In the Juniper Networks Junos operating system, the management of routing information is partitioned into several distinct routing tables (RIBs), each serving a specific architectural purpose. When dealing withMultiprotocol Label Switching (MPLS), understanding the distinction between inet.0 and inet.3 is fundamental for troubleshooting and traffic engineering.
Theinet.3routing table is specifically designed to store the egress IPv4 addresses ofLabel-Switched Paths (LSPs). When an ingress router successfully establishes an LSP (via RSVP or LDP), it places the host address of the egress router (the tail-end) into the inet.3 table. This table is not used for general packet forwarding; instead, it is primarily used by theBorder Gateway Protocol (BGP)for next-hop resolution. When BGP receives a route, it checks both inet.0 and inet.3 to resolve the next hop. If a matching entry exists in inet.3, the router knows it can reach that destination via an MPLS tunnel, allowing for the encapsulation of BGP traffic within MPLS.
In contrast,inet.0is the default unicast routing table used for standard IPv4 forwarding and contains routes learned via IGPs (OSPF, IS-IS) or static routing.inet.1is utilized for multicast forwarding (MBGP), andinet.2is typically used for Multicast Source Discovery Protocol (MSDP) or RPF checks in multicast environments. By isolating LSP egress points in inet.3, Junos prevents MPLS-specific paths from interfering with standard IGP path selection unless the administrator explicitly chooses to merge them (e.g., using the traffic-engineering bgp-igp command). Therefore, by default, the ingress router maintains its list of reachable LSP endpoints in inet.3.
