Question: Traceroute to a subnet broadcast address – what happens?

certskills
By certskills June 16, 2015 09:05

Here’s a new question that makes you think about LAN broadcasts. I made this one up because of a post on the Cisco Learning Network about whether routers always choose to not forward broadcasts, or not. Traditionally, routers have been called “broadcast firewalls” because they stop broadcasts, which switches do not. However, when is a broadcast really a broadcast, or more specifically, when might a router actually forward a broadcast? This is frankly pretty deep for ICND1/CCENT, but it does have the nice side-effect of forcing you to think about lots of fundamentals, so I figured its worth discussing for that reason alone. Feel free to theorize out loud here!

PC A issues a ping 192.168.1.30 command, and it succeeds. Then, PC A issues a tracert -d 192.168.1.31 command, which traces the route to 192.168.1.31 and lists only the IP addresses (and not hostnames, due to the -d flag) of the routers. However, 192.168.1.31 happens to be the subnet broadcast address (aka directed broadcast address) of the subnet on the right side of the figure.  Which answer most accurately describes the results of this command? Assume all default settings on the router, other than what’s implied to be configured to support the working ping from A to B.

A. The command fails, listing no router IP addresses, because A gets no ARP response from its ARP looking for 192.168.1.31’s MAC address

B. The command fails, listing no router IP addresses, because R2 notices that the destination is a broadcast address, and discards the packet.

C. The command lists R2’s 172.16.0.1 IP address, but no others, because R2 notices that the destination is a broadcast address, and discards the packet.

D. The command lists R2’s 172.16.0.1 address, and then R3’s 172.16.6.2 address, but no other IP addresses, because R3 discards the packet based on its default of no ip directed-broadcast on its F0/0 interface

E. The command lists R2’s 172.16.0.1 address, and then R3’s 172.16.6.2 address, but no other IP addresses, because R3 gets no response to its ARP request for 192.168.1.31 on its F0/0 interface

Answers: Serial Link Interview Question
traceroute to a subnet broadcast - Answer (1)
certskills
By certskills June 16, 2015 09:05
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6 Comments

  1. Dadash June 16, 10:12

    Hi Mr Odom, i think answer is “E”. Thanks for interesting questions!

    Reply to this comment
  2. Joy June 16, 15:07

    answer D

    Reply to this comment
  3. Victor June 16, 21:46

    e
    ttl expired will answer the first two, it’s only until it reach the local subnet that it becomes a broadcast packet

    Reply to this comment
  4. Chad June 18, 14:10

    E

    Windows traceroute uses icmp. R2 will forward icmp request to R3 and R3 will have the route for 192.168.1.31 but it will not have the ARP entry for 192.168.1.31 hence it can’t forward the packet and traceroute will fail

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