Troubleshooting guide

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ATM and Layer 3 Switch Router Troubleshooting Guide
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Chapter 11 Troubleshooting Layer 3 Network Connections
Troubleshooting Layer 3 IP Multicast Switching
One way to start is to place (preserve) the default route processor for all global groups at or near the
border of your routing domain, while placing another route processor in a more centrally located switch
router for all local groups using the administratively scoped addresses (239.x.x.x).
Note If you configure PIM in sparse mode or sparse-dense mode and do not configure Auto-RP, you must
statically configure an RP.
Distance Vector Multicast Routing Protocol
DVMRP is the first-generation multicast routing protocol most known for its use in the Multicast
Backbone (MBONE). DVMRP uses a flood-and-prune approach to multicast packet delivery. This
means that DVMRP assumes that all other switch routers in a network want to forward multicast packets
for a group. This creates huge scalability problems, as switch routers must now maintain state for
multicast paths that may not require or want to handle multicast traffic. For that reason, the Cisco switch
router does not support DVMRP, but does support DVMRP interoperability with PIM. This allows the
Cisco switch router to interoperate with non-Cisco multicast switch routers that use DVMRP.
Cisco IOS software in the Catalyst 8500 supports dynamic discovery of DVMRP switch routers, and can
interoperate with them over traditional media or over DVMRP-specific tunnels. When a DVMRP
neighbor has been discovered, the switch router periodically transmits DVMRP report messages
advertising the unicast sources reachable in the PIM domain.
When a Cisco switch router runs DVMRP over a tunnel, it advertises sources in DVMRP Report
messages much as it does on real networks. In addition, the software caches DVMRP Report messages
it receives and uses them in its Reverse Path Forwarding (RPF) calculation. This allows the software to
forward multicast packets received over the tunnel.
Essential to multicast routing is the idea of spanning trees. Multicast routing procedures, for example
PIM, construct these trees (with receivers as leafs), while multicast forwarding forwards multicast
packets a long the trees.
To support a multicast forwarding function with tag switching, each tag switch associates a tag with a
multicast tree as follows. When a tag switch creates a multicast forwarding entry (either for a shared or
for a source-specific tree), and the list of outgoing interfaces for the entry, the switch also creates local
tags (one per outgoing interface). The switch creates an entry in its TIB and populates (outgoing tag,
outgoing interface, outgoing MAC header) with this information for each outgoing interface, placing a
locally generated tag in the outgoing tag field. This creates a binding between a multicast tree and the
tags. The switch then advertises over each outgoing interface associated with the entry the binding
between the tag (associated with this interface) and the tree.
When a tag switch receives a binding between a multicast tree and a tag from another tag switch, if the
other switch is the upstream neighbor (with respect to the multicast tree), the local switch places the tag
carried in the binding into the incoming tag component of the TIB entry associated with the tree. When
a set of tag switches are interconnected via a multiple-access subnetwork, the tag allocation procedure
for multicast has to be coordinated among the switches. In all other cases tag allocation procedure for
multicast could be the same as for tags used with destination-based routing.
Cisco Group Membership Protocol
Cisco Group Membership Protocol (CGMP) addresses the issue of efficiently forwarding IP multicast
packets across Layer 2 switches. CGMP allows Layer 2 switches to leverage IGMP information recorded
on the Catalyst 8500 to make intelligent Layer 2 forwarding decisions based on the destinations
requesting the multicast traffic. The net result is that with CGMP, IP multicast traffic is delivered only
to those Layer 2 switch ports that are interested in multicast traffic. All Layer 2 switch ports that have