Cisco Systems 3560 사용자 설명서
44-8
Catalyst 3560 Switch Software Configuration Guide
OL-8553-06
Chapter 44 Configuring IP Multicast Routing
Understanding Cisco’s Implementation of IP Multicast Routing
3.
If the RPF check fails, the packet is discarded.
Some multicast routing protocols, such as DVMRP, maintain a separate multicast routing table and use
it for the RPF check. However, PIM uses the unicast routing table to perform the RPF check.
it for the RPF check. However, PIM uses the unicast routing table to perform the RPF check.
shows port 2 receiving a multicast packet from source 151.10.3.21.
shows that
the port on the reverse path to the source is port 1, not port 2. Because the RPF check fails, the multilayer
switch discards the packet. Another multicast packet from source 151.10.3.21 is received on port 1, and
the routing table shows this port is on the reverse path to the source. Because the RPF check passes, the
switch forwards the packet to all port in the outgoing port list.
switch discards the packet. Another multicast packet from source 151.10.3.21 is received on port 1, and
the routing table shows this port is on the reverse path to the source. Because the RPF check passes, the
switch forwards the packet to all port in the outgoing port list.
Figure 44-3
RPF Check
PIM uses both source trees and RP-rooted shared trees to forward datagrams (described in the
and the
). The RPF check is performed
differently for each:
•
If a PIM router or multilayer switch has a source-tree state (that is, an (S,G) entry is present in the
multicast routing table), it performs the RPF check against the IP address of the source of the
multicast packet.
multicast routing table), it performs the RPF check against the IP address of the source of the
multicast packet.
•
If a PIM router or multilayer switch has a shared-tree state (and no explicit source-tree state), it
performs the RPF check on the RP address (which is known when members join the group).
performs the RPF check on the RP address (which is known when members join the group).
Sparse-mode PIM uses the RPF lookup function to decide where it needs to send joins and prunes:
•
(S,G) joins (which are source-tree states) are sent toward the source.
•
(*,G) joins (which are shared-tree states) are sent toward the RP.
DVMRP and dense-mode PIM use only source trees and use RPF as previously described.
Understanding DVMRP
DVMRP is implemented in the equipment of many vendors and is based on the public-domain mrouted
program. This protocol has been deployed in the MBONE and in other intradomain multicast networks.
program. This protocol has been deployed in the MBONE and in other intradomain multicast networks.
Table 44-1
Routing Table Example for an RPF Check
Network
Port
151.10.0.0/16
Gigabit Ethernet 0/1
198.14.32.0/32
Gigabit Ethernet 0/3
204.1.16.0/24
Gigabit Ethernet 0/4
Multicast
packet from
source 151.10.3.21
is forwarded.
Multicast
packet from
source 151.10.3.21
packet is discarded.
Gigabit Ethernet 0/1
Gigabit Ethernet 0/3
Gigabit Ethernet 0/4
Gigabit Ethernet 0/2
101242
Layer 3 switch