISL: Reserved
Page 17
ISL:
ETHER: ----- Ethernet Header -----
ETHER:
ETHER: Destination = Multicast 01005E0A0A0A
!--- Destination is the GDA MAC. ETHER: Source = Station Cisco176DCCA !--- Sourced by
the PC connected in 3/1. ETHER: Ethertype = 0800 (IP) ETHER: IP: ----- IP Header ----
- IP: IP: Version = 4, header length = 20 bytes IP: Type of service = C0 IP: 110.
.... = internetwork control IP: ...0 .... = normal delay IP: .... 0... = normal
throughput IP: .... .0.. = normal reliability IP: Total length = 28 bytes IP:
Identification = 0 IP: Flags = 0X IP: .0.. .... = may fragment IP: ..0. .... = last
fragment IP: Fragment offset = 0 bytes IP: Time to live = 1 seconds/hops IP: Protocol
= 2 (IGMP) IP: Header checksum = CC09 (correct) IP: Source address = [10.1.1.2] IP:
Destination address = [224.10.10.10] IP: No options IP: IGMP: ----- IGMP header -----
IGMP: IGMP: Version = 1 IGMP: Type = 6 (Ver2 Membership Report) IGMP: Unused = 0x00
IGMP: Checksum = FFEA (correct) IGMP: Group Address = [224.10.10.10] IGMP:
Frame 3
Frame 3 is the CGMP frame sent by the router to the switch to tell the switch to add a static entry
for 01-00-5e-0a-0a-0a.
ISL: ----- ISL Protocol Packet -----
ISL:
ISL: Destination Address = 01000C0000
ISL: Type = 0 (Ethernet)
ISL: User = 0 (Normal)
ISL: Source Address = 8C958B7B1000
ISL: Length = 76
ISL: Constant value = 0xAAAA03
ISL: Vendor ID = 0x8C958B
ISL: Virtual LAN ID (VLAN) = 2
ISL: Bridge Protocol Data Unit (BPDU) = 0
ISL: Port Index = 193
ISL: Reserved
ISL:
ETHER: ----- Ethernet Header -----
ETHER:
ETHER: Destination = Multicast 01000CDDDDDD
ETHER: Source = Station Cisco11411E1
ETHER: 802.3 length = 24
ETHER:
LLC: ----- LLC Header -----
LLC:
LLC: DSAP Address = AA, DSAP IG Bit = 00 (Individual Address)
LLC: SSAP Address = AA, SSAP CR Bit = 00 (Command)
LLC: Unnumbered frame: UI
LLC:
SNAP: ----- SNAP Header -----
SNAP:
SNAP: Vendor ID = Cisco1
SNAP: Type = 2001 (CGMP)
SNAP:
CGMP: ----- CGMP -----
CGMP:
CGMP: Version = 16
CGMP: Type = 0 (Join)
CGMP: Reserved
CGMP: Count = 1
CGMP:
CGMP: Group Destination Address and Unicast Source Address
CGMP:
Page 18
CGMP: GDA =0100.5E0A.0A0A
!--- GDA MAC added in show cam static
!--- command output.
CGMP: USA =0000.0C76.DCCA
!--- MAC of the PC in 3/1. CGMP:
Below is the configuration of the router and the switch.
Router_A (router) Configuration:
Router_A#write terminal
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname Router_A
!
!
ip subnet-zero
ip multicast-routing
ip dvmrp route-limit 20000
interface FastEthernet0
no ip address
no ip directed-broadcast
!
interface FastEthernet0.1
encapsulation isl 1
ip address 10.1.1.1 255.255.255.0
no ip redirects
no ip directed-broadcast
!
interface FastEthernet0.2
encapsulation isl 2
ip address 10.2.2.1 255.255.255.0
no ip redirects
no ip directed-broadcast
ip pim dense-mode
ip cgmp
!
interface FastEthernet0.3
encapsulation isl 3
ip address 10.3.3.1 255.255.255.0
no ip redirects
no ip directed-broadcast
ip pim dense-mode
ip cgmp
!
Switch_B configuration for CGMP:
#cgmp
set cgmp enable
set cgmp leave enable
!
