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Zorluk: OrtaRouting Table Components

A network device attempts to forward an IP packet toward the target address 172.31.25.140. The routing table contains four matching prefix entries:

- 172.31.25.0/24 [1/0] via 10.1.1.1
- 172.31.25.128/26 [90/307200] via 10.2.2.2
- 172.31.25.128/27 [110/40] via 10.3.3.3
- 172.31.25.136/29 [120/4] via 10.4.4.4

Which next-hop IP address will the device select to forward this packet?

  1. 10.4.4.4Cevap
  2. B
    10.1.1.1
  3. C
    10.2.2.2
  4. D
    10.3.3.3

Cevap

The router forwards the packet to next-hop address 10.4.4.4 because 172.31.25.136/29 has the longest matching prefix length (/29).
When a router receives a packet, it compares the destination IP address against all entries in its routing table and selects the route with the longest matching prefix length (the most specific subnet mask). Here, 172.31.25.136/29 matches 172.31.25.140 with a 29-bit prefix length, which is longer than /27, /26, or /24. The next-hop associated with this route is 10.4.4.4.

Adım Adım Çözüm

1
Determine which routing entries match destination IP 172.31.25.140.
All four routes (172.31.25.0/24, 172.31.25.128/26, 172.31.25.128/27, and 172.31.25.136/29) contain the IP address 172.31.25.140 within their range.
For 172.31.25.136/29, the usable IP host range spans 172.31.25.137 through 172.31.25.142, which includes 172.31.25.140.
2
Compare the prefix lengths of all matching routes.
The prefix lengths are /24, /26, /27, and /29.
Routers use the Longest Prefix Match (LPM) rule to select the forwarding route regardless of Administrative Distance.
3
Select the route with the highest prefix length (most specific mask).
/29 is the longest prefix match, corresponding to route 172.31.25.136/29 via 10.4.4.4.
Administrative distance is only evaluated when comparing identical prefixes from different routing sources. For different prefix lengths, LPM always governs.

Anahtar Kavram

Longest Prefix Match (LPM)
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Konular

Routing Table Components

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