Reading a routing table you did not build

You inherit networks more often than you build them. The person who designed it has left, the diagram is three years stale, and something is broken. The routing table is the most honest document you have, because it describes what the device is actually doing rather than what someone intended.

Start with the default

Find the default route first. It tells you where this device believes the rest of the world is. If there are two defaults with different metrics, you have just found either a deliberate failover design or an accident, and it is worth knowing which before you touch anything.

Then the connected routes

Connected and local routes tell you what this device is directly attached to. This is ground truth: no protocol, no neighbour, no trust involved. Every subnet here is a broadcast domain someone decided to create. Count them. If there are eleven and the diagram shows six, the diagram is wrong.

Then everything learned

Now look at what was learned dynamically and from whom. Group routes by next hop rather than by prefix. Three next hops means three neighbours matter; twenty prefixes through one next hop is one relationship, not twenty.

show ip route | include ^O|^B|^D
show ip route summary

The questions that follow

Look for prefixes that overlap. A /16 learned by OSPF and a /24 learned statically for part of the same space is a deliberate override, or a leftover from an incident nobody removed. Look for routes to networks that no longer exist. Look for anything pointing at a next hop that does not answer.

What this gets you

After twenty minutes you can describe the network’s shape without a diagram: how many sites, which links are primary, where the exits are, and which parts were bolted on later. That is worth more than any inherited document, because you derived it from the running configuration rather than from someone’s memory.

Write down what you found before you change anything. You are the only person who has looked at this recently, and in six months that will include you.