Page 19
CGMP statistics for VLAN 3:
Switch_B (enable) show cgmp sta 3
CGMP enabled
CGMP statistics for vlan 3:
valid rx pkts received 109
invalid rx pkts received 0
valid cgmp joins received 108
valid cgmp leaves received 1
valid igmp leaves received 1
valid igmp queries received 63
igmp gs queries transmitted 1
igmp leaves transmitted 1
failures to add GDA to EARL 0
topology notifications received 0
Switch_B (enable)
IGMP Snooping
IGMP snooping is another feature that allows you to directly capture IGMP frames. For IGMP
snooping support on Catalyst switches, refer to Multicast Catalyst Switches Support Matrix.
IGMP Snooping Overview
IGMP snooping, as implied by the name, is a feature that allows the switch to "listen in" on the
IGMP conversation between hosts and routers. When a switch hears an IGMP report from a host
for a given multicast group, the switch adds the host's port number to the GDA list for that group.
And, when the switch hears an IGMP Leave, it removes the host's port from the CAM table entry.
Learning the Router Port
The switch listens to the following messages in order to detect router ports with IGMP snooping:
IGMP Membership query send to 01-00-5e-00-00-01
G
PIMv1 hello send to 01-00-5e-00-00-02
G
PIMv2 hello send to 01-00-5e-00-00-0d
G
DVMRP probes send to 01-00-5e-00-04
G
MOSPF message send to 01-00-5e-00-05 or 06
G
By enabling IGMP snooping on a switch, all the above MAC entries are added to the show cam
system command output of the snooping switch. Once a router port is detected, it is added to the
port list of all GDAs in that VLAN.
Joining a Group With IGMP Snooping
The following are two joining scenarios:
Scenario A: Host A is the first host to join a group in the segment.
Host A sends an unsolicited IGMP Membership report.
1.
Page 20
The switch intercepts the IGMP Membership report that was sent by the host that wanted to
join the group.
2.
The switch creates a multicast entry for that group and links it to the port on which it has
received the report and to all router ports.
3.
The switch forwards the IGMP report to all router ports. This is so the router also receives the
IGMP report, and updates its multicast routing table accordingly.
4.
Scenario B: Host B now is the second host to join the same group.
Host B sends an unsolicited IGMP Membership report.
1.
The switch intercepts the IGMP Membership report sent by the host that wants to join the
group.
2.
The switch does not necessarily forward the IGMP report to all router ports. Actually, the
switch forwards IGMP reports to router ports using proxy reporting, and only forwards one
report per group within 10s.
3.
Note: In order to maintain group membership, the multicast router sends a IGMP query
every 60 seconds. This query is intercepted by the switch, and forwarded to all ports on the
switch. All hosts that are members of the group answer that query. But, given the fact that
the switch intercepts the reply report as well, the other host does not see each of the other
reports, and thus, all hosts send a report (instead of one per group). The switch then uses
Proxy Reporting as well, to forward only one report per group among all received responses.
Assume Host A wants to leave the group, but Host B still wants to receive the group.
The switch captures the IGMP Leave message from Host A.
G
The switch issues a group-specific IGMP Query for the group on that port (and only on that
port).
G
If the switch does not receive a report, it discards this port from the entry. If it receives a
response from that port, it does nothing and discards the leave.
G
Host B is still interested by that group on that switch. This would not be the last non-router port
in the entry. Therefore, the switch does not forward the Leave message.
G
Now, assume Host B wants to leave the group and Host B is the last user interested by this group
in this segment.
The switch captures the IGMP Leave message from Host A.
G
The switch issues a group-specific IGMP Query for that group on that port.
G
If the switch does not receives a report, it discards this port from the entry.
G
This is the last non-router port for that GDA. The switch forwards the IGMP Leave message to
all router ports and removes the entry from its table.
G
IGMP / CGMP Interaction
In some networks, due to hardware limitations, you might not be able to run IGMP snooping on all
switches. In this case, you might need to run CGMP on some switches in the same network.
Note that this is a special case. The switch running IGMP snooping detects CGMP messages and
detects that some switches in the network are running CGMP. Therefore, it moves to a special
IGMP-CGMP mode and disables the proxy reporting. This is absolutely necessary for the proper
operation of CGMP, because routers use the source MAC address of the IGMP report in order to
Page 21
create a CGMP Join. Routers running CGMP need to see all IGMP reports, so proxy reporting
must be disabled. Any reports sent to the router should only be those strictly needed for IGMP
snooping.
