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
not requested the traffic do not receive it. When a Layer 3 enabled ATM switch router receives an IGMP
join message, it records the source MAC address of the IGMP message, and turns around and issues a
CGMP join message downstream, to a Layer 2 switch. The switch uses the CGMP message to
dynamically build an entry in the switching table that maps the multicast traffic to the client switch port.
The Catalyst 8500 uses PIM, not CGMP, for multicast forwarding determination. However, the Catalyst
8510 does function as a CGMP server, meaning that on a per-interface basis, it informs the connected
LAN switch of multicast groups that it needs to be aware of. The Catalyst 8500 responds to IGMP
version 1 and 2 multicast join and leave (for IGMP v2) requests and forwards them on the multicast tree
via PIM.
The Multicast Routing Table
The Cisco IOS software running on the switch router uses PIM and DVMRP interoperability to exchange
IP multicast network information. Each routing protocol runs as a separate IOS process in the SRP. The
multicast routing table is a centralized routing information database that is resident on the SRP. The
packet forwarding engine consults the routing table to route the packets to appropriate destinations.
A multicast routing table is different than a unicast routing table. A multicast routing table maps an
ordered pair consisting of a source IP address and a multicast group to an ordered pair consisting of an
input interface and a set of output interfaces. Packets from the given source to the given multicast group
that arrives over an input interface are appropriate output interfaces.
Packets that arrive on the wrong input interface are discarded.
The switch router maintains the central multicast routing table at the SRP. By using CEF and the
associated distribution of the forwarding information base (FIB), the line cards can forward multicast
traffic intelligently, based on the multicast topology of the network. This feature allows the input port to
decide which output interfaces require the multicast traffic, and inform the switching fabric about which
output ports to direct that packet to. Any change in the multicast routing table is instantly downloaded
to the line cards, allowing the switch router to maintain a constant, up-to-date map of the network.
MSDP
In the PIM-SM model, multicast sources and receivers must register with their local RP. Actually, the
switch router closest to the sources or receivers registers with the RP, but the key point to note is that the
RP knows about all the sources and receivers for any particular group. RPs in other domains have no way
of knowing about sources located in other domains. MSDP solves this problem.
MSDP allows RPs to share information about active sources. RPs know about the receivers in their local
domain. When they hear about active sources through MSDP, they can pass on that information to their
local receivers and multicast data can be forwarded between the domains directly. A useful feature of
MSDP is that it allows each domain to maintain an independent RP that does not rely on other domains.
The RP in each domain establishes an MSDP peering session using a TCP connection with the RPs in
other domains, or with border switch routers leading to the other domains. When the RP learns about a
new multicast source within its own domain (through the normal PIM register mechanism), the RP
encapsulates the first data packet in a Source Active (SA) message and sends the SA to all MSDP peers.
The SA is forwarded by each receiving peer using a modified RPF check, until the SA reaches every
MSDP switch router in the interconnected networks—theoretically the entire multicast internet. If the
receiving MSDP peer is an RP, and the RP has a (*, G) entry for the group in the SA (there is an interested
receiver), the RP will create (S, G) state for the source and join to the shortest path tree for the source.
The encapsulated data will be decapsulated and forwarded down that shared tree of that RP. When the
packet is received by the last-hop switch router of a receiver, the last-hop switch router may also join the
shortest path tree to the source. The MSDP speaker periodically sends source addresses that include all
sources within that RP domain.