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Two questions about switch and IEEE 802.1q

Hello! I am a student of the Department of Computer Science Information Engineering. Recently, I am learing VLAN and IEEE 802.1Q protocol, and here are two questions that confused me :

1. A packet with 802.1q VLAN tag 424 is sent out from the switch via port Gi1/0/1. Later,
the same packet is sent out again, but via port Gi1/0/2. What is the difference in 802.1q header
between the two output packets from distinct source ports?
2. Three Packets with no 802.1q VLAN tag are sent out from different end users to the
switch via ports Gi1/0/3, Gi1/0/4, Gi1/0/5, respectively. What is the difference in 802.1q
header among the three packets from distinct incoming ports?

Could anyone please help me figure out these problems? Thank you!

3 REPLIES 3
VIP Expert

Re: Two questions about switch and IEEE 802.1q

#1 If the identical frame is being sent out different ports, it's just that, identical. VLAN frames don't have port information.

#2 Insufficient information to say. The VLAN tagging might be alike for all 3 hosts, or each might be different, or some other combination. Depends on what VLAN tags the hosts are using for that frame. Again, VLAN frames don't have port information.
Hall of Fame Master

Re: Two questions about switch and IEEE 802.1q

It seems to me that #2 borders on being a trick question. If the 3 packets (should we understand that this really means frames, since packets usually denotes layer 3 orientation while frames denotes layer 2 orientation) have no 802.1q vlan tags then how could they have 802.11 headers? Frames from users where the frame has no 802.1q tag implies that the users are connected to access ports. 802.1q headers are only applied to frames on trunk ports.

 

And I wonder about #1. If it is literally true that "later the same packet is sent again" then it implies that there is a loop in the topology. And certainly if it is the same frame then there can be no difference in the header.

 

HTH

 

Rick

VIP Expert

Re: Two questions about switch and IEEE 802.1q

Rick, great catch on #2. I missed the "with no 802.1q VLAN tag". That does sort of negate any possible difference in .q tags, when there are none. ;)

My read of the questions was the OP appears to be looking to determine if VLAN tag somehow relates to specific switch ports, beyond possible adding a VLAN ID or accepting one. If so, it does not.

To the OP, assuming the switch ports in question are configured alike, VLAN tags will be processed alike. For example, just as you can generally swap two hosts connected to access ports (that are configured alike), you can generally swap connections between two trunk ports (also configured alike). The frames for egress and ingress will be processed alike and the frames will look alike, with or without the VLAN tag.

BTW . . .

"Frames from users where the frame has no 802.1q tag implies that the users are connected to access ports. 802.1q headers are only applied to frames on trunk ports." As Rick notes, one would expect the switch's ports, for untagged frames, would be access ports, but "implies", perhaps, might be a tad too strong as an expectation. The switch port might be a trunk port accepting untagged frames on a "native" VLAN. It might also be a access port with a Voice (tagged) VLAN defined. There's just too much unknown, which I agree with Rick, can make these questions appear like "trick" questions.

Regarding number 1, other possibilities include perhaps the switch doesn't "know" the destination MAC so it might be flooding the same frame to multiple ports, or perhaps the one port is "spanning" the other; this in addition to Rick's mention of some topology issue. Again, there's insufficient information.

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