Multicast Source-Only Network
If the segment contains only one multicast server (multicast source) and no client, you might end
up with a situation where you do not have any IGMP packets in that segment, but you do have a
lot of multicast traffic. In this case, the switch simply forwards the traffic from that group to
everybody in the segment. Fortunately, a switch running IGMP snooping is able to detect these
multicast streams and adds a multicast entry for that group with only the router port. These entries
are flagged internally as mcast_source_only and are aged out each 5 minutes, or when the router
port goes away. Note that even after this aging, the address is relearned within a few seconds if
the traffic continues. Within the relearning period, momentary flooding can occur in the VLAN. In
order to avoid this and retain the entries, use the set igmp flooding enable | disable command. After the
flooding is disabled, the switch does not age out the source-only entries.
Limitations
As with CGMP, GDAs that map to a MAC that falls in the range 01-00-5e-00-00-xx is never pruned
by IGMP snooping.
Configuration of IGMP Snooping on Cisco Switches
To enable/disable IGMP snooping, issue the following command:
set igmp <enable/disable>
G
To configure the multicast router (static), issue the following command:
set multicast router <mod/port>
G
clear multicast router <mod |port | all>
G
To monitor and check IGMP statistics, issue the following commands:
show igmp statistics <VLAN_id>
G
show multicast router
G
Practical Example of IGMP Snooping
The setup for this example is similar to that CGMP testing, used earlier in this document. The only
difference is that port 3/2 and 3/3 are both connected to the same VLAN and are both client-
configured to join group 224.10.10.10.
The following example explains several manipulations, looks at what the switch does, and
examines the resulting output. In the following example, Switch_B is a Catalyst 5500 running
IGMP snooping, and Router_A is the multicast router connected to port 3/1.
Enable IGMP snooping on the switch and see the result by issuing the debug command.
Notice that each set of entries has been added to the show cam sys command output, allowing
for the detection of the router port through PIM, MOSPF, and so on.
Switch_B (enable) set igmp en
MCAST-IGMP: Set Sys Entries
1.
Page 22
MCAST-SYS-ENTRIES: Add system Entries in vlan 1
MCAST-IGMP: Set Sys Entries
MCAST-SYS-ENTRIES: Add system Entries in vlan 2
MCAST-IGMP: Set Sys Entries
MCAST-SYS-ENTRIES: Add system Entries in vlan 3
IGMP feature for IP multicast enabled
Switch_B (enable) show cam sys
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
1 00-10-2f-00-14-00 # 7/1
1 00-e0-fe-4b-f3-ff # 1/9
1 01-00-0c-cc-cc-cc # 1/9
1 01-00-0c-cc-cc-cd # 1/9
1 01-00-0c-dd-dd-dd # 1/9
1 01-00-0c-ee-ee-ee # 1/9
1 01-00-5e-00-00-01 # 1/9
1 01-00-5e-00-00-04 # 1/9
1 01-00-5e-00-00-05 # 1/9
1 01-00-5e-00-00-06 # 1/9
1 01-00-5e-00-00-0d # 1/9
1 01-80-c2-00-00-00 # 1/9
1 01-80-c2-00-00-01 # 1/9
2 00-10-2f-00-14-00 # 7/1
2 01-00-0c-cc-cc-cc # 1/9
2 01-00-0c-cc-cc-cd # 1/9
2 01-00-0c-dd-dd-dd # 1/9
2 01-00-5e-00-00-01 # 1/9
2 01-00-5e-00-00-04 # 1/9
2 01-00-5e-00-00-05 # 1/9
2 01-00-5e-00-00-06 # 1/9
2 01-00-5e-00-00-0d # 1/9
The switch receives a PIMv2 packet from router Router_A and adds the router port.
MCAST-IGMPQ:recvd a PIM V2 packet of type HELLO on the port 3/1 vlanNo 2
MCAST-ROUTER: Adding port 3/1, vlanNo 2
MCAST-ROUTER: Creating RouterPortTimer for port 3/1, vlanNo 2
MCAST-IGMPQ:recvd a PIM V2 packet of type HELLO on the port 3/1 vlanNo 3
MCAST-ROUTER: Adding port 3/1, vlanNo 3
MCAST-ROUTER: Creating RouterPortTimer for port 3/1, vlanNo 3
Switch_B (enable) show multi router
CGMP disabled
IGMP enabled
Port Vlan
3/1 2-3
Total Number of Entries = 1
'*' - Configured
Switch_B (enable)
2.
Connect a new host in group 224.10.10.10 (on port 3/2). This host sends an IGMP
membership report. The report is received, snooped by the switch, the entry is added, and
the IGMP report is forwarded to the router.On Switch_B
3.
