Service Provider Routing and Switching, Specialist (JNCIS-SP) Free Sample Questions

To leak interface routes between routing tables, a RIB group must be created specifying the primary and secondary routing tables using the 'import-rib' statement. The primary table (inet.0) is listed first, followed by the secondary table (cust-A.inet.0). This RIB group is then applied globally under the [edit routing-options interface-routes] hierarchy to instruct the router to place direct interface routes into both tables.

Filter-Based Forwarding (FBF) relies on a firewall filter applied to the ingress interface. The filter evaluates traffic and uses the 'then routing-instance' action for matched packets, directing them to a custom routing instance. To isolate this forwarding plane without creating full virtual routers, the instance is typically configured as 'instance-type forwarding' and populated with the necessary next-hop routes (e.g., pointing to ISP-1). Traffic not matching the filter falls through to the global routing table (inet.0).

Both aggregate and generated routes are used to summarize routing information. By default, an aggregate route is assigned a next-hop of reject (or discard, depending on configuration), preventing routing loops for subnets within the summary that do not exist. Conversely, a generated route can inherit the real next-hop of the primary contributing route, allowing it to forward traffic dynamically.

In Junos OS, applying a load-balancing policy to the forwarding table using the 'export' statement under [edit routing-options forwarding-table] enables ECMP. By default, this implementation uses a hash algorithm based on IP headers (and optionally Layer 4 headers) to perform per-flow load balancing. This ensures that all packets belonging to the same flow take the same path, preventing out-of-order delivery issues common with strict per-packet load balancing.

A Type 4 LSA (ASBR Summary LSA) is generated by an ABR to advertise the presence and routing cost to an ASBR to routers in other areas. While Type 5 LSAs carry the actual external routes, routers outside the ASBR's area need the Type 4 LSA to know how to reach the ASBR itself, as Type 1 (Router) LSAs do not cross area boundaries.

If an OSPF adjacency is stuck in the ExStart or Exchange state, the most common cause is an MTU mismatch between the two neighbors. During the ExStart/Exchange phase, routers exchange Database Description (DBD) packets. If the interface MTUs do not match, the routers will negotiate the master/slave relationship but fail to properly exchange the DBD packets, halting the adjacency process. Timer mismatches or area ID mismatches would prevent the adjacency from even reaching the Init/2-Way states.