3.2 Router Decision Making

Question 1

What three key factors does a router consider when forwarding?

Answer 1

1. Longest Prefix Match (LPM)
2. Administrative Distance (AD)
3. Routing Protocol Metric

Longest Prefix Match (LPM) – The router selects the most specific route that matches the destination IP.

Administrative Distance (AD) – If multiple routes exist, the one with the lowest AD is preferred.

Routing Protocol Metric – If ADs are equal, the router chooses the route with the lowest metric from the routing protocol.

Question 2

What is the default process a router follows to determine the best route?

Answer 2

1. It uses Longest Prefix Match (LPM) to find the most specific route.
2. It chooses the route with the lowest Administrative Distance (AD).
3. It selects the path with the best Routing Protocol Metric if AD is equal.

A router prioritizes the most specific route, then the lowest AD, and finally the best metric if needed.

Question 3

List three different metric calculations used by routing protocols.

Answer 3

1. RIP
2. OSPF
3. EIGRP

RIP (Routing Information Protocol) – Hop count

OSPF (Open Shortest Path First) – Cost (based on bandwidth)

EIGRP (Enhanced Interior Gateway Routing Protocol) – Composite metric (bandwidth, delay, reliability, load)

Question 4

What routing metric counts the number of routers a packet passes through?

Answer 4

Hop

The hop count metric only considers the number of times a data packet is forwarded by routers along its path.

Question 5

Define:

Longest Prefix Match

(LPM)

Answer 5

The process of selecting the most specific route in the routing table.

A more specific subnet mask (e.g., /30 over /24) takes precedence over a broader one when multiple matches exist.

Question 6

A packet destined for 192.168.1.50 matches both 192.168.1.0/24 and 192.168.1.0/26. Which route is selected?

Answer 6

192.168.1.0/26

Since /26 is more specific than /24, the router chooses it first.

Question 7

True or False:

A router always prefers the route with the smallest subnet mask.

Answer 7

False

A router chooses the longest prefix (most specific subnet), meaning /30 is preferred over /24.

Question 8

List three subnet mask lengths with host counts.

Answer 8

1. /30 → 2 usable hosts
2. /28 → 14 usable hosts
3. /24 → 254 usable hosts

The smaller the subnet mask (higher prefix length), the fewer available hosts.

Question 9

Define:

Administrative Distance

(AD)

Answer 9

The measure of trustworthiness of a route.

A lower AD means higher reliability—directly connected routes (AD = 0) are most trusted, while external BGP (AD = 20) is more preferred than RIP (AD = 120).

Question 10

A router has two routes to 10.1.1.0/24: RIP (AD 120) and OSPF (AD 110). Which route will be chosen?

Answer 10

OSPF (AD 110)

Lower AD is preferred, so OSPF is selected over RIP.

Question 11

A router has two routes to 10.1.1.0/24:

OSPF Route (AD = 110, Metric = 10)
Static Route (AD = 1, Metric = 20)

Which route will be chosen?

Answer 11

Static Route (AD = 1) since AD is prioritized over the metric.

AD is evaluated before the metric when selecting a route.

Question 12

List the Administrative Distance (AD) of three common routing sources.

Answer 12

1. Connected Route – 0
2. Static Route – 1
3. OSPF – 110

Static and connected routes are more trusted than dynamic routing protocols due to manual configuration.

Question 13

What route type has the best administrative distance?

Answer 13

Connected

Routes marked as connected have the lowest administrative distance of 0, making them the most preferred routes.

Question 14

Which type of route is prioritized by default on a router?

Answer 14

Static

Static routes have a default administrative distance of 1, making them preferred unless their AD is altered.

Question 15

Define:

Floating Static Route

Answer 15

A static route with an increased AD value, making it a backup route.

Floating static route is used to create failover routes if the primary route goes down.

Question 16

What are two recognized values for AD in routing?

Answer 16

1. 0
2. 110

AD values help routers select the best path among multiple routing protocols, with 0 indicating a directly connected route.

Question 17

Which administrative distance indicates the best route to a subnet?

Answer 17

1

An administrative distance of 1 signifies that a static route is the most favorable option for directing traffic to a subnet.

Question 18

Why might a router choose a higher AD route over a lower AD one?

Answer 18

1. The lower AD route is unreachable (e.g., next-hop failure or interface down).
2. There are administrative restrictions (e.g., filtering or policies).

A route with lower AD is only used if it is reachable.

Question 19

Which metric is used for links with high interface faults and data loss?

Answer 19

Reliability

The reliability metric assesses link performance by factoring in failures and losses, making it suitable for routing decisions in poor-performing links.

Question 20

True or False:

RIP (AD 120) is preferred over EIGRP (AD 90).

Answer 20

False

Since EIGRP has a lower AD (90), it is preferred over RIP (120).

Question 21

Fill in the blank:

The metric value used by OSPF is called ______.

Answer 21

cost

OSPF calculates cost based on interface bandwidth (higher bandwidth = lower cost).

Question 22

True or False:

Between two OSPF routes, the higher-cost one is always ignored.

Answer 22

False

Higher-cost routes are only considered if the lower-cost route becomes unavailable.

Question 23

What role do metrics play within a routing table?

Answer 23

1. They help choose between paths within the same routing protocol.
2. They are the second value in brackets in the ‘show ip route’ command.

Metrics assess the cost of paths to determine the best route for data transmission.

Question 24

Which routing metric prioritizes paths based on data transfer capacity?

Answer 24

Bandwidth

Bandwidth-based metrics select routes with the highest capacity for data transmission.

Question 25

# Fill in the blank: \_\_\_\_\_\_ protocol **exclusively uses hop count** as its metric.

Answer 25

RIP ## Footnote **RIP** relies solely on hop count to determine the *best path for data transmission*, contrasting with other protocols that may consider different metrics.

Question 26

What does the **delay metric** measure in routing?

Answer 26

The **time required** for a router to process and send out a datagram. ## Footnote *Lower cumulative delay* across all links is **favored** when selecting a routing path.

Question 27

What if **multiple routes** share prefix length, AD, and metric?

Answer 27

Equal-Cost Multi-Path (ECMP) Routing is used. ## Footnote **ECMP (Equal-Cost Multi-Path)** allows a router to *load-balance traffic* over multiple equal-cost paths.