5.1 Routing & Forwarding Tables

Why Tables Matter on JNCIA-Junos

The Routing Fundamentals domain is roughly 18% of the JNCIA-Junos exam, and almost every routing question depends on understanding how Junos stores and uses routes. Junos cleanly separates the control plane, which decides routes, from the data plane, which forwards packets. Knowing which table answers which question is the single most reusable skill in this domain.

RIB vs FIB

The Routing Information Base (RIB) is the control-plane database of every route the device has learned, from every source. The main IPv4 unicast RIB in Junos is named inet.0. The Forwarding Information Base (FIB) is the data-plane copy used to actually forward packets; it lives in the Packet Forwarding Engine (PFE).

The Routing Engine (RE) owns the RIB. It gathers routes, runs the active-route selection algorithm, and then copies only the active routes into the FIB. The PFE never re-evaluates routes; it simply forwards based on the FIB the RE gave it.

During convergence the two tables can briefly differ, because the RE selects a new active route before the updated FIB is fully programmed into the PFE. Exam questions often hinge on this distinction.

Common Junos RIBs

Junos uses many routing tables. The most exam-relevant ones are listed below; the table name encodes both the address family and the table number.

For JNCIA-Junos, focus on inet.0 and inet6.0. Routing instances create additional tables named like instance-name.inet.0.

Route Resolution and Longest-Prefix Match

When a packet arrives, the PFE looks up the destination IP address in the FIB and selects the longest-prefix match (LPM): the most specific prefix that contains the destination address. Longest match always wins on a per-packet forwarding decision, and it is independent of route preference.

Consider a FIB that contains all three of these routes:

0.0.0.0/0        -> next-hop 203.0.113.1   (default route)
10.0.0.0/8       -> next-hop 198.51.100.1
10.1.1.0/24      -> next-hop 198.51.100.9

A packet to 10.1.1.50 matches all three prefixes, but 10.1.1.0/24 is the longest (most specific) match, so the packet is forwarded to 198.51.100.9. A packet to 10.9.9.9 matches 10.0.0.0/8 and the default, so the /8 wins. A packet to 172.16.5.5 matches only 0.0.0.0/0, so it uses the default route.

Route resolution is the related process where Junos resolves an indirect next hop (for example a BGP next hop that is not directly connected) by looking it up recursively in the RIB until it finds a reachable forwarding next hop. If the next hop cannot be resolved, the route is not usable and is not installed as active.

Active Route Selection

When multiple routes exist for the same destination prefix, Junos must choose exactly one active route to install into the FIB. The decision order is:

1. Lowest route preference (Junos administrative distance) wins first.
2. If preference ties, the protocol's own tiebreakers apply (for example, lowest OSPF metric or BGP path selection).
3. Junos can install multiple equal-cost paths for load balancing only when explicitly configured.

In show route output the active route is marked with a * and the word Active; inactive candidates are still listed but are not forwarded on. Remember the key contrast: longest-prefix match chooses between different prefixes at forwarding time, while route preference chooses between competing routes to the same prefix at selection time.

Reading show route Output

The primary command is show route. Useful variants you should recognize:

A simplified entry looks like this:

10.1.1.0/24  *[Static/5] 1d 02:14:00
                > to 198.51.100.9 via ge-0/0/1.0

Here * marks the active route, Static is the source protocol, 5 is the route preference, and > marks the selected forwarding next hop.