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Routing Table Components

48 questions

All types48Multiple choice24Multiple select13Matching11
Question 41Question →

Match each Cisco IPv4 routing table entry format on the left with its corresponding operational description on the right.

Click a left item, then click its matching right item

Items

D 192.168.10.0/24 [90/2170112]
O 10.1.1.0/24 [110/65]
C 172.16.1.0/24
S 0.0.0.0/0 [1/0] via 192.168.1.1

Matches

Show answer & explanation

Answer

D 192.168.10.0/24 [90/2170112] matches the EIGRP-learned route description; O 10.1.1.0/24 [110/65] matches the OSPF-learned route description; C 172.16.1.0/24 matches the directly connected route description; S 0.0.0.0/0 [1/0] via 192.168.1.1 matches the static default route description.
Each routing table notation correctly matches its protocol source code ('D' for EIGRP, 'O' for OSPF, 'C' for connected, 'S' for static) and accurately decodes the bracketed administrative distance and metric values.

Step-by-Step Solution

1
Identify routing protocol source codes
Map 'D' to EIGRP, 'O' to OSPF, 'C' to Directly Connected, and 'S' to Static.
Cisco IOS routing tables use standard single-letter source codes to indicate route origins.
2
Interpret bracket notation [AD/Metric]
Extract administrative distance as the first value and route metric as the second value inside the brackets.
The bracketed syntax standardizes administrative distance (route trustworthiness) and metric (path cost).
3
Pair routing lines to functional descriptions
Match each left routing entry string with its corresponding description on the right.
Ensures all routing table components are correctly identified and defined.

Key Concept

Cisco IPv4 Routing Table Component Interpretation
Estimated Time:1m 30s
Question 42Question →

An administrator examines the following routing table output on a Cisco router:

text
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
O 10.15.0.0/16 [110/20] via 192.168.1.1, 00:10:22, GigabitEthernet0/0
D 10.15.10.0/24 [90/307200] via 192.168.2.1, 00:05:14, GigabitEthernet0/1
S 10.15.10.128/25 [1/0] via 192.168.3.1, 01:12:05, GigabitEthernet0/2
C 10.15.10.0/28 is directly connected, GigabitEthernet0/3

Which two statements regarding route selection and routing table components in this output are correct?

Select all that apply

Show answer & explanation

Answer: Packets destined for host 10.15.10.130 are forwarded out interface GigabitEthernet0/2 because the subnet mask /25 provides the longest matching prefix for this destination.; The bracketed notation [90/307200] in the EIGRP entry indicates an administrative distance of 90 and a routing metric of 307200.

Answer

The two correct statements are that packets destined for host 10.15.10.130 are forwarded out interface GigabitEthernet0/2 due to longest prefix match (/25), and that the bracketed notation [90/307200] represents an administrative distance of 90 and a metric of 307200.
Longest prefix match is the primary rule for IP packet forwarding. Destination 10.15.10.130 matches 10.15.10.128/25 with a 25-bit mask, which is longer than /24 or /16, causing forwarding via GigabitEthernet0/2. Additionally, standard Cisco routing table format displays administrative distance first and metric second inside square brackets, making [90/307200] represent an AD of 90 and a metric of 307200.

Step-by-Step Solution

1
Evaluate destination host 10.15.10.130 against all matching routes in the routing table.
Matches 10.15.0.0/16, 10.15.10.0/24, and 10.15.10.128/25.
The router applies the longest prefix match rule, selecting /25 (10.15.10.128/25 via GigabitEthernet0/2).
2
Interpret routing table bracketed components [AD/Metric].
[90/307200] corresponds to AD 90 and Metric 307200.
The first number inside the brackets is always administrative distance and the second number is the routing metric.

