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Zorluk: OrtaRouting Table Components

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?

  1. 10.0.0.3 via GigabitEthernet0/2Cevap
  2. B
    10.0.0.1 via GigabitEthernet0/0
  3. C
    10.0.0.2 via GigabitEthernet0/1
  4. D
    10.0.0.4 via GigabitEthernet0/3

Cevap

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.

Adım Adım Çözüm

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.

Anahtar Kavram

Longest Prefix Match (LPM) Routing Decision Logic
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Konular

Routing Table Components

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