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Difficulty: MediumRouter Forwarding Decision Logic

Refer to the following partial IPv4 routing table output from a Cisco router:

text
Codes: C - connected, S - static, D - EIGRP, O - OSPF

Gateway of last resort is 10.88.0.1 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 10.88.0.1
O 10.88.0.0/16 [110/20] via 10.88.1.1, GigabitEthernet0/0/1
D 10.88.15.0/24 [90/307200] via 10.88.2.1, GigabitEthernet0/0/2
O 10.88.15.192/27 [110/50] via 10.88.3.1, GigabitEthernet0/0/3

The router receives a unicast IPv4 packet addressed to destination host 10.88.15.200. Which next-hop IP address will the router use to forward this packet?

  1. 10.88.3.1Answer
  2. B
    10.88.2.1
  3. C
    10.88.0.1
  4. D
    10.88.1.1

Answer

10.88.3.1
When a router receives a packet, it compares the destination IP address against all routing table entries. If multiple routes match, the router selects the route with the longest prefix length (most specific subnet mask). Here, 10.88.15.200 matches 0.0.0.0/0, 10.88.0.0/16, 10.88.15.0/24, and 10.88.15.192/27. The /27 prefix is the longest match, so the router forwards the packet to its next-hop address 10.88.3.1.

Step-by-Step Solution

1
Evaluate all matching routes for destination IP 10.88.15.200.
Matching entries found: 0.0.0.0/0, 10.88.0.0/16, 10.88.15.0/24, and 10.88.15.192/27.
10.88.15.200 falls within the range 10.88.15.192 to 10.88.15.223 defined by the /27 subnet mask.
2
Apply the Longest Prefix Match (LPM) rule.
The /27 prefix length is the longest (most specific) among all matching routes.
The router always prioritizes the most specific prefix match first. Administrative distance and metrics are only evaluated when comparing identical prefix lengths from different sources.
3
Identify the next-hop IP associated with the winning route 10.88.15.192/27.
Next-hop IP is 10.88.3.1.
The routing table entry points to 'via 10.88.3.1'.

Key Concept

Longest Prefix Match (LPM) rule in IPv4 routing lookup decision logic
Estimated Time:1m 30s
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Topics

Router Forwarding Decision Logic

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