9-1. IGMP Snooping Some Catalyst switches can be configured to intercept IGMP join requests as hosts ask to join IP multicast groups. IGMP join requests can occur as the following happen: - Hosts send unsolicited membership reports to join specific multicast groups. - Multicast routers acting as IGMP queriers send IGMP membership query messages to the all-hosts multicast group 224.0.0.1 every 60 seconds. Interested hosts respond with membership reports to join specific multicast groups.
The switch keeps a record of the IP multicast group, its Layer 2 MAC address, and the switch ports that connect to the requesting host and the multicast router. Multicast routers cannot keep a detailed list of all hosts belonging to a multicast group. Rather, a router knows only which multicast groups are active on specific subnets. The switch also relays the initial join request for a multicast group to all its known multicast routers. If no multicast routers are present in the network, the switch can be configured to act as an IGMP querier. When a host wants to leave a multicast group, IGMPv1 detects only the absence of its membership reports. IGMPv2, however, allows the host to send an IGMP leave group message to the "all-routers" multicast group 224.0.0.2 at any time. When the switch intercepts an IGMP leave group message on a switch port, it normally sends a query to that multicast group back out the same switch port. If no hosts respond to the query and no multicast routers have been discovered on the switch port, that port is removed from the multicast group. IGMP Fast-Leave Processing can be used to allow the switch to immediately remove a port from a multicast group after a Leave Group message is received. Spanning tree topology changes that occur in a VLAN also cause the switch to purge any multicast group information learned through IGMP snooping. That information must be relearned.
TIP IGMP snooping is supported on all Catalyst switch platforms except the 2900XL and 3500XL families. IGMP snooping requires specialized hardware to intercept and inspect every IGMP multicast packethardware that is not available on the lower-end switch models.
Configuration 1. | (Optional) Enable IGMP snooping:
COS | set igmp {enable | disable}
| IOS | (interface) ip igmp snooping
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IGMP snooping is enabled by default, except on the Catalyst 5000 family.
| 2. | (Optional; Catalyst 6000 IOS only) Allow snooping to learn from another source:
COS | N/A | IOS | [View full width] (interface) ip igmp snooping mrouter learn {cgmp  |) pim-dvmrp}
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In addition to normal IGMP snooping, the switch can also learn by listening to CGMP messages (cgmp) or PIM-DVMRP messages (pim-dvmrp).
| 3. | (Optional) Use IGMP Fast-Leave Processing:
COS | set igmp fastleave {enable | disable}
| IOS | (interface) ip igmp snooping fast-leave
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By default, Fast-Leave Processing is disabled. Fast-Leave improves the latency of multicast group removal, but should be used only on VLANs where single hosts are connected to each switch port.
| 4. | (Optional) Statically identify a multicast router port:
COS | set multicast router mod/port
| IOS | [View full width] (interface) ip igmp snooping mrouter {interface {interface interface-number} | {Port-channel number}}
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IGMP snooping automatically detects ports where multicast routers are connected. You can also give a static definition of a multicast router port.
| 5. | (Optional) Define a static multicast host entry:
COS | set cam {static | permanent} {mac-address} {mod/port}
| IOS | [View full width] (interface) ip igmp snooping static {mac-address} {interface {interface interface-number}} | {Port-channel number}}
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The host connected to the specified interface is statically joined to multicast group mac-address (dotted-triplet format) on the current VLAN interface. On the COS switch, the static entry can be used until the next reboot (static) or even across the next reboot (permanent).
| 6. | (Optional) Act as an IGMP querier.
- a. Enable the querier:
COS | set igmp querier {enable | disable} vlan
| IOS | (interface) ip igmp snooping querier
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By default, the IGMP querier function is disabled. If no other multicast routers are available and there is no need to route multicast packets on the local network, the switch can provide the IGMP querier function. Use the enable keyword and specify the vlan number where the querier will be used.
- b. (Optional) Adjust the query interval:
COS | set igmp querier vlan qi seconds
| IOS | (global) ip igmp query-interval seconds
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The time between general IGMP queries or the query interval, on the vlan number can be set to seconds. (The default is 125 seconds.)
- c. (Optional) Adjust the self-election interval:
COS | set igmp querier vlan oqi seconds
| IOS | (global) ip igmp query-timeout seconds
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If more than one querier is present on a VLAN, only one of them is elected to remain the querier. If no other general IGMP queries are overheard on the vlan number for seconds (default 300 seconds), the switch will elect itself as the querier.
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TIP Querier election takes place by using the source IP address in the general query messages. For a specific VLAN, switches use the IP address from their VLAN interface as the IGMP source address. The lowest IP address wins the querier election.
IGMP Snooping Example IGMP snooping is enabled globally on the COS switch and on specific interfaces on the IOS switch. IGMP Fast-Leave Processing is allowed. A static entry for IP multicast group address 224.100.1.35 (MAC address 01-00-5e-64-01-23) is configured that lists switch ports 2/1 and 2/3 as permanent members. These switch ports are assigned to a common VLAN 199. COS | set igmp enable set igmp fastleave enable set vlan 199 2/1-48 set cam permanent 01-00-5e-64-01-23 2/1,2/3
| IOS | [View full width] (global) interface fastethernet 2/1 (interface) ip igmp snooping (interface) ip igmp snooping fast-leave (interface) switchport access vlan 199 (global) interface fastethernet 2/3 (interface) ip igmp snooping (interface) ip igmp snooping fast-leave (interface) switchport access vlan 199 (global) interface vlan 199 (interface) ip igmp snooping static 0100.5364.0123 interface fastethernet 2/1 (interface) ip igmp snooping static 0100.5364.0123 interface fastethernet 2/3
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Displaying Information About IGMP Snooping Table 9-2 lists some switch commands that you can use to display helpful information about IGMP snooping. Table 9-2. Switch Commands to Display IGMP Snooping InformationDisplay Function | Switch OS | Command |
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Multicast protocols in use | COS | show multicast protocols status
| IOS | N/A | IGMP statistics | COS | show igmp statistics [vlan-id]
| IOS | [View full width] (exec) show ip igmp interface interface interface-number
| Multicast routers discovered | COS | show multicast router igmp [mod/port] [vlan-id]
| IOS | [View full width] (exec) show ip igmp snooping mrouter interface vlan vlan-id
| Number of multicast groups in a VLAN | COS | show multicast group count [vlan-id]
| IOS | (exec) show mac-address-table multicast vlan-id count
| Multicast group information | COS | show multicast group [mac-addr] [vlan-id]
-or- show mac multicast [{Vlan vlan-id | grp-mac-addr}]
| IOS | [View full width] (exec) show mac-address-table multicast {mac-group-address [vlan-id]}
| IGMP snooping on an interface | COS | N/A | IOS | (exec) show ip igmp interface vlan-id
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