Examkin

Soru

Zorluk: OrtaRouting Table Components

A Cisco router displays the following routing table output:

text
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
O 10.75.0.0/16 [110/20] via 192.168.12.2, 00:14:22, GigabitEthernet0/0
D 10.75.96.0/19 [90/307200] via 192.168.13.2, 01:42:05, GigabitEthernet0/1
S 10.75.100.0/24 [1/0] via 192.168.14.2, 03:02:10, GigabitEthernet0/2
O IA 10.75.100.32/27 [110/65] via 192.168.15.2, 00:05:10, GigabitEthernet0/3

The router receives an IP packet destined for 10.75.100.5010.75.100.50. Which next-hop IP address and outgoing interface will the router select to forward this packet?

  1. A
    192.168.14.2 via GigabitEthernet0/2
  2. 192.168.15.2 via GigabitEthernet0/3Cevap
  3. C
    192.168.13.2 via GigabitEthernet0/1
  4. D
    192.168.12.2 via GigabitEthernet0/0

Cevap

Next-hop address 192.168.15.2 via interface GigabitEthernet0/3
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 prefix match (most specific subnet mask). The destination address 10.75.100.50 matches four routes in the routing table: /16, /19, /24, and /27. The /27 route (10.75.100.32/27) has the most matching network bits (27 bits), so the router forwards the packet to next-hop 192.168.15.2 out interface GigabitEthernet0/3.

Adım Adım Çözüm

1
Identify all candidate routes that encompass destination IP 10.75.100.50
The address 10.75.100.50 falls within 10.75.0.0/16, 10.75.96.0/19 (10.75.96.0 - 10.75.127.255), 10.75.100.0/24 (10.75.100.0 - 10.75.100.255), and 10.75.100.32/27 (10.75.100.32 - 10.75.100.63).
A packet destination must match the subnet range of a candidate route entry.
2
Apply the Longest Prefix Match (LPM) rule across all matching routes
The prefix lengths are /16, /19, /24, and /27. The longest prefix match is /27 (10.75.100.32/27).
Cisco routers always prefer the route with the most specific prefix (longest subnet mask) regardless of administrative distance or metric.
3
Extract the next-hop IP and outgoing interface from the longest prefix match route entry
The entry 10.75.100.32/27 specifies next-hop 192.168.15.2 via interface GigabitEthernet0/3.
The router uses the forward destination components listed in the winning routing table entry.

Anahtar Kavram

Longest Prefix Match Rule
Tahmini Süre:1m 15s
Bu soruyu puanla

Konular

Routing Table Components

Bu konudan daha fazla soru