Troubleshooting guide

11-43
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-01
Chapter 11 Troubleshooting Layer 3 Network Connections
Troubleshooting Layer 3 IP Multicast Switching
If you have any problems with these fields, check the interface configuration. For information about
configuring interfaces, refer to the Layer 3 Software Feature and Configuration Guide.
Troubleshooting Layer 3 IP Multicast Switching
IP multicast allows IP traffic to be sent from one source or multiple sources and delivered to multiple
destinations. Instead of sending individual packets to each destination, which is highly taxing to the
switch fabric, a single packet is sent to a multicast group, which is identified by a single IP destination
group address. That IP destination group consists of a number of IP destinations that require that frame.
From a router perspective, an input multicast feed from a given source must be sent out through
(possibly) multiple output interfaces based on the information received by the multicast routing
protocols such as PIM.
Layer 3 IP Multicast Overview
The Layer 3 enabled ATM switch router supports IP multicast at wire speed for all ports, allowing for
high-speed switching of packets from input source ports to multiple destination ports. The Layer 3
enabled ATM switch router also supports IP multicast routing protocols such as PIM dense and sparse
modes, as well as DVMRP interoperability.
Internet Group Management Protocol
The Internet Group Management Protocol (IGMP) provides a method for end stations to request
multicast traffic as well as for switch router to determine who on a locally attached segment is requesting
traffic. IGMP uses IP datagrams to allow IP multicast applications to join a multicast group. IGMP relies
on Class D IP addresses for the creation of multicast groups and is defined in RFC 1112. Membership
in a multicast group is dynamic, meaning that it changes over time as hosts join and leave the group.
Multicast switch routers use IGMP host-query messages (sent to the group address 224.0.0.1 with a TTL
of 1) to keep track of the hosts that belong to multicast groups. When switch router receives a packet
addressed to a multicast group, it forwards the packet to those interfaces that have hosts belonging to
that group. Switch routers periodically send host-query messages to refresh their multicast group
membership knowledge.
The Catalyst 8500 supports both IGMP version 1, which most end stations currently support, and IGMP
version 2, which, unlike version 1, provides support for clients informing the network that they are
leaving a multicast group.
Protocol Independent Multicast
As networks increase in size, multicast routing becomes critically important in order to determine, in a
large routed network, which segments require multicast traffic and which do not. PIM is a routing
protocol for multicast that uses existing unicast routing protocols such as RIP or OSPF for path
forwarding determination and network location. PIM can be operated in two modes. PIM dense mode
and PIM sparse mode. The mode selected determines how the switch router populates its multicast
routing table, and how the it forwards multicast packets it receives from its directly connected LANs.
Note Enabling PIM on an interface also enables IGMP operation on that interface.