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Zorluk: Çok zorRouting Table Components

An administrator examines the following output from a Cisco router's IPv4 routing table:

text
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 6 subnets, 3 masks
O 10.20.30.0/24 [110/20] via 192.168.12.2, 00:15:32, GigabitEthernet0/0
D 10.20.30.32/28 [90/307200] via 192.168.23.2, 01:42:10, GigabitEthernet0/1
S 10.20.30.40/29 [1/0] via 192.168.34.2
O IA 10.20.30.44/30 [110/50] via 192.168.45.2, 00:08:19, GigabitEthernet0/2

Which two statements accurately interpret the components and forwarding behavior of this routing table? (Select two.)

  1. Packets destined for host 10.20.30.45 will be egressed via interface GigabitEthernet0/2 because 10.20.30.44/30 provides the longest matching prefix for that destination address.Cevap
  2. The bracketed notation [90/307200] in the 10.20.30.32/28 route entry specifies an Administrative Distance of 90 and an EIGRP composite metric of 307200.Cevap
  3. C
    Packets destined for host 10.20.30.45 will be routed via next-hop 192.168.34.2 because the static route entry 10.20.30.40/29 features the lowest Administrative Distance.
  4. D
    Host 10.20.30.45 cannot match the route entry 10.20.30.44/30 because 10.20.30.45 is the subnet broadcast address of that /30 network.

Cevap

The correct statements are that packets for destination 10.20.30.45 will exit via GigabitEthernet0/2 due to longest prefix match (/30), and that [90/307200] defines an Administrative Distance of 90 and metric of 307200.
The router uses the Longest Prefix Match rule during packet lookup. Since destination 10.20.30.45 falls into 10.20.30.44/30 (range 10.20.30.44 to 10.20.30.47), the /30 prefix length is the longest and takes precedence over /29, /28, and /24. Additionally, routing table bracket notation [X/Y] represents [Administrative Distance / Metric], correctly mapping [90/307200] to AD 90 and EIGRP metric 307200.

Adım Adım Çözüm

1
Evaluate subnet range matching for target IP address 10.20.30.45 across all routing table entries.
The target address 10.20.30.45 matches 10.20.30.0/24, 10.20.30.32/28 (32-47), 10.20.30.40/29 (40-47), and 10.20.30.44/30 (44-47).
Before making forwarding decisions, the router determines which routing table prefixes encompass the destination address.
2
Apply the Longest Prefix Match (LPM) rule to select the forwarding route.
Prefix length /30 is more specific than /29, /28, and /24. The route 10.20.30.44/30 is selected.
Routers always prioritize the longest (most specific) network mask when choosing between matching routing entries, regardless of Administrative Distance.
3
Interpret routing table component metrics in the brackets [X/Y].
[90/307200] corresponds to AD=90 (Internal EIGRP) and Metric=307200.
The first number inside the brackets is Administrative Distance (trustworthiness), and the second number is the path metric.

Anahtar Kavram

Routing Table Components and Longest Prefix Match Forwarding Logic
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Routing Table Components

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