WB1 8.23 IGMP Timers

8.23 IGMP Timers

• Configure the designated IGMP querier on the VLAN 146 segment so that
failed multicast traffic receivers are detected and removed within 60 seconds.
• Every active receiver should respond to general IGMP queries within 4 seconds.
• Designated querier failures should be detected two times faster than by default.
• Since there is just one receiver on R3’s connection to SW1, configure the
router to remove multicast group state immediately after a corresponding
IGMP leave report has been received.

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IGMP reports are sent asynchronously, and thus might be missed by the router.
Therefore, one of the multicast-capable routers sharing the same segment will be
elected as the designated IGMP querier, and will periodically send IGMP
Membership queries to all hosts on the segment. Every host receiving the
Membership query should respond with an IGMP report containing all joined
groups. The designated querier is the router with the lowest IP address on the
segment, if the running IGMP version is 2 or 3. Notice that the PIM DR on the
segment is elected based on the highest IP address by default, and thus some
functional decoupling and load-balancing may occur if there is more than one
router on the segment. In addition to sending the periodic membership queries,
the designated querier also builds PIM SM shared trees, on behalf of the
receivers signaling multicast group membership.

Periodic queries are sent at the interval defined by the interface-level command
ip igmp query-interval <seconds>. If a non-designated multicast router
does not hear the membership queries for more than the interval defined by the
command ip igmp querier-timeout <seconds> it will attempt to become
a designated querier itself, assuming that the old one failed. If the latter
command is not configured on the interface, the querier timeout equals twice the
query-interval configured on the same interface. The default query-interval value
is 60 seconds (though the RFC recommends 125 seconds).

When IGMPv1 is configured on an interface explicitly, the router times out the
group state if there is no report for this group during 3 consecutive membership
queries (180 seconds by default). This might result in very high leave latency, as
IGMPv1 has no explicit group leave message. IGMPv2 significantly enhances the
procedure of leaving a multicast group as detailed below:

1) Every membership query message contains a special timer value, defined by
the command ip igmp query-max-response-time [time-inseconds].
Every host on the segment is supposed to send an IGMP report
during the time-window defined by this interval. All hosts count to the interval
value and randomly fire the IGMP report. If a host hears another report for the
same group, it suppresses its own message. Thus, excessive report flooding is
avoided. If there is no report for a given group during the query-max-response
interval, the router removes the mroute state for this group.

2) Every host, willing to leave a group, may send out a special IGMP Leave
report for this particular group. As soon as the router receives a leave report, it
generates a special IGMP group specific query for the multicast channel in
question. This query is needed to check if there are any other hosts on the
segment that still need this group. The number of special queries generated is
defined by the command ip igmp last-member-query-count <N> which
is set to 2 by default. The Queries are sent at the intervals specified by the
command ip igmp last-member-query-interval <milliseconds>
which is 1000ms by default. If there is no answer to the special queries, the
group state is removed from the mroute table.

3) If there is just one receiver for a particular group on the segment, e.g. if there
is just one host connected to the router, the leave latency could be further
reduced by configuring the ip igmp immediate-leave group-list
<access-list> command. If there is an IGMP leave message received on the
interface for a group matching the access-list, the mroute state is immediately
removed, without any further delays. The access-list is a standard ACL that
defines the groups eligible for this feature.
Notice that the explicit leave feature is also available for IGMPv3, with the
extension to the multicast group source. That is to say, a host may leave a
particular sender for the group, while continuing to listen to other senders.

For our scenario, R1 is the designated querier for the segment. Since R4 has the
next lowest IP address, it is the backup querier. In real life, you would configure
every router on the segment with the same settings, but this task’s goal is to
check your understanding of the querier election. For the immediate leave
feature we configure only the groups permitted by the IGMP access-group. You
may configure it for all groups, as the resulting effect would be the same.

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R1:
interface FastEthernet0/0
ip igmp query-interval 20
ip igmp query-max-response-time 4

R3:
ip access-list standard IMMEDIATE_LEAVE
permit 239.1.1.0 0.0.0.255
!
interface FastEthernet 0/0
ip igmp immediate-leave group-list IMMEDIATE_LEAVE

R4:
interface FastEthernet0/1
ip igmp querier-timeout 60

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Verify timers using the show igmp interface command.

Rack1R1#show ip igmp interface fastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
  Internet address is 155.1.146.1/24
  IGMP is enabled on interface
  Current IGMP host version is 2
  Current IGMP router version is 2
  IGMP query interval is 20 seconds
  IGMP querier timeout is 40 seconds
  IGMP max query response time is 4 seconds
  Last member query count is 2
  Last member query response interval is 1000 ms
  Inbound IGMP access group is not set
  IGMP activity: 4 joins, 2 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is 155.1.146.1 (this system)
  IGMP querying router is 155.1.146.1 (this system)
  No multicast groups joined by this system
Rack1R1#

Rack1R4#sh ip igmp interface fastEthernet 0/1
FastEthernet0/1 is up, line protocol is up
  Internet address is 155.1.146.4/24
  IGMP is enabled on interface
  Current IGMP host version is 2
  Current IGMP router version is 2
  IGMP query interval is 60 seconds
  IGMP querier timeout is 60 seconds
  IGMP max query response time is 10 seconds
  Last member query count is 2
  Last member query response interval is 1000 ms
  Inbound IGMP access group is not set
  IGMP activity: 4 joins, 2 leaves
  Multicast routing is enabled on interface
  Multicast TTL threshold is 0
  Multicast designated router (DR) is 155.1.146.1 
  IGMP querying router is 155.1.146.1 
  Multicast groups joined by this system (number of users):
      224.0.1.40(1)
Rack1R4#

Now enable IGMP debugging on R3 and unsubscribe SW1 from the group
239.1.1.7. Notice that the group state is removed immediately, without any
additional last member queries.

SW1:
interface FastEthernet 0/3
no ip igmp join-group 239.1.1.7

Rack1R3#debug ip igmp
IGMP(0): Received Leave from 155.1.37.7 (FastEthernet0/0) for 239.1.1.7
IGMP(0): Leave group 239.1.1.7 immediately on FastEthernet0/0
IGMP_ACL(0): Group 239.1.1.7 unaccounted on FastEthernet0/0
IGMP(0): Send v2 general Query on FastEthernet0/0
IGMP(0): Deleting 239.1.1.7 on FastEthernet0/0
IGMP(0): Helper Leave for group 239.1.1.7 out Serial1/0.1
IGMP(0): Received v2 Query on Serial1/0.1 from 155.1.0.5
IGMP(0): Previous querier timed out, v2 querier for Serial1/0.1 is this
systemv2 querier for Serial1/0.1 is this system.
IGMP(0): Received v2 Query on Serial1/0.1 from 155.1.0.5
IGMP(0): Received v2 Query on Serial1/0.1 from 155.1.0.5
IGMP(0): Previous querier timed out, v2 querier for Serial1/0.1 is this
systemv2 querier for Serial1/0.1 is this system.
IGMP(0): Received v2 Report on FastEthernet0/0 from 155.1.37.7 for
239.2.2.7
IGMP(*): Group 239.2.2.7 access denied on FastEthernet0/0