8.32 Catalyst IGMP Snooping
• Configure R6’s Fa0/0 interface with the ip address 155.1.146.6/24
• Join the multicast group 239.1.1.100 on the VLAN 146 interfaces of R4
and R6.
• Configure the switches to assist in optimal traffic flooding across the
switched topology for VLAN 146.
• Ensure a switch stops flooding out of a given port as soon as it receives
an IGMPv2 report.
--------------------------------------------------
Multicast poses special difficulties in switched Ethernet networks. By default, IP
Multicast addresses are mapped to Ethernet multicast MAC addresses per the
following procedure:
1) The multicast Ethernet MAC address range starts at 01:00:5E:00:00:00 and
goes through 01:00:5E:7F:FF:FF. The highest bit of the 4th byte in these
addresses is fixed at 1, and thus only low-order 23 bits can vary. This allocation
is historical.
2) Based on this range, the low-order 23 bits of the multicast IP address are
mapped into the low-order 23 bits of the MAC address.
3) The high order 4 bits of the Layer 3 IP address is fixed to 1110 to indicate the
Class D address space between 224.0.0.0 and 239.255.255.255. Thus, 28 bits
remain for IP Multicast group numbering.
Therefore, there are 2^5 multicast groups mapped to the same multicast MAC
address. However, this is only a part of the puzzle. As you remember, Ethernet
switches flood multicast traffic out of all ports by default. In heavily loaded
networks, this might result in excessive bandwidth usage. In order to overcome
this issue, it’s possible for switches to listen to IGMP and PIM messages
received on Layer 2 ports. Based on the information snooped from these
messages, switches may selectively prune some ports from unneeded multicast
traffic. This procedure is called IGMP snooping and allows for selective multicast
flooding in switched networks.
Notice that in order to work effectively, IGMP snooping must be implemented on
Layer 3 capable switches. This is because Layer 2 devices cannot distinguish
IGMP and PIM packets from any other multicast frames, and must process all
multicast traffic at the CPU level. Thus, a layer 2 switch’s performance might be
severely impacted by intense multicast flows.
IGMP snooping is enabled by default on Catalyst multi-layer switches. Snooping
is enabled globally and on a per-VLAN basis. If you want to disable IGMP
snooping globally on all VLANs, use the command no ip igmp snooping.
Use the command no ip igmp snooping vlan <VLAN-ID> to disable it for
a single VLAN. Generally, switches automatically discover switchports connected
to multicast-capable routers by listening to PIM and DVMRP messages, and
flood all multicast groups on such ports. If you want to statically configure a port
as connected to a multicast router, use the command ip igmp snooping
vlan <vlan-id> mrouter interface <interface-id>.
If your switch has just one host connected to every layer 2 switch-port, you may
want to enable the IGMP Snooping Immediate Leave feature using the command
ip igmp snooping vlan <vlan-id> immediate-leave. When you
enable this feature, the switch will immediately remove a port from the flood list
for a given group once it receives the IGMPv2 Leave message on an interface
that is part of a particular group.
---------------------------------------------------------------
R1:
ip multicast-routing
!
interface FastEthernet0/0
ip address 155.1.146.1 255.255.255.0
ip pim dense-mode
ip igmp join-group 239.1.1.100
R4:
ip multicast-routing
!
interface FastEthernet0/1
ip address 155.1.146.4 255.255.255.0
ip pim dense-mode
ip igmp join-group 239.1.1.100
R6:
ip multicast-routing
!
interface FastEthernet0/0
ip address 155.1.146.6 255.255.255.0
ip pim dense-mode
ip igmp join-group 239.1.1.100
SW1:
ip igmp snooping vlan 146 immediate-leave
!
interface FastEthernet0/1
switchport access vlan 146
SW2:
ip igmp snooping vlan 146 immediate-leave
!
interface FastEthernet0/6
switchport access vlan 146
SW4:
ip igmp snooping vlan 146 immediate-leave
!
interface FastEthernet0/4
switchport access vlan 146
!
---------------------------------------------------------------
First, check the IGMP Snooping general settings for VLAN 146. Notice that
“IGMPv2 immediate leave” is enabled for this VLAN.
Rack1SW1#show ip igmp snooping vlan 146
Global IGMP Snooping configuration:
-------------------------------------------
IGMP snooping : Enabled
IGMPv3 snooping (minimal) : Enabled
Report suppression : Enabled
TCN solicit query : Disabled
TCN flood query count : 2
Robustness variable : 2
Last member query count : 2
Last member query interval : 1000
Vlan 146:
--------
IGMP snooping : Enabled
IGMPv2 immediate leave : Enabled
Multicast router learning mode : pim-dvmrp
CGMP interoperability mode : IGMP_ONLY
Robustness variable : 2
Last member query count : 2
Last member query interval : 1000
Rack1SW1#
---------------------------------------------------------------
Now join R1’s VLAN 146 interface to the IGMP group 239.1.1.100 and check that
IGMP snooping actually processes the IGMP packets. From the commands
output, you can see that there are two ports in the flood list for the group – one
for the receiver, and the other for the multicast router (R4).
