Junos OS utilizes the inet.0 routing table by default for Reverse Path Forwarding (RPF) checks. This table contains the active unicast routes, and the RPF check validates that multicast traffic is arriving on an interface that the router would use to send unicast traffic back to the source.
When a network design requires the multicast forwarding path to be different from the unicast forwarding path, the inet.2 routing table is used. This table can be populated with routes (for example, from MBGP or copied from inet.0 via a rib-group) to create a separate multicast topology, which is then used for RPF checks instead of inet.0.
Why Incorrect Options are Wrong
B. MBGP and IS-IS place routes in met. l directly.
inet.1 is the multicast forwarding cache, which stores active source-group (S,G) state information, not routes from routing protocols.
D. IS-IS and OSPFv3 place routes in met. 2 directly.
Interior Gateway Protocols (IGPs) like IS-IS and OSPF populate the primary unicast routing table, inet.0, not the inet.2 table.
References
1. Juniper Networks TechLibrary, Junos OS Routing Protocols Library for Routing Devices, "Junos OS Routing Tables" section.
This document specifies the default routing tables. It states: "inet.0: Stores IPv4 unicast routes... This is the main IPv4 routing table." It also defines "inet.2: Stores MBGP routes used for multicast reverse-path-forwarding (RPF) checks." This supports the correctness of options A and C, as inet.0 is the default (unicast) table for RPF, and inet.2 is used for a divergent multicast topology.
2. Juniper Networks TechLibrary, Multicast Protocols User Guide, "Multicast Reverse-Path Forwarding" section.
This guide details the RPF mechanism. It explicitly states, "By default, the router performs the RPF check for a multicast source address against the unicast routing table (inet.0)." This directly confirms that inet.0 is used for RPF checks (Option C). The section also discusses using alternative tables like inet.2 for RPF, which aligns with the purpose described in Option A.
3. Juniper Networks TechLibrary, Junos OS Routing Protocols Library for Routing Devices, "show route table" command reference.
The documentation for this command describes the output for various tables. For inet.1, it is described as the "Multicast forwarding cache." This confirms that inet.1 is not a table for IGP or BGP routes, making option B incorrect.
