Reading a Routing Table – Detailed Guide

1. Types of Routes

Route Type	Description	Code Example
Directly Connected	Routes learned via physically connected interfaces	`C`
Static	Manually configured routes	`S`
Dynamic	Learned via routing protocols	`O` (OSPF), `D` (EIGRP), `R` (RIP), `B` (BGP)

2. Route Codes and Protocol Sources

Code	Protocol	Meaning	Administrative Distance (AD)
C	Connected	Directly connected network	0
S	Static	Manually configured	1
O	OSPF	Intra-area route	110
O IA	OSPF	Inter-area route	110
O E1	OSPF	External Type 1	110
O E2	OSPF	External Type 2	110
D	EIGRP	Internal EIGRP	90
D EX	EIGRP	External EIGRP	170
R	RIP	Routing Information Protocol	120
B	BGP	Border Gateway Protocol	20 (external), 200 (internal)

3. Destination Networks and Masks

Displayed as network prefix/mask (e.g., 192.168.1.0/24).
Default route shown as 0.0.0.0/0.

4. Next-Hop Information

Next-hop IP: Router IP to forward packets.
Exit Interface: Local outgoing interface.

5. Metric Values

OSPF: Cost (based on bandwidth).
EIGRP: Composite (bandwidth, delay).
RIP: Hop count.
BGP: Path attributes.

6. Route Tags and Filters

Tags help identify and filter routes, especially during redistribution.

7. Administrative Distance (AD)

Determines route preference; lower AD = preferred route.

8. Subnet vs. Supernet Entries

/32: Single host route.
Summarized routes aggregate multiple networks.

9. Route Age and Last Update

Shows how long since last route update (e.g., 00:00:12 = 12 seconds ago).

10. Fallback Routes and Candidate Defaults

Floating static routes have higher AD and serve as backups.

11. Parsing Output Line by Line

O 10.0.0.0/24 [110/20] via 192.168.1.2, 00:00:12, GigabitEthernet0/1

Part	Explanation
`O`	Learned via OSPF
`10.0.0.0/24`	Destination network
`[110/20]`	[AD/Metric]
`via 192.168.1.2`	Next-hop IP
`00:00:12`	Route age
`GigabitEthernet0/1`	Exit interface

12. Verifying Reachability

show ip route [network]
ping [IP], traceroute [IP]

13. Common Troubleshooting Clues

Symptom	Possible Cause
Missing routes	Incorrect network statements, route not learned
Unexpected AD or metric	Misconfigured protocols or redistribution
Incorrect next-hop	Topology or configuration errors

14. IPv6 Routing Table Differences

Uses show ipv6 route.
Similar route codes with IPv6 specifics.

✅ Summary Table

(See Section 11)

Reading a Routing Table Quiz

1. What does the code C represent in a routing table?

A. Static Route B. Directly Connected Route C. RIP Route D. OSPF Route

Correct answer is B. 'C' stands for directly connected routes, learned via interfaces physically connected to the router.

2. What is the Administrative Distance (AD) of an EIGRP internal route?

A. 110 B. 120 C. 0 D. 90

Correct answer is D. EIGRP internal routes have an AD of 90, making them preferred over OSPF and RIP.

3. Which routing protocol uses hop count as its metric?

A. RIP B. OSPF C. EIGRP D. BGP

Correct answer is A. RIP uses hop count as its metric to determine the best path.

4. In the routing table entry `O 10.0.0.0/24 [110/20] via 192.168.1.2`, what does the number 110 represent?

A. Metric value B. Interface number C. Administrative Distance D. Route age

Correct answer is C. The 110 is the Administrative Distance (AD) for OSPF routes.

5. What does the next-hop IP address indicate in a routing table?

A. The destination network address B. The IP address of the router to forward packets to C. The source IP address of packets D. The IP address of the local host

Correct answer is B. The next-hop IP is the router address where packets are forwarded next.

6. Which of the following is TRUE about floating static routes?

A. They have the lowest Administrative Distance B. They are preferred over directly connected routes C. They are used as primary routes D. They have higher Administrative Distance and act as backup routes

Correct answer is D. Floating static routes have higher AD and only become active if the primary route fails.

7. What does the code O IA stand for in an OSPF routing table?

A. OSPF Intra-Area route B. OSPF External Type 1 C. OSPF Inter-Area route D. OSPF Internal route

Correct answer is C. 'O IA' refers to routes between different OSPF areas (inter-area).

8. Which show command would you use to display current routes on a Cisco router?

A. show ip route B. show running-config C. show interfaces D. show ip protocols

Correct answer is A. 'show ip route' displays the current routing table entries.

9. What metric does OSPF use to determine the best route?

A. Hop count B. Cost (based on bandwidth) C. Delay D. Path length

Correct answer is B. OSPF uses cost, which is calculated based on bandwidth, as its routing metric.

10. What does the route age field indicate in a routing table?

A. The number of routers on the path B. The time since the router was rebooted C. Time since the last update was received for the route D. Time since the route was manually configured

Correct answer is C. Route age shows how long ago the last routing update was received.

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