Answer to Question: Destination MAC Only?
The previous post posed a question that appeared straightforward – but one that set up a couple of the more common mistakes that people make with the subject of Layer 2 frame forwarding. The goal was twofold: yet another practice question, but also to give you a reason to check out the “CCENT ICND1 100-105 Exam Prep LiveLessons” product in case you wanted more background info. This post details the answers, focusing on the trickier answers.
First – the Literal Answers
That is, a frame sent by PC2 to PC4’s MAC address (0200.4444.4444) will be forwarded out SW3’s ports F0/1 and G0/1, but not the other ports. Here’s a copy of the figure for reference, and a copy of the exhibit.
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
Analysis: Forwarding in VLAN 2
One key to answering this question is to determine which VLAN the switch considers the frame to be in when processing the frame. Reducing the facts:
- The question stem states: frame is sent by PC2
- The Figure shows PC2 off switch SW3’s port F0/2
- The stem states that all ports connected to PCs are access ports
- The exhibit shows port F0/2 as an access port in VLAN 2
So, switch SW3 processes the frame as part of VLAN 2.
Ruling Out Two Incorrect Answers
Next consider the incorrect answer about port F0/2. Will a switch receive a frame in one access interface (F0/2) and send it back out that same access interface? No. Simple enough.
Next, consider the answer for interface F0/3. Why wouldn’t the switch forward the frame out F0/3? That port appears to be up and working, and SW3 needs to flood the frame as it turns out.
The reason is simple: the port is not part of VLAN 2, but the frame arrived in VLAN 2. Following the facts again:
- From the earlier list, switch SW3 considers the frame to be part of VLAN 2
- The question stem states that the ports connected to PCs are access ports
- The exhibit shows port F0/3 as being assigned to VLAN 3
Therefore, switch SW3 will not forward the frame out port F0/3 – even though SW3 will flood the frame.
Flooding the Frame… In VLAN 2
Now turn to the correct answers. The frame arrives in an access port assigned to VLAN 2 (port F0/2). As it turns out, switch SW3 floods the frame in VLAN 2. Here’s why:
- The destination MAC – 0200.4444.4444 – is not in SW3’s MAC table at all. More importantly, there is no entry for that MAC address that also lists VLAN 2.
- In other words, the destination is unknown to SW3 in VLAN 2, so SW3 treats the frame as an unknown unicast frame
- SW3 floods the frame out:
- All ports in VLAN 2 (access ports and trunks)
- …Except the incoming port (F0/2 in this case)
Per the exhibit, those interfaces include F0/1 (an access port) and G0/1 (a trunk port). Those are the two correct answers.
Extra Logic… That Does NOT Apply
Did you think that SW3 would NOT forward the frame out port F0/1?
Couple of times over the last few months I’ve heard from readers who have wanted to add a bit of switching logic to SW3, logic that would have made SW3 not forward the frame out F0/1. Here’s the logic, which is NOT true – but you can easily see why someone might go there mentally.
- SW3 has learned the MAC address of the one device off its F0/1 access port (0200.1111.1111)
- When flooding the frame in VLAN 2, SW3 does NOT flood the frame out port F0/1, because SW3 has already learned of one MAC
- The logic (which is, again, not true) is that because SW3 knows of one MAC on that port, it should NOT flood the frame on the port
Switches don’t add that bit of logic, because there might be more than one device connected off that port (for instance, through a hub or even through another switch). So, don’t add the logic!
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