Examkin

Soru

Zorluk: OrtaRouting Table Components

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.)

  1. 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.Cevap
  2. The router evaluates administrative distance only among routes that share the exact same longest prefix match length.Cevap
  3. C
    The router forwards the packet to next-hop 192.168.1.10 because static routes have a lower administrative distance (1) than OSPF or EIGRP routes.
  4. D
    The router forwards the packet via 192.168.1.2 because EIGRP (AD 90) is preferred over OSPF (AD 110) for destination prefix 10.10.20.0.

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

Longest Prefix Match Rule vs. Administrative Distance
Bu soruyu puanla

Konular

Routing Table Components

Bu konudan daha fazla soru