Page 23
MCAST-IGMPQ:recvd an IGMP V2 Report on the port 3/2 vlanNo 3
GDA 224.10.10.10
MCAST-RELAY:Relaying packet on port 3/1 vlanNo 3
MCAST-SEND: Inband Transmit Succeeded for IGMP RELAY msg on port 3/1
vlanNo 3
Switch_B (enable) show cam static
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
3 01-00-5e-0a-0a-0a 3/1-2
Add one more user in VLAN 3 on port 3/3, as shown below.
Switch_B (enable) show cam static
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
3 01-00-5e-0a-0a-0a 3/1-3
4.
Remove port 3/2. Port 3/2 sends an IGMP Leave message; the switch sends back an IGMP
group-specific query on port 3/2 and starts a timer. When the timer expires without receiving
a response, it deletes the port from the group.
MCAST-IGMPQ:recvd an IGMP Leave on the port 3/2 vlanNo 3 GDA 224.10.10.10
MCAST-IGMPQ-LEAVE:router_port_tbl[vlanNo].QueryTime = 0
MCAST-DEL-TIMER: Deletion Timer Value set to Random Value 1
MCAST-SEND:Transmitting IGMP Mac Based GS Query msg on port 3/2 vlanNo 3
MCAST-SEND: Transmit Succeeded for IGMP Group Specific Query msg on port 3/2 vlanNo 3
MCAST-TIMER:IGMPLeaveTimer expired on port 3/2 vlanNo 3 GDA 01-00-5e-0a-0a-0a
MCAST-TIMER:IGMPLeaveTimer:delete leave timer
Switch_B (enable) show cam static
- = Static Entry. + = Permanent Entry. # = System Entry. R = Router Entry.
X = Port Security Entry
VLAN Dest MAC/Route Des [CoS] Destination Ports or VCs / [Protocol Type]
3 01-00-5e-0a-0a-0a 3/1,3/3
5.
The host on port 3/3 leaves the group and sends an IGMP Leave message. The only
difference from the previous point is that the IGMP Leave message is finally forwarded to the
router port.
MCAST-IGMPQ:recvd an IGMP Leave on the port 3/3 vlanNo 3 GDA 224.10.10.10
MCAST-SEND:Transmitting IGMP Mac Based GS Query msg on port 3/3 vlanNo 3
MCAST-SEND: Transmit Succeeded for IGMP Group Specific Query msg on
port 3/3 vlanNo 3
MCAST-TIMER:IGMPLeaveTimer expired on port 3/3 vlanNo 3 GDA
01-00-5e-0a-0a-0a
MCAST-TIMER:IGMPLeaveTimer expiry: Transmit IGMP Leave on port 3/1
vlanNo 3
MCAST-SEND:Transmitting IGMP Leave msg on port 3/1 vlanNo 3
6.
Page 24
MCAST-SEND: Inband Transmit Succeeded for IGMP Leave Message on port 3/1
vlanNo 3
MCAST-TIMER:IGMPLeaveTimer:delete leave timer
The subnet configuration is now back at the beginning, its state in Step 1. The multicast entry has
disappeared from the show cam static command output.
To finish, view an example of the show igmp static command output, as shown below.
Switch_B (enable) show igmp stat 2
IGMP enabled
IGMP statistics for vlan 2:
Total valid pkts rcvd: 329
Total invalid pkts recvd 0
General Queries recvd 82
Group Specific Queries recvd 0
MAC-Based General Queries recvd 0
Leaves recvd 0
Reports recvd 82
Queries Xmitted 0
GS Queries Xmitted 0
Reports Xmitted 0
Leaves Xmitted 0
Failures to add GDA to EARL 0
Topology Notifications rcvd 0
Switch_B (enable) show igmp stat 3
IGMP enabled
IGMP statistics for vlan 3:
Total valid pkts rcvd: 360
Total invalid pkts recvd 0
General Queries recvd 93
Group Specific Queries recvd 6
MAC-Based General Queries recvd 0
Leaves recvd 11
Reports recvd 64
Queries Xmitted 0
GS Queries Xmitted 14
Reports Xmitted 0
Leaves Xmitted 10
Failures to add GDA to EARL 0
Topology Notifications rcvd 1
Switch_B (enable)
Related Information
Multicast Catalyst Switches Support Matrix
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IP Multicast Support Page
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Cisco Technology Support
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Cisco Product Support
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Technical Support & Documentation - Cisco Systems
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