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Zorluk: KolayRouter Forwarding Decision Logic

Refer to the following routing table snippet from a Cisco router:

text
Codes: C - connected, S - static, O - OSPF, D - EIGRP
Gateway of last resort is not set

D 172.16.10.0/24 [90/2172416] via 10.1.1.1, 00:05:12, GigabitEthernet0/0
O 172.16.10.0/26 [110/20] via 10.1.1.2, 00:12:45, GigabitEthernet0/1
S 172.16.0.0/16 [1/0] via 10.1.1.3
C 172.16.10.128/27 is directly connected, GigabitEthernet0/2

When the router receives an IP packet destined for host 172.16.10.33, which next-hop IP address or interface will be selected to forward the packet?

  1. 10.1.1.2Cevap
  2. B
    10.1.1.3
  3. C
    10.1.1.1
  4. D
    GigabitEthernet0/2

Cevap

10.1.1.2 via GigabitEthernet0/1 based on Longest Prefix Match (/26).
When forwarding IP packets, a router always evaluates candidate routes using Longest Prefix Match (LPM) first. For the target IP address 172.16.10.33, three candidate routes match: 172.16.0.0/16, 172.16.10.0/24, and 172.16.10.0/26. The OSPF route 172.16.10.0/26 provides the longest matching prefix mask (/26), so the router forwards the packet to next-hop IP 10.1.1.2.

Adım Adım Çözüm

1
Identify all candidate routes that match destination IP 172.16.10.33.
Three matching routes exist: 172.16.0.0/16, 172.16.10.0/24, and 172.16.10.0/26. The route 172.16.10.128/27 covers range 172.16.10.128 to 172.16.10.159 and does not match.
Routing decision logic only evaluates routes whose network address and subnet mask encompass the target IP.
2
Compare prefix lengths among all matching candidate routes.
172.16.10.0/26 has a 26-bit prefix match, which is longer (more specific) than /24 and /16.
Longest Prefix Match (LPM) is always the primary criterion used by a router when selecting a route from the routing table.
3
Determine the next-hop associated with the longest prefix match route.
The route 172.16.10.0/26 points to next-hop IP 10.1.1.2.
Administrative Distance and Metric are only evaluated when comparing routes with identical prefix lengths from different sources.

Anahtar Kavram

Router Forwarding Decision Logic (Longest Prefix Match Precedence)
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Router Forwarding Decision Logic

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