Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Software Configuration Guide, Release 12.2(33)MRB
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Chapter 1 Cisco MWR 2941 Router Overview
Features
Shared Tree Advantage
Shared trees have the advantage of requiring the minimum amount of state in each router. This advantage
lowers the overall memory requirements for a network that only allows shared trees. The disadvantage
of shared trees is that under certain circumstances the paths between the source and receivers might not
be the optimal paths, which might introduce some latency in packet delivery. For example, in Figure 8
the shortest path between Host A (source 1) and Host B (a receiver) would be Router A and Router C.
Because we are using Router D as the root for a shared tree, the traffic must traverse Routers A, B, D
and then C. Network designers must carefully consider the placement of the rendezvous point (RP) when
implementing a shared tree-only environment.
In unicast routing, traffic is routed through the network along a single path from the source to the
destination host. A unicast router does not consider the source address; it considers only the destination
address and how to forward the traffic toward that destination. The router scans through its routing table
for the destination address and then forwards a single copy of the unicast packet out the correct interface
in the direction of the destination.
In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that are represented
by a multicast group address. The multicast router must determine which direction is the upstream
direction (toward the source) and which one is the downstream direction (or directions) toward the
receivers. If there are multiple downstream paths, the router replicates the packet and forwards it down
the appropriate downstream paths (best unicast route metric)—which is not necessarily all paths.
Forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path
Forwarding (RPF). RPF is described in the following section.
Reverse Path Forwarding (RPF)
In unicast routing, traffic is routed through the network along a single path from the source to the
destination host. A unicast router does not consider the source address; it considers only the destination
address and how to forward the traffic toward that destination. The router scans through its routing table
for the destination network and then forwards a single copy of the unicast packet out the correct interface
in the direction of the destination.
In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that are represented
by a multicast group address. The multicast router must determine which direction is the upstream
direction (toward the source) and which one is the downstream direction (or directions) toward the
receivers. If there are multiple downstream paths, the router replicates the packet and forwards it down
the appropriate downstream paths (best unicast route metric)—which is not necessarily all paths.
Forwarding multicast traffic away from the source, rather than to the receiver, is called Reverse Path
Forwarding (RPF). RPF is an algorithm used for forwarding multicast datagrams.
Protocol Independent Multicast (PIM) uses the unicast routing information to create a distribution tree
along the reverse path from the receivers towards the source. The multicast routers then forward packets
along the distribution tree from the source to the receivers. RPF is a key concept in multicast forwarding.
It enables routers to correctly forward multicast traffic down the distribution tree. RPF makes use of the
existing unicast routing table to determine the upstream and downstream neighbors. A router will
forward a multicast packet only if it is received on the upstream interface. This RPF check helps to
guarantee that the distribution tree will be loop-free.