Reference Guide
Multicast Features | 637
In Figure 33-2, VLAN 400 is configured with an access list to permit only IGMP reports for group
239.0.0.1. Though Receiver 2 sends a membership report for groups 239.0.0.1 and 239.0.0.2, a multicast
routing table entry is created only for group 239.0.0.1. VLAN 300 has no access list limiting Receiver 1, so
both IGMP reports are accepted, and two corresponding entries are created in the routing table.
Figure 33-2. Preventing a Host from Joining a Group
Rate Limit IGMP Join Requests
If you expect a burst of IGMP Joins, protect the IGMP process from overload by limiting that rate at which
new groups can be joined using the command
ip igmp group-join-limit from INTERFACE mode. Hosts
whose IGMP requests are denied will use the retry mechanism built-in to IGMP so that they’re
membership is delayed rather than permanently denied.
View the enable status of this feature using the command
show ip igmp interface from EXEC Privilege
mode.
FTOS Behavior: Do not enter the command ip igmp access-group before creating the access-list. If
you do, upon entering your first deny rule, FTOS clears multicast routing table and re-learns all groups,
even those not covered by the rules in the access-list, because there is an implicit deny all rule at the
end of all access-lists. Therefore, configuring an IGMP join request filter in this order might result in
data loss. If you must enter the command ip igmp access-group before creating the access-list,
prevent FTOS from clearing the routing table by entering a permit any rule with high sequence number
before you enter any other rules.
R1
R2
R3
Source 2
10.11.1.2
Source 1
10.11.5.2
Receiver 2
10.11.4.2
Receiver 1
10.11.3.2
Group: 239.0.0.1, 239.0.0.2
1/31
3/11
2/11
2/31 3/21
interface Vlan 300
ip pim sparse-mode
ip address 10.11.3.1/24
untagged GigabitEthernet 1/1
no shutdown
interface GigabitEthernet 1/21
ip pim sparse-mode
ip address 10.11.12.1/24
no shutdown
interface GigabitEthernet 1/31
ip pim sparse-mode
ip address 10.11.13.1/24
no shutdown
interface GigabitEthernet 2/1
ip pim sparse-mode
ip address 10.11.1.1/24
no shutdown
interface GigabitEthernet 2/11
ip pim sparse-mode
ip address 10.11.12.2/24
no shutdown
interface GigabitEthernet 3/11
ip pim sparse-mode
ip address 10.11.13.2/24
no shutdown
interface GigabitEthernet 3/21
ip pim sparse-mode
ip address 10.11.23.2/24
no shutdown
interface GigabitEthernet 2/31
ip pim sparse-mode
ip address 10.11.23.1/24
no shutdown
3/1
ip multicast-routing
ip pim rp-address 10.11.12.2 group-address 224.0.0.0/4
router rip
network 10.0.0.0
1/21
ip igmp snooping enable
2/1
2/1
interface Vlan 400
ip pim sparse-mode
ip address 10.11.4.1/24
untagged GigabitEthernet 1/2
ip igmp access-group igmpjoinfilR2G2
no shutdown
interface GigabitEthernet 3/1
ip pim sparse-mode
ip address 10.11.5.1/24
no shutdown
RP
Group: 239.0.0.1, 239.0.0.2
R1(conf-if-vl-300)# do show ip pim tib
PIM 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,
M - MSDP created entry, A - Candidate for MSDP Advertisement
K - Ack-Pending State
Timers: Uptime/Expires
Interface state: Interface, next-Hop, State/Mode
(*, 239.0.0.1), uptime 00:00:07, expires 00:00:00, RP 10.11.12.2, flags: S
CJ
Incoming interface: GigabitEthernet 1/21, RPF neighbor 10.11.12.2
Outgoing interface list:
Vlan 300 Forward/Sparse 00:00:07/Never
(*, 239.0.0.2), uptime 00:01:10, expires 00:00:00, RP 10.11.12.2, flags: S
CJ
Incoming interface: GigabitEthernet 1/21, RPF neighbor 10.11.12.2
Outgoing interface list:
Vlan 300 Forward/Sparse 00:01:10/Never
R1(conf-if-vl-400)# do show ip pim tib
PIM 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,
M - MSDP created entry, A - Candidate for MSDP Advertisement
K - Ack-Pending State
Timers: Uptime/Expires
Interface state: Interface, next-Hop, State/Mode
(*, 239.0.0.1), uptime 00:00:06, expires 00:00:00, RP 10.11.12.2, flags: SCJ
Incoming interface: GigabitEthernet 1/21, RPF neighbor 10.11.12.2
Outgoing interface list:
Vlan 400 Forward/Sparse 00:00:06/Never
R1(conf)#do show run acl
!
ip access-list extended igmpjoinfilR2G2
seq 5 permit ip any host 239.0.0.1
239.0.0.1
239.0.0.2