Interpret route codes, prefixes, next hops, administrative distance, metrics and the gateway of last resort.
Route codesPrefixNext hopADMetric
01 // Mental model
Start with the big picture.
A code such as C, L, S or O tells you how IOS learned the route.
CCNA focus: Read left to right: source code, destination prefix, bracketed AD/metric, then next hop, age and exit interface.
02 // Building blocks
Know what each part does.
01
Route source
A code such as C, L, S or O tells you how IOS learned the route.
02
Prefix
The destination network and mask define which packet addresses match.
03
Preference
Administrative distance compares different sources; metric compares routes from the same protocol.
04
Forwarding detail
A next hop or exit interface tells the router where to send a matching packet.
03 // Compare and recognise
Read the clues.
Item
What to remember
C / L
Connected network / local interface address.
S
Static route configured by an administrator.
O
Route learned by OSPF.
Gateway of last resort
The next hop used by the selected default route.
04 // Annotated route output
Annotated route output.
R1# show ip routeS* 0.0.0.0/0 [1/0] via 192.0.2.1O 10.20.0.0/16 [110/20] via 10.0.12.2, 00:02:11, Gi0/1C 10.0.12.0/30 is directly connected, Gi0/1L 10.0.12.1/32 is directly connected, Gi0/1
Read the example from top to bottom, then verify the resulting state. Configuration is only complete when the output matches the intended design.
05 // Exam and troubleshooting
Turn facts into a method.
Connected network / local interface address.
Static route configured by an administrator.
Route learned by OSPF.
The next hop used by the selected default route.
Exam checkpoint: Read left to right: source code, destination prefix, bracketed AD/metric, then next hop, age and exit interface.
06 // Check yourself
Reading the Routing Table quiz.
1. What does the first number in [110/20] represent?
2. Which code identifies an OSPF route?
3. What does an asterisk beside a route code commonly indicate?