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Zorluk: ZorRouter Forwarding Decision Logic

Refer to the following partial routing table output from a Cisco router:

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 10.4.4.4 to network 0.0.0.0

O 172.16.32.0/20 [110/20] via 10.1.1.1, 00:10:05, GigabitEthernet0/0
D 172.16.35.64/28 [90/307200] via 10.2.2.2, 00:05:12, GigabitEthernet0/1
S 172.16.35.64/26 [1/0] via 10.3.3.3
C 172.16.35.0/24 is directly connected, GigabitEthernet0/2
S* 0.0.0.0/0 [1/0] via 10.4.4.4

The router receives an ingress IPv4 packet with a destination address of 172.16.35.73. Which statements accurately describe the router forwarding decision for this packet? (Select TWO.)

  1. The router forwards the packet to next-hop IP 10.2.2.2 via GigabitEthernet0/1 because the route to 172.16.35.64/28 provides the longest prefix match.Cevap
  2. The router bypasses the static route to 172.16.35.64/26 despite its lower Administrative Distance because Administrative Distance is only compared among routes with identical prefix lengths.Cevap
  3. C
    The router selects the static route via 10.3.3.3 because static routes have an Administrative Distance of 1, which takes precedence over EIGRP routes.
  4. D
    The router forwards the packet out GigabitEthernet0/2 because directly connected subnets take priority over dynamically learned routes.

Cevap

The router forwards the packet to next-hop IP 10.2.2.2 via GigabitEthernet0/1 because 172.16.35.64/28 is the longest prefix match, and it ignores lower administrative distances on shorter prefixes because AD is only evaluated when prefix lengths are equal.
When forwarding an IPv4 packet, a Cisco router evaluates the Forwarding Information Base using Longest Prefix Match (LPM) as the primary criteria. For destination address 172.16.35.73, the route 172.16.35.64/28 (host range 172.16.35.64 to 172.16.35.79) has a 28-bit prefix length, making it more specific than the /26 static route, the /24 directly connected subnet, or the /20 OSPF route. Administrative Distance (AD) is only evaluated when multiple routing protocols install identical network prefixes into the routing table, so the router does not consider the lower AD of the static route.

Adım Adım Çözüm

1
Identify all candidate routes matching destination IP 172.16.35.73
Candidate routes are 172.16.32.0/20, 172.16.35.0/24, 172.16.35.64/26, 172.16.35.64/28, and 0.0.0.0/0.
The destination host IP 172.16.35.73 falls within the IP address range of all these configured route subnets.
2
Determine the Longest Prefix Match (LPM) among candidates
172.16.35.64/28 has the longest prefix mask length (/28 = 28 bits).
Cisco IOS forwarding logic always selects the route with the most specific subnet mask matching the target destination.
3
Evaluate Administrative Distance and Metric applicability
Administrative Distance and Metric are not compared across different prefix lengths.
AD is only used when multiple routing protocols offer routes to the exact same network prefix length.

Anahtar Kavram

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

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