Switch Forwarding Question: Destination MAC Only?

By certskills December 13, 2016 12:05

How does a Layer 2 switch choose where to send a frame? Does it think about more than the destination MAC address? One of the common mistakes videos in the new “CCENT ICND1 100-105 Exam Prep LiveLessons” product talks about just that; you can watch that one for free as one of the sample videos on the web site. I thought I’d add another question to the site, one that the video would help you answer. Answer on your own, and if you struggle with the question, go check out the video for some more background!

A Question to Consider

The links between switches in the figure are VLAN trunks, and the links to servers are access ports. PC2 sends a frame to PC4’s MAC address, namely 0200.4444.4444. Out which of its interfaces will switch SW3 forward the frame?

  1. F0/1
  2. F0/2
  3. F0/3
  4. G0/1
  5. None; SW3 will discard the frame without forwarding it


Figure for This Question

SW3# show interfaces status
Port      Name         Status       Vlan     Duplex  Speed Type
Fa0/1                  connected    2        a-full  a-100 10/100BaseTX
Fa0/2                  connected    2        a-full  a-100 10/100BaseTX
Fa0/3                  connected    3        a-full  a-100 10/100BaseTX
G0/1                   connected    trunk    a-full a-1000 10/100/1000BaseTx 
! Only pertinent lines shown

SW3# show mac address-table dynamic
        Mac Address Table
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
2       0200.1111.1111    DYNAMIC     Fa0/1
3       0200.5555.5555    DYNAMIC     Fa0/3
2       0200.6666.6666    DYNAMIC     Gi0/1

Example 1: Command Output


Answers next post! And if you get stuck but want a little background before looking at the answer, go here and watch Lesson 7.2.






Common Mistakes in ICND1 100-105 / CCENT Domain 1
Answer to Question: Destination MAC Only?
By certskills December 13, 2016 12:05
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December 13, 2016 8:42 pm

I’m going to take a shot at this since (haven’t watched the video yet) I’m lurking about, so let’s start eliminating stuff:

– switches don’t forward frames out on the interface that the frame came from, taking out f0/2 out of the equation.
– analyzing the output of the show interface status, we see that f0/3 is in a different vlan, also excluding this interface.
– since the switch (by analyzing the mac address table output) hasn’t learned destination ports for 0200.2222.2222 (pc2) and 0200.4444.4444 (pc4), it suggests that a broadcast will happen (comming from pc2), flooding the frame out on f0/1 and g0/1

Comments will be welcome, and if I’m wrong, try to point me in the right direction without giving away the answer please! TYVM

Reply to  Ruben
November 24, 2017 2:32 pm

I believe it will only forward it out Gi0/1 as that is a trunk port and the other vlans connecting fa0/1 and fa0/2 are on vlan 2 and access ports.

Punya Athma
May 8, 2020 12:40 am

Answers are: A, and D. Ports fa0/1, fa0/2, and, Gi0/1, are in the same VLAN 2. For SW3 the frame from PC2 intended to PC4 is an unknown uni cast frame as the mac address of PC4 is not in the table of SW3. So the frame flooded out fa0/1, and, Gi0/1 except the port fa0/2 from it the frame entered SW3.