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392 Nokia Network Voyager for IPSO 4.0 Reference Guide
6. (Optional) Enter a value between 20 and 4000 in the Route expiration time text box to set
the interval, in seconds, after which a route that has not been refreshed is placed in the route
hold-down queue.
The default is 140 seconds.
7. (Optional) Enter a value between 0 and 8000 in the Route hold-down period text box to set
the interval, in seconds, for which an expired route is kept in the hold-down queue before it
is deleted from the route database. Set this interval to twice the value of the route report
interval.
The default is 120 seconds.
8. (Optional) Enter a value between 60 and 86400 in the Cache lifetime text box to set the
interval, in seconds that a cached multicast forwarding entry is maintained in the kernel
forwarding table before it is timed out because of inactivity.
The default is 300 seconds.
9. Click Apply, and then click Save to make your changes permanent.
IGMP
Internet Group Management Protocol (IGMP) allows hosts on multiaccess networks to inform
locally attached routers of their group membership information. Hosts share their group
membership information by multicasting IGMP host membership reports. Multicast routers
listen for these host membership reports, and then exchange this information with other
multicast routers.
The group membership reporting protocol includes two types of messages: host membership
query and host membership report. IGMP messages are encapsulated in IP datagrams, with an IP
protocol number of 2. Protocol operation requires that a designated querier router be elected on
each subnet and that it periodically multicast a host membership query to the all-hosts group.
Hosts respond to a query by generating host membership reports for each multicast group to
which they belong. These reports are sent to the group being reported, which allows other active
members on the subnet to cancel their reports. This behavior limits the number of reports
generated to one for each active group on the subnet. This exchange allows the multicast routers
to maintain a database of all active host groups on each of their attached subnets. A group is
declared inactive (expired) when no report is received for several query intervals.
The IGMPv2 protocol adds a leave group message and uses an unused field in the IGMPv.1 host
membership query message to specify a maximum response time. The leave group message
allows a host to report when its membership in a multicast group terminates. Then, the IGMP
querier router can send a group-directed query with a very small maximum response time to
probe for any remaining active group members. This accelerated leave extension can reduce the
time required to expire a group and prune the multicast distribution tree from minutes, down to
several seconds
The unicast traceroute program allows the tracing of a path from one device to another, using
mechanisms that already exist in IP. Unfortunately, you cannot apply such mechanisms to IP
multicast packets. The key mechanism for unicast traceroute is the ICMP TTL exceeded
message that is specifically precluded as a response to multicast packets. The traceroute facility