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Zorluk: ZorRouter Forwarding Decision Logic

Examine the following Cisco router IP routing table output:

text
R1# show ip route
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 not set

172.24.0.0/16 is subnetted, 4 subnets
S 172.24.0.0/16 [1/0] via 10.10.10.1
D 172.24.16.0/24 [90/307200] via 10.10.20.1
O 172.24.16.64/26 [110/40] via 10.10.30.1
R 172.24.16.80/28 [120/2] via 10.10.40.1

Router R1 receives an IP packet destined for 172.24.16.85172.24.16.85. Which next-hop IP address will router R1 select to forward this packet?

  1. 10.10.40.1Cevap
  2. B
    10.10.10.1
  3. C
    10.10.20.1
  4. D
    10.10.30.1

Cevap

10.10.40.1
When a router makes a forwarding decision, it evaluates matching routes using Longest Prefix Match (LPM) as the top priority criterion. Because 172.24.16.85172.24.16.85 matches 172.24.16.80/28172.24.16.80/28 (range .80–.95), the /28/28 route has the longest prefix length (28 bits) among all candidates. Thus, next-hop 10.10.40.1 is selected regardless of RIP having a higher Administrative Distance (120) than Static (1), EIGRP (90), or OSPF (110).

Adım Adım Çözüm

1
Determine which routing table entries match the target destination IP 172.24.16.85172.24.16.85.
Matches found: 172.24.0.0/16172.24.0.0/16, 172.24.16.0/24172.24.16.0/24, 172.24.16.64/26172.24.16.64/26 (range .64 to .127), and 172.24.16.80/28172.24.16.80/28 (range .80 to .95).
The destination address 172.24.16.85172.24.16.85 falls within the address ranges of all four configured subnets.
2
Apply the router forwarding lookup order logic (Longest Prefix Match).
The /28/28 prefix is selected over /26/26, /24/24, and /16/16 because 2828 is the maximum number of matching prefix bits.
Routers always evaluate prefix length first. Administrative Distance is only evaluated when comparing identical prefixes learned from different sources.
3
Identify the next-hop IP associated with the winning /28/28 route entry.
The next-hop address for 172.24.16.80/28172.24.16.80/28 is 10.10.40.110.10.40.1.
The packet is forwarded out to the next hop specified in the /28/28 RIP route entry despite its higher AD.

Anahtar Kavram

Longest Prefix Match (LPM) precedence in Cisco IPv4 routing lookup decision logic
Tahmini Süre:2m 0s
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Konular

Router Forwarding Decision Logic

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