Sparse-Dense Mode for Auto-RP
A prerequisite of Auto-RP is that all interfaces must be configured in
sparse-dense mode using the ip pim sparse-dense-mode interface
configuration command. An interface configured in sparse-dense mode is
treated in either sparse mode or dense mode of operation, depending on
which mode the multicast group operates. If a multicast group has a
known RP, the interface is treated in sparse mode. If a group has no
known RP, the interface is treated in dense mode and data will be
flooded over this interface.
To successfully implement Auto-RP and prevent any groups other than
224.0.1.39 and 224.0.1.40 from operating in dense mode, we recommend
configuring a "sink RP" (also known as "RP of last resort"). A sink RP
is a statically configured RP that may or may not actually exist in
the network. Configuring a sink RP does not interfere with Auto-RP
operation because, by default, Auto-RP messages supersede static RP
configurations. We recommend configuring a sink RP for all possible
multicast groups in your network, because it is possible for an
unknown or unexpected source to become active. If no RP is configured
to limit source registration, the group may revert to dense mode
operation and be flooded with data.
Auto-RP Filters
When using Auto-RP, configure the ip pim rp-announce-filter global
configuration command on Auto-RP mapping agent routers to filter
Auto-RP announcement messages that arrive on group 224.0.1.39 from
candidate RP routers. This command prevents unwanted candidate RP
announcement messages from being processed by the mapping agent.
Unwanted messages could interfere with the RP election mechanism of
the mapping agent.
interface Ethernet3/2
ip address 20.0.2.3 255.255.255.0
ip pim sparse-dense-mode
ip pim rp-address 1.1.1.1 20
ip pim send-rp-announce Loopback0 scope 32 group-list 10
ip pim send-rp-discovery Loopback0 scope 32
access-list 10 permit 224.0.0.0 15.255.255.255
access-list 20 deny 224.0.1.39
access-list 20 deny 224.0.1.40
access-list 20 permit 224.0.0.0 15.255.255.255
This example pulled from Omar's reference link can be confusing at
first glance. When we are defining the sink, 1.1.1.1, and denying
224.0.1.39, 224.0.1.40 and our gropp-list that was defined in the
example confused me. Until I realized they are defining the sink and
blocking the auto-rp addresses from being defined by the static ip pim
rp address command. Nicely done. The strangest piece it that 1.1.1.1
is not defined at all in the configuration. This takes some getting
used to but is correct since the sink does not need to exist.
Blogs and organic groups at http://www.ccie.net
Received on Fri Sep 25 2009 - 19:10:30 ART
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