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Difficulty: HardRouter Forwarding Decision Logic

Refer to the following partial output from the `show ip route` command on a Cisco router:

text
Gateway of last resort is 10.0.0.1 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 10.0.0.1, GigabitEthernet0/0
S 172.20.0.0/16 [1/0] via 10.1.1.1, GigabitEthernet0/1
O 172.20.12.0/24 [110/30] via 10.2.2.2, GigabitEthernet0/2
D 172.20.12.64/27 [90/3072] via 10.3.3.3, GigabitEthernet0/3
S 172.20.12.64/30 [1/0] via 10.4.4.4, GigabitEthernet0/4

The router receives an IP packet destined for 172.20.12.70172.20.12.70. Which TWO statements correctly describe how the router processes and forwards this packet?

  1. The router forwards the packet out interface GigabitEthernet0/3 toward next-hop IP 10.3.3.3.Answer
  2. The route 172.20.12.64/27 is selected over 172.20.0.0/16 because Longest Prefix Match takes precedence over Administrative Distance.Answer
  3. C
    The router forwards the packet out interface GigabitEthernet0/4 because static routes with an Administrative Distance of 1 are preferred over EIGRP routes.
  4. D
    The router forwards the packet out interface GigabitEthernet0/1 because the static route 172.20.0.0/16 has a lower Administrative Distance than the EIGRP route.

Answer

The router forwards the packet out interface GigabitEthernet0/3 toward next-hop IP 10.3.3.3, and the route 172.20.12.64/27 is selected over 172.20.0.0/16 because Longest Prefix Match takes precedence over Administrative Distance.
When a router receives a packet, it compares the destination IP address against all entries in its routing table. The matching route with the longest prefix length (most specific mask) is selected. Here, 172.20.12.70 matches 0.0.0.0/0 (/0), 172.20.0.0/16 (/16), 172.20.12.0/24 (/24), and 172.20.12.64/27 (/27). The /27 route is the longest match, so traffic is sent via next-hop 10.3.3.3 out GigabitEthernet0/3. Administrative Distance is ignored because prefix lengths differ.

Step-by-Step Solution

1
Determine subnet boundaries for all matching route entries against destination IP 172.20.12.70
0.0.0.0/0 matches (/0); 172.20.0.0/16 matches (/16); 172.20.12.0/24 matches (/24); 172.20.12.64/27 covers 172.20.12.64 - 172.20.12.95 and matches (/27); 172.20.12.64/30 covers 172.20.12.64 - 172.20.12.67 and DOES NOT match.
Only routes whose network range actually encompasses the destination IP address can be considered for packet forwarding.
2
Compare prefix lengths among all valid candidate matching routes
Candidate prefix lengths are /0, /16, /24, and /27. The longest prefix match is 172.20.12.64/27.
Cisco forwarding decision logic strictly prefers the route with the most specific (longest) subnet mask regardless of routing protocol origin or Administrative Distance.
3
Identify the egress interface and next-hop address for the winning route 172.20.12.64/27
Next-hop IP is 10.3.3.3 via egress interface GigabitEthernet0/3.
The routing table entry `D 172.20.12.64/27 [90/3072] via 10.3.3.3, GigabitEthernet0/3` directly specifies these parameters.

Key Concept

Router Forwarding Decision Logic and Longest Prefix Match (LPM) Precedence
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Topics

Router Forwarding Decision Logic

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