Switch Forwarding Question: Destination MAC Only?
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?
- F0/1
- F0/2
- F0/3
- G0/1
- 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 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.
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
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.
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.
Hello Wendell,
This is wishing you a happy & prosperous new year!. Please a question. This is not related to this topic but AP Architecture.
Are you able to explain to me the difference between Cloud-based AP Architecture & Split-MAC Architecture. Both of these architectures have their management functions removed from the APs, but what I don’t understand is why in cloud-based architecture the APs are still refered as Autonomous APs instead of Lightweight APs. Does it mean that in the cloud architectures, not all of the management functions are removed from the APs?
Hi Tewa,
Happy new year to you as well!
Watching your comments over these months is like watching your progression from start to finish in the books, by the way. Nice to see!
Anyway, if you were to break down every management, control plane, and data plane function in wireless, indeed there are differences between the cloud-based model (which comes from Cisco’s Meraki product line) versus the Split Mac architecture. It’s more about the data plane. In the cloud model, the data plane on the AP does all the work. It receives 802.11 frames on the wireless side and translates to 802.3. No CAPWAP encapsulation to a controller. So, while all config happens in the cloud, forwarding is stand alone. The split MAC architecture requires the lightweight APs to cooperatively implement data plane forwarding with the WLC. Receiving an 802.11 frame, it encapsulates the 802.11 frame in CAPWAP. The WLC de-encapsulates, translates to 802.3, and then forwards. That’s the primary difference.
Wendell
Hello Wendell,
Thank you for your kind words. I just want to let you know that I attempted the CCNA exam and I passed. I wouldn’t have done it without God and your support in answering most of my crazy/stupid questions. I am now more knowledgable about networking than I was when I first picked up your book in March 2023. Keep up with the good work of maintaining this blog posts, they were a life saver for me and I am sure they have helped an awful lot in the past.
The funny thing though is that I don’t know where to go from here. I picked up this book because I wanted to have a basic understanding of networking. After a few pages into the book, I got hooked with the way you simplified the concepts for novice like me and I decided to take it up a notch and attempt the exam. Now I don’t know what to do. Would be nice to get some networking job but with recruiters requiring donkey years of experience, I don’t know if I stand any chance of landing a decent networking job. Time will tell, I am praying for God’s guidance on the next steps.
Thanks once again and do keep up the good work. Watch out for my stupid questions every once in a while…lol
Hi Tewa,
Woohoo!!!! Congratulations!
You certainly dug in more in this blog than most. You’re quite welcome for the help.
God will order your steps, Tewa. Keep taking them! Pick something that you find interesting and keep learning – and don’t lose that momentum!
All the best,
Wendell