Key Concept

Routing Table Lookup Logic & Component Interpretation
Question 43Question →

A Cisco router receives an IPv4 packet with a destination IP address of 172.16.50.170. The router inspects its routing table and finds the following route entries:

S 172.16.50.0/24 [1/0] via 10.0.0.1, GigabitEthernet0/0
D 172.16.50.128/25 [90/2560] via 10.0.0.2, GigabitEthernet0/1
O 172.16.50.160/27 [110/50] via 10.0.0.3, GigabitEthernet0/2
S 172.16.50.176/28 [1/0] via 10.0.0.4, GigabitEthernet0/3

Which next-hop IP address and outgoing interface will the router select to forward this packet?

Show answer & explanation

Answer: 10.0.0.3 via GigabitEthernet0/2

Answer

10.0.0.3 via GigabitEthernet0/2
When a router performs a forwarding lookup for destination 172.16.50.170, it checks all installed routes to find matches. The prefix 172.16.50.160/27 includes addresses from 172.16.50.160 to 172.16.50.191. Because /27 is the longest (most specific) prefix length among all matching routes, the router selects this entry (next-hop 10.0.0.3 via GigabitEthernet0/2). Administrative distance is not evaluated during lookup across routes of different prefix lengths.

Step-by-Step Solution

1
Identify all candidate routing table entries that match the destination IP address 172.16.50.170.
Three routes match: 172.16.50.0/24 (range .0 - .255), 172.16.50.128/25 (range .128 - .255), and 172.16.50.160/27 (range .160 - .191). The route 172.16.50.176/28 (range .176 - .191) does not match.
Only matching routes are eligible for forwarding decisions.
2
Compare the prefix lengths of all matching candidate routes.
Prefix lengths are /24, /25, and /27. The longest prefix match is /27.
Routers use the Longest Prefix Match (LPM) rule first when making forwarding decisions.
3
Determine the next-hop and exit interface associated with the longest matching route.
The route 172.16.50.160/27 specifies next-hop 10.0.0.3 out interface GigabitEthernet0/2.
Administrative distance is only evaluated when comparing identical prefixes from different sources, not when comparing routes of different prefix lengths.

Key Concept

Longest Prefix Match (LPM) Routing Decision Logic
Question 44Question →

Match each element of a Cisco IPv4 routing table entry on the left with its corresponding operational role on the right.

Click a left item, then click its matching right item

Items

First integer inside brackets (e.g., 110 in [110/65])
Second integer inside brackets (e.g., 65 in [110/65])
IP address following 'via' (e.g., 10.1.1.2)
Interface designation at the end of entry (e.g., GigabitEthernet0/0/1)

Matches

Show answer & explanation

Answer

The first integer inside the brackets matches the Administrative Distance (trustworthiness of the route source). The second integer inside the brackets matches the Metric (routing protocol path cost). The IP address following 'via' matches the next-hop address. The interface designation matches the local egress interface.
In Cisco IOS routing table output entries (such as O 192.168.10.0/24 [110/65] via 10.1.1.2, GigabitEthernet0/0/1), the components strictly follow a defined syntax: the first bracketed integer is the Administrative Distance (believability/trustworthiness rating), the second bracketed integer is the routing protocol metric (cost of the path), the IP address following 'via' is the next-hop address of the adjacent router, and the ending interface identifier is the local exit port.

Step-by-Step Solution

1
Analyze the Cisco routing table entry bracket notation [AD/Metric].
Identify that the first number represents Administrative Distance and the second number represents Metric cost.
Cisco IOS routes display bracketed values formatted strictly as [Administrative Distance / Metric].
2
Examine the forwarding path descriptors ('via IP' and interface name).
Map 'via IP' to the next-hop router's address and the interface name to the local outbound port.
'via' indicates the immediate upstream router address, while the final parameter indicates the local exit interface.

Key Concept

Routing Table Components & Cisco IOS Route Entry Structure
Question 45Question →

A Cisco router receives an IPv4 packet destined for host 192.168.4.66. The routing table contains the following entries:

text
Gateway of last resort is not set

192.168.4.0/24 is variably subnetted, 4 subnets, 4 masks
S 192.168.4.0/24 [1/0] via 172.16.4.4
B 192.168.4.64/26 [20/0] via 172.16.3.3, 01:45:10, GigabitEthernet0/2
D 192.168.4.64/27 [90/307200] via 172.16.2.2, 00:15:30, GigabitEthernet0/1
O 192.168.4.64/28 [110/20] via 172.16.1.1, 00:08:12, GigabitEthernet0/0

Which route will the router select to forward the packet?

