3.1 Routing Tables and Forwarding Decisions

Figure 1 - Routes can enter the RIB from connected interfaces, static configuration, dynamic routing, and a default route.

2. How Routes Enter the Routing Table

Connected and local routes appear only when the interface is operational. If the interface is administratively shut or line protocol is down, the associated connected/local routes are removed. Static routes depend on the configured next hop or interface and may also disappear if their resolution fails. Dynamic routes depend on routing-protocol operation and neighbor/path availability.

3. Reading show ip route

Example routing table

Figure 3 - Anatomy of an OSPF route entry.

For a learned route such as O 10.20.20.0/24 [110/2] via 10.0.12.2, the first value inside brackets is administrative distance and the second is the routing protocol metric. The next-hop address identifies the immediate Layer 3 neighbor to which the packet should be sent. The exit interface is the local interface used to reach that next hop.

4. Routing Protocol Code

The route code is the symbol at the beginning of a routing-table entry. It identifies the route source. The code is essential because it tells you whether the route came from the local device, a static configuration, OSPF, EIGRP, RIP, BGP, or another source.

5. Prefix

A prefix is the destination network portion of a route. In 10.20.20.0/24, the destination prefix is 10.20.20.0 and the prefix length is /24. A packet does not need to have the exact destination address 10.20.20.0; any destination address that falls inside that prefix can match the route.

Longer prefix lengths are more specific. This becomes critical during forwarding because a router can have several routes that all match the same destination address; the longest matching prefix is selected.

6. Network Mask and Prefix Length

The network mask identifies which bits of an IPv4 address represent the network prefix. Cisco routing output usually represents this as CIDR prefix length. For example, /24 is equivalent to 255.255.255.0, /30 is 255.255.255.252, and /32 identifies a single host address.

7. Next Hop and Exit Interface

The next hop is the address of the neighboring Layer 3 device that should receive the packet next. The next hop is not normally the final destination; it is the next router on the path. For directly connected routes, Cisco often displays “is directly connected” instead of a next-hop address.

Figure 7 - Recursive next-hop resolution.

A router must be able to resolve the next-hop IP to a reachable connected path. In the example, the route to 172.16.50.0/24 points to 10.0.12.2. R1 also has a connected route to 10.0.12.0/30, so it knows that 10.0.12.2 is reachable through Gi0/1. ARP can then resolve the next-hop MAC address on Ethernet.

Verification

8. Administrative Distance (AD)

Administrative distance is a local Cisco preference value used when the device knows the same destination prefix from different route sources. Lower AD is more trusted. AD does not travel with the packet and is not exchanged as a universal network-wide value; it is a local decision parameter on the router.

Figure 8 - Same prefix, different route sources: lower administrative distance wins.

9. Routing Protocol Metric

A metric is a protocol-specific value used by a routing protocol to decide which path is best when multiple paths to the same prefix exist within that protocol. Lower is generally better for OSPF, RIP, and EIGRP, although the metric formula and meaning are different for each protocol.

Figure 9 - OSPF compares OSPF cost for paths to the same prefix.

Never compare an OSPF cost of 20 directly with a RIP hop count of 2. The router first selects the preferred route source using administrative distance for the same prefix; metric is meaningful within the route source/protocol calculation.

10. Gateway of Last Resort

The gateway of last resort is the next hop associated with a default route. The default prefix 0.0.0.0/0 matches every IPv4 address, but…