3com 5500-ei pwr Instruccion De Instalación

 

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In the network, multicast packet transmission is based on the guidance of the multicast forwarding 

table derived from the unicast routing table or the multicast routing table specially provided for 

multicast.  

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To process the same multicast information from different peers received on different interfaces of 

the same device, every multicast packet is subject to a Reverse Path Forwarding (RPF) check on 

the incoming interface. The result of the RPF check determines whether the packet will be 

forwarded or discarded. The RPF check mechanism is the basis for most multicast routing 

protocols to implement multicast forwarding.  

The RPF mechanism enables multicast devices to forward multicast packets correctly based on the 

multicast route configuration. In addition, the RPF mechanism also helps avoid data loops caused by 

various reasons.  

Upon receiving a multicast packet that a multicast source S sends to a multicast group G, the multicast 

device first searches its multicast forwarding table: 

1)  If the corresponding (S, G) entry exists, and the interface on which the packet actually arrived is the 

incoming interface in the multicast forwarding table, the router forwards the packet to all the 

outgoing interfaces. 

2)  If the corresponding (S, G) entry exists, but the interface on which the packet actually arrived is not 

the incoming interface in the multicast forwarding table, the multicast packet is subject to an RPF 

check.  

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If the result of the RPF check shows that the RPF interface is the incoming interface of the existing 

(S, G) entry, this means that the (S, G) entry is correct but the packet arrived from a wrong path and 

is to be discarded.  

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If the result of the RPF check shows that the RPF interface is not the incoming interface of the 

existing (S, G) entry, this means that the (S, G) entry is no longer valid. The router replaces the 

incoming interface of the (S, G) entry with the interface on which the packet actually arrived and 

forwards the packet to all the outgoing interfaces.  

3)  If no corresponding (S, G) entry exists in the multicast forwarding table, the packet is also subject to 

an RPF check. The router creates an (S, G) entry based on the relevant routing information and 

using the RPF interface as the incoming interface, and installs the entry into the multicast 

forwarding table.  

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If the interface on which the packet actually arrived is the RPF interface, the RPF check is 

successful and the router forwards the packet to all the outgoing interfaces.  

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If the interface on which the packet actually arrived is not the RPF interface, the RPF check fails 

and the router discards the packet.  

The basis for an RPF check is a unicast route. A unicast routing table contains the shortest path to each 

destination subnet. A multicast routing protocol does not independently maintain any type of unicast 

route; instead, it relies on the existing unicast routing information in creating multicast routing entries.  

When performing an RPF check, a router searches its unicast routing table. The specific process is as 

follows: The router automatically chooses an optimal unicast route by searching its unicast routing table, 

using the IP address of the “packet source” as the destination address. The outgoing interface in the 

corresponding routing entry is the RPF interface and the next hop is the RPF neighbor. The router