Show answer & explanation

Answer: The OSPF route 192.168.4.64/28 via 172.16.1.1 because it provides the longest matching prefix for the destination IP address.

Answer

The router selects the OSPF route `192.168.4.64/28` via 172.16.1.1 because it provides the longest prefix match (/28) for the destination IP address 192.168.4.66.
When a router receives a packet, it compares the destination IP address against all routes in its routing table. When multiple routes match the destination IP address, the router selects the route with the longest prefix length (most network bits / longest subnet mask). In this case, 192.168.4.66 matches all four prefixes, but the /28 route is the most specific match (28 bits vs 27, 26, or 24 bits). Administrative distance plays no role here because the prefixes are of different lengths.

Step-by-Step Solution

1
Identify the destination IP address of the incoming packet
Destination IP is 192.168.4.66
Routing lookup is driven by matching the packet's destination IP against routing table entries.
2
Determine which routing table prefixes encompass the destination IP address
192.168.4.0/24 (range .0-.255), 192.168.4.64/26 (range .64-.127), 192.168.4.64/27 (range .64-.95), and 192.168.4.64/28 (range .64-.79) all match the destination address 192.168.4.66.
All four configured subnets contain the IP address 192.168.4.66.
3
Apply the Longest Prefix Match (LPM) rule
The /28 prefix length (28 subnet bits) is the longest and most specific match compared to /27, /26, and /24.
Cisco routers always prioritize the most specific route (longest subnet mask) when forwarding traffic.

Key Concept

Longest Prefix Match (LPM) in IPv4 Route Determination
Estimated Time:1m 0s
Question 46Question →

An administrator examines the following routing table output from a Cisco router:

text
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D 10.10.20.0/24 [90/2170112] via 192.168.1.2, 00:15:42, GigabitEthernet0/0
O 10.10.20.0/26 [110/20] via 192.168.1.6, 00:08:12, GigabitEthernet0/1
S 10.10.0.0/16 [1/0] via 192.168.1.10
C 10.10.20.128/25 is directly connected, GigabitEthernet0/2

If the router receives a packet with a destination IP address of 10.10.20.45, which two statements correctly describe how the router processes this packet? (Select two.)

Select all that apply

Show answer & explanation

Answer: The router forwards the packet out GigabitEthernet0/1 because the prefix 10.10.20.0/26 represents the longest matching prefix for the destination IP address.; The router evaluates administrative distance only among routes that share the exact same longest prefix match length.

Answer

The router selects the OSPF route (10.10.20.0/26) and forwards the packet out interface GigabitEthernet0/1 because 26 bits is the longest matching prefix for host 10.10.20.45. Administrative distance is only evaluated when multiple routing protocols advertise the exact same destination network prefix length.
The destination address 10.10.20.45 matches three routes in the routing table: 10.10.0.0/16, 10.10.20.0/24, and 10.10.20.0/26. Router forwarding logic mandates that the route with the longest matching prefix (/26) is chosen first. Consequently, the packet is egressed out interface GigabitEthernet0/1. Furthermore, administrative distance is only referenced when comparing multiple sources advertising the exact same network prefix and subnet mask.

Step-by-Step Solution

1
Determine which routing table entries match the destination IP address 10.10.20.45.
Three entries match: 10.10.0.0/16 (range 10.10.0.0-10.10.255.255), 10.10.20.0/24 (range 10.10.20.0-10.10.20.255), and 10.10.20.0/26 (range 10.10.20.0-10.10.20.63). Entry 10.10.20.128/25 does not match.
Routing decision logic requires evaluating all active prefix matches for a given destination host address.
2
Apply the Longest Prefix Match (LPM) rule among matching routes.
The route 10.10.20.0/26 has the longest prefix length (/26 > /24 > /16).
Routers always select the route with the most specific (longest) subnet mask before comparing administrative distance.
3
Identify the forwarding interface and understand administrative distance behavior.
Packet is egressed via GigabitEthernet0/1. Administrative distance is only compared when identical prefixes with identical mask lengths exist from different protocol sources.
Administrative distance compares routing protocol reliability for the same prefix, not across prefixes of different specificity.