R1:
interface FastEthernet 0/0
ip igmp join-group 239.1.1.100
Rack1SW1#show ip igmp snooping groups vlan 146
Vlan Group Type Version Port List
-----------------------------------------------------------------------
146 224.0.1.40 igmp v2 Fa1/0/1, Fa1/0/13,
Fa1/0/19
146 239.1.1.100 igmp v2 Fa1/0/1, Fa1/0/13,
Fa1/0/19
Rack1SW1#
Rack1SW1#sh ip igmp snooping mrouter vlan 146
Vlan ports
---- -----
146 Fa1/0/1(dynamic), Fa1/0/13(dynamic), Fa1/0/19(dynamic)
Rack1SW1#
Rack1SW4#show ip igmp snooping groups vlan 146
Vlan Group Type Version Port List
-----------------------------------------------------------------------
146 224.0.1.40 igmp v2 Fa1/0/4, Fa1/0/13
146 239.1.1.100 igmp v2 Fa1/0/4, Fa1/0/13
Rack1SW4#
Rack1SW4#show ip igmp snooping mrouter vlan 146
Vlan ports
---- -----
146 Fa1/0/4(dynamic), Fa1/0/13(dynamic)
Rack1SW4#
---------------------------------------------------------------
From R1 ping the multicast group:
R1:
Ping 239.1.1.100 repeat 1000
Multicast router information is learned via PIM messages. There are two
multicast routers on VLAN 146 – R4 and R6. Confirm that R4 actually receives
the IGMP report from R1.
Rack1R1#ping 239.1.1.100 repeat 10
Type escape sequence to abort.
Sending 10, 100-byte ICMP Echos to 239.1.1.100, timeout is 2 seconds:
Reply to request 0 from 155.1.146.1, 16 ms
Reply to request 1 from 155.1.146.1, 12 ms
Reply to request 1 from 155.1.146.4, 24 ms
Reply to request 1 from 155.1.146.6, 24 ms
Reply to request 2 from 155.1.146.4, 12 ms
Reply to request 2 from 155.1.146.6, 28 ms
Reply to request 2 from 155.1.146.1, 16 ms
Reply to request 3 from 155.1.146.1, 12 ms
Reply to request 3 from 155.1.146.6, 28 ms
Reply to request 3 from 155.1.146.4, 28 ms
Reply to request 4 from 155.1.146.1, 12 ms
Reply to request 4 from 155.1.146.4, 24 ms
Reply to request 4 from 155.1.146.6, 24 ms
Reply to request 5 from 155.1.146.1, 12 ms
Reply to request 5 from 155.1.146.4, 56 ms
Reply to request 5 from 155.1.146.6, 52 ms
Reply to request 6 from 155.1.146.1, 12 ms
Reply to request 6 from 155.1.146.4, 52 ms
Reply to request 6 from 155.1.146.6, 32 ms
Reply to request 7 from 155.1.146.1, 8 ms
Reply to request 7 from 155.1.146.4, 48 ms
Reply to request 7 from 155.1.146.6, 48 ms
Reply to request 8 from 155.1.146.1, 12 ms
Reply to request 8 from 155.1.146.4, 56 ms
Reply to request 8 from 155.1.146.6, 52 ms
Reply to request 9 from 155.1.146.1, 12 ms
Reply to request 9 from 155.1.146.4, 36 ms
Reply to request 9 from 155.1.146.6, 32 ms
Rack1R1#
Rack1R4#show ip igmp groups
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter Group Accounted
239.1.1.100 FastEthernet0/1 00:09:13 00:02:22 155.1.146.1
224.0.1.40 FastEthernet0/1 00:09:15 00:02:19 155.1.146.4
Rack1R4#
Rack1R6#show ip igmp groups
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter Group Accounted
239.1.1.100 FastEthernet0/0 00:09:26 00:01:55 155.1.146.1
224.0.1.40 FastEthernet0/0 00:09:28 00:02:59 155.1.146.6
Rack1R6#
Rack1R1#show ip igmp groups
IGMP Connected Group Membership
Group Address Interface Uptime Expires Last Reporter Group Accounted
239.1.1.100 FastEthernet0/0 00:10:06 00:02:38 155.1.146.6
224.0.1.40 FastEthernet0/0 00:10:16 00:02:31 155.1.146.6
Rack1R1#
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