Viewing the Routing Table – Understanding Network Paths

Purpose of the Routing Table

Definition: The routing table is a database on routers and Layer 3 switches listing known network destinations, next hops, and how to reach them.

Why View the Routing Table?

To understand how the device will forward network traffic.
To diagnose routing issues (e.g., missing, incorrect, or duplicate routes).
To verify protocol operation, advertisements, and configuration changes.

Common Commands to View the Routing Table

IPv4 Routing Table: show ip route
IPv6 Routing Table: show ipv6 route
Filter for specific networks/protocols:
show ip route | include 192.168
show ip route ospf

Reading Routing Table Entries

Router# show ip route

Codes: C - connected, S - static, O - OSPF, D - EIGRP, R - RIP, B - BGP, ...
Gateway of last resort is 192.168.1.1 to network 0.0.0.0

C    192.168.10.0/24 is directly connected, GigabitEthernet0/1
O    10.0.0.0/24 [110/20] via 192.168.1.2, 00:00:12, GigabitEthernet0/2
S*   0.0.0.0/0 [1/0] via 192.168.1.1

Fields explained:

Route Code: (C, S, O, D, etc.) – Indicates how the route was learned.
Destination network: (e.g., 192.168.10.0/24)
Next-hop IP address: (e.g., via 192.168.1.2)
Exit Interface: (e.g., GigabitEthernet0/2)
Metric & AD: (e.g., [110/20])
Route Age: (e.g., 00:00:12)

Route Types and Codes

Code	Meaning	How Learned/Configured
C	Connected	Directly on interface
S	Static	Manually configured
O	OSPF	OSPF dynamic protocol
D	EIGRP	EIGRP protocol
R	RIP	RIP protocol
B	BGP	Border Gateway Protocol
S*	Static Default	Default route (0.0.0.0/0)

Administrative Distance (AD) and Metrics

Administrative Distance (AD): Trustworthiness of a route (lower = more preferred).
Common values:

Source AD Value

Connected 0

Static 1

EIGRP 90

OSPF 110

RIP 120
Metric: Route cost (protocol-dependent; lower is better). Used if AD is equal between routes.

Source	AD Value
Connected	0
Static	1
EIGRP	90
OSPF	110
RIP	120

Route Selection Process

Router compares AD values; picks the route with the lowest AD for each destination.
If AD is the same, selects the route with the lowest metric.

Default Routes and Gateway of Last Resort

Default Route: 0.0.0.0/0 (IPv4) or ::/0 (IPv6). Used when no specific match exists.
Gateway of Last Resort: Next-hop for the default route. Critical for Internet access and unknown destinations.
```
S* 0.0.0.0/0 [1/0] via 192.168.1.1
```

Summary Routes and Aggregation

Summary route: Represents a group of subnets as one entry (e.g., 10.0.0.0/16 summarizes 10.0.1.0/24, 10.0.2.0/24).
Benefits: Reduces routing table size, improves scalability, and limits advertisements.

Floating Static Routes

Floating static route: Backup route with a higher AD than the primary.
Used only if the main route is lost.

ip route 10.0.0.0 255.255.255.0 192.168.2.1 200

Route Aging and Timers

Dynamic routes will age out if not refreshed (e.g., neighbor failure).
Route age shown in table (e.g., 00:01:22 for last update time).

Troubleshooting with the Routing Table

Check for missing routes: May indicate configuration/protocol issues.
Verify correct next-hop: Use ping and traceroute for neighbor reachability.
Identify route flapping: Instability; check interfaces, links, protocol configuration.
Use filtering:
show ip route | include 10.10.10.0

Filtering and Searching the Routing Table

show ip route | include 192.168 — Only show entries matching 192.168 networks.
show ip route | begin OSPF — Start display from OSPF routes.

Differences Across Platforms

Cisco IOS: Standard codes and output as above.
Other vendors: Codes and formatting may differ (Juniper, MikroTik, etc.). Always consult device documentation.

Example: Real-World Use Case

Scenario: John’s PC (192.168.10.20) can’t reach the internet.

Check the routing table: show ip route
Is there a default route (0.0.0.0/0) pointing to the gateway? If not, add one.
Is there a route to John’s subnet? If not, fix dynamic or static route config.
Is the next-hop reachable? Use ping and traceroute.

Exam Tips and Key Points

Be able to interpret all routing table fields: code, destination, next-hop, interface, metric, age.
Know the protocol codes (C, S, O, D, etc.) and what they mean.
Understand route selection: lowest AD first, then lowest metric.
Recognize the role of default route and gateway of last resort.
Troubleshoot missing or incorrect routes by consulting the table and protocol status.
Use command modifiers and filtering to quickly find information.

View Routing Table Quiz

1. What is the main purpose of viewing the routing table on a router?

A. To display VLAN assignments B. To configure IP addresses C. To understand how packets will be forwarded and diagnose routing issues D. To monitor switch port status

Correct answer is C. Viewing the routing table helps in understanding packet forwarding and troubleshooting routing problems.

2. Which command displays the IPv4 routing table on Cisco devices?

A. show ip route B. show ipv6 route C. show interfaces D. show mac address-table

Correct answer is A. "show ip route" displays the IPv4 routing table.

3. What does the route code 'O' represent in the routing table?

A. Static route B. Connected route C. BGP route D. OSPF learned route

Correct answer is D. 'O' indicates a route learned via OSPF.

4. What does the administrative distance (AD) indicate?

A. Network speed B. Trustworthiness or preference of a route (lower is better) C. Packet loss percentage D. Route age in seconds

Correct answer is B. AD determines how trusted a route is, lower values are preferred.

5. What is the default route shown as in the routing table?

A. 255.255.255.0/24 B. 192.168.1.0/24 C. 0.0.0.0/0 D. ::/0

Correct answer is C. The default IPv4 route is represented as 0.0.0.0/0.

6. How does a router select the best route when multiple routes exist to the same destination?

A. Based on the highest IP address B. Random selection C. Based on the lowest route age D. Based on the lowest administrative distance and then lowest metric

Correct answer is D. The router chooses the route with the lowest AD, and if tied, the lowest metric.

7. What does the 'C' route code indicate in the routing table?

A. Connected route (directly attached interface) B. Static route C. RIP learned route D. BGP route

Correct answer is A. 'C' means the route is directly connected to the router.

8. What is the purpose of a floating static route?

A. It is the primary route for all traffic B. It acts as a backup route with a higher administrative distance C. It is used for VLAN trunking D. It disables routing on an interface

Correct answer is B. Floating static routes have higher AD to be used only if the primary route fails.

9. Which command filters routing table output to show only routes learned via OSPF?

A. show ip route connected B. show ip route static C. show ip route rip D. show ip route ospf

Correct answer is D. The filter "show ip route ospf" shows only OSPF routes.

10. If a PC cannot reach the internet, what should be checked first in the routing table?

A. VLAN membership B. MAC address table C. Presence of a default route (0.0.0.0/0) pointing to the correct gateway D. IP address of the DNS server

Correct answer is C. The default route is essential for internet traffic; if missing or incorrect, connectivity fails.

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