Key Concept

Longest Prefix Match Rule vs. Administrative Distance
Question 47Question →

A network device attempts to forward an IP packet toward the target address 172.31.25.140. The routing table contains four matching prefix entries:

- 172.31.25.0/24 [1/0] via 10.1.1.1
- 172.31.25.128/26 [90/307200] via 10.2.2.2
- 172.31.25.128/27 [110/40] via 10.3.3.3
- 172.31.25.136/29 [120/4] via 10.4.4.4

Which next-hop IP address will the device select to forward this packet?

Show answer & explanation

Answer: 10.4.4.4

Answer

The router forwards the packet to next-hop address 10.4.4.4 because 172.31.25.136/29 has the longest matching prefix length (/29).
When a router receives a packet, it compares the destination IP address against all entries in its routing table and selects the route with the longest matching prefix length (the most specific subnet mask). Here, 172.31.25.136/29 matches 172.31.25.140 with a 29-bit prefix length, which is longer than /27, /26, or /24. The next-hop associated with this route is 10.4.4.4.

Step-by-Step Solution

1
Determine which routing entries match destination IP 172.31.25.140.
All four routes (172.31.25.0/24, 172.31.25.128/26, 172.31.25.128/27, and 172.31.25.136/29) contain the IP address 172.31.25.140 within their range.
For 172.31.25.136/29, the usable IP host range spans 172.31.25.137 through 172.31.25.142, which includes 172.31.25.140.
2
Compare the prefix lengths of all matching routes.
The prefix lengths are /24, /26, /27, and /29.
Routers use the Longest Prefix Match (LPM) rule to select the forwarding route regardless of Administrative Distance.
3
Select the route with the highest prefix length (most specific mask).
/29 is the longest prefix match, corresponding to route 172.31.25.136/29 via 10.4.4.4.
Administrative distance is only evaluated when comparing identical prefixes from different routing sources. For different prefix lengths, LPM always governs.

Key Concept

Longest Prefix Match (LPM)
Question 48Question →

A network administrator is reviewing the output of the 'show ip route' command on a Cisco IOS router. Match each routing table component or source code on the left with its correct functional definition on the right.

Click a left item, then click its matching right item

Items

Administrative Distance
Metric
Prefix Length
Source Code 'D'

Matches

Show answer & explanation

Answer

Administrative Distance matches the trustworthiness of the route source; Metric matches the protocol-specific path cost; Prefix Length matches the subnet mask bit count used for longest prefix matching; Source Code 'D' matches routes dynamically learned via EIGRP.
Each field in a Cisco IPv4 routing table entry provides distinct information: Administrative Distance prioritizes between routing protocols; Metric determines the optimal path within a single protocol; Prefix Length defines mask length for longest prefix match logic; and the code 'D' identifies routes learned via EIGRP.

Step-by-Step Solution

1
Identify the definition of Administrative Distance.
Administrative Distance rates the trustworthiness of routing protocol sources.
It allows the router to choose the preferred route source when the same network prefix is advertised by different routing sources.
2
Identify the role of Metric in a routing table entry.
Metric represents path cost calculated by an individual routing protocol.
When multiple candidate paths to a destination exist within the same protocol, the route with the lowest metric value is selected.
3
Determine the function of Prefix Length.
Prefix length denotes the subnet mask bit count and determines match specificity.
Routers evaluate prefix lengths to perform longest prefix matching as the primary criteria during packet lookup.
4
Identify the Cisco IOS routing table source code indicator 'D'.
'D' corresponds to EIGRP-learned routes.
Cisco utilizes 'D' for EIGRP (referencing its DUAL algorithm) because 'E' was historically assigned to EGP.

Key Concept

Routing Table Components and Operational Roles
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