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So, the question is this. I have a Cisco router with multiple VLANS assigned to a single Gigabit port. 100,200,300, etc. as well as the normal connection without any vlan tagging.

Now, everything goes over that one ethernet cable.

My question is this,

Assuming that you have 4 identical streams of the same protocol and same port happening at the same time, how is information send over the cable?

When the VLANS communicate with the router, do they just timestamp the regular network, then move on to vlan 100 and timestamp is, etc in serial?

I guess what I'm saying is if this is a hypothetical latency sensitive application, and these networks are utilizing the same resource, but need to stay on separate VLANS, what takes priority?

Whose packets are sent first? If all the VLANS needs to access this at the exact same time how is that done?

I hope this makes sense, thank you.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 15 '19 at 5:14
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Every router interface has a small hardware queue, which is FIFO (First In, First Out).

There is no timestamp or anything like that. Packets are queued in the hardware queue, and processed in the order in which they were enqueued.

That is how it works by default. Cisco routers also have a comprehensive QoS. This can be configured to change many things in the way traffic is handled, but it is a subject of entire books, so it is too large to explain here.

  • Hey there, thanks for the response, I appreciate it. I have QoS configured but that's not exactly what i'm confused about. For example, on the ios cli, there's a delay settings under each interface, measured in tens of microseconds. Assuming microsecond scale, would this be on the computer side as opposed to the router? I guess I'm asking if microseconds count, and I want all VLANS to access the resource at, lets says, 100 microseconds. So @ timestamp of 100 microseconds, every vlan pulls from the stream. Now it's serial and not parallel