3com S7906E 설치 설명서

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multicast data delivery along the correct path. In addition, the RPF check mechanism also helps avoid

data loops caused by various reasons.

RPF check process

The basis for an RPF check is a unicast route, an MBGP route, or a multicast static route.

A unicast routing table contains the shortest path to each destination subnet,

An MBGP routing table contains multicast routing information, and

A multicast static routing table contains the RPF routing information defined by the user through

static configuration.

When performing an RPF check, a router searches its unicast routing table and multicast static routing

table at the same time. The specific process is as follows:

1) The router first chooses an optimal route from the unicast routing table, MBGP routing table, and

multicast static routing table:

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 considers the path along which the packet from the RPF neighbor arrived on the RPF

interface to be the shortest path that leads back to the source.

The router automatically chooses an optimal MBGP route by searching its MBGP 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 automatically chooses an optimal multicast static route by searching its multicast static

routing table, using the IP address of the “packet source” as the destination address. The

corresponding routing entry explicitly defines the RPF interface and the RPF neighbor.

2) Then, the router selects one from these three optimal routes as the RPF route. The selection

process is as follows:

If configured to use the longest match principle, the router selects the longest match route from the

three; if these three routes have the same mask, the router selects the route with the highest

priority; if the three routes have the same priority, the router selects the RPF route according to the

sequence of multicast static route, MBGP route, and unicast route.

If not configured to use the longest match principle, the router selects the route with the highest

priority; if the three routes have the same priority, the router selects the RPF route according to the

sequence of multicast static route, MBGP route, and unicast route.

The above-mentioned “packet source” can mean different things in different situations:

For a packet traveling along the shortest path tree (SPT) from the multicast source to the receivers

or the rendezvous point (RP), the “packet source” for RPF check is the multicast source.

For a packet traveling along the rendezvous point tree (RPT) from the RP to the receivers, the

“packet source” for RPF check is the RP.

For a bootstrap message from the bootstrap router (BSR), the “packet source” for RPF check is the

BSR.

For details about the concepts of SPT, RPT and BSR, refer to PIM Configuration in the IP Multicast

Volume.