Components of the Routing Table

What it is

The routing table is the list of networks a router knows how to reach, and where to send packets for each one.

Every forwarding decision a router makes starts here. If you can read a routing table line by line, you can tell how a router learned a route, how trustworthy that route is and where it will send the traffic.

You can display the table with show ip route. For IPv6, use show ipv6 route.

How it works

Each line in the table is one route, made of a few parts that you read from left to right.

Reading a route entry

Here is an example of a table, trimmed down:

Codes: L - local, C - connected, S - static, O - OSPF, D - EIGRP, * - candidate default

Gateway of last resort is 203.0.113.2 to network 0.0.0.0

S*    0.0.0.0/0 [1/0] via 203.0.113.2
      10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C        10.1.1.0/24 is directly connected, GigabitEthernet0/0
L        10.1.1.1/32 is directly connected, GigabitEthernet0/0
O        10.2.2.0/24 [110/20] via 10.1.1.2, 00:05:12, GigabitEthernet0/0

Take the OSPF line apart:

O 10.2.2.0/24 [110/20] via 10.1.1.2, 00:05:12, GigabitEthernet0/0

• Route code (O): how the router learned the route. Here, by OSPF.
• Prefix (10.2.2.0): the destination network.
• Network mask (/24): the length of the prefix. Together with the prefix, it defines the destination range.
• Administrative distance (110): the first number in the brackets. It says how trustworthy the source of the route is.
• Metric (20): the second number in the brackets. It says how good the route is within its own protocol.
• Next hop (via 10.1.1.2): the address of the next router to send the packet to.
• Age (00:05:12): how long the route has been known.
• Outgoing interface (GigabitEthernet0/0): the interface the router uses to send the packet.

Route codes

These are the codes you will meet most often.

• C: a directly connected network, from an interface with an address that is up.
• L: the local address of the interface itself, shown as a /32 host route.
• S: a static route that an administrator configured.
• O: learned through OSPF. Variants such as O IA (inter-area) and O E2 (external) also exist.
• D: learned through EIGRP.
• R: learned through RIP.
• B: learned through BGP.
• \*: a candidate default route. It appears with another letter, as in S*.

Administrative distance

When a router learns the same destination from different sources, it must choose between them.

Administrative distance (AD) is the tie-breaker between sources. A lower number is more trusted.

The table lists the values worth memorising. Other sources, such as EIGRP summary routes, have their own values.

Metric

The metric compares routes that came from the same protocol, and each protocol calculates it differently.

• RIP counts hops.
• OSPF uses cost, which is based on bandwidth.
• EIGRP uses a combined value that mainly depends on bandwidth and delay.

The cost of an OSPF interface is the reference bandwidth divided by the interface bandwidth. The default reference bandwidth is 100 Mbps, so a 100 Mbps link costs 1, and a 10 Mbps link costs 10. A 1 Gbps link also costs 1, because the lowest cost is 1.

A metric from one protocol means nothing to another. You cannot compare an OSPF cost of 20 with a RIP hop count of 3. That is why AD exists.

The gateway of last resort

The line Gateway of last resort is 203.0.113.2 to network 0.0.0.0 names the default route, which is where the router sends packets that match no other route.

It is usually a route to 0.0.0.0/0. If no default route exists, the line says the gateway of last resort is not set, and the router drops packets with an unknown destination.

Useful variations

A few forms of the command narrow the output.

show ip route
show ip route ospf
show ip route 10.2.2.0
show ip route connected

Adding a protocol shows only those routes. Adding an address shows the route that would match that destination, including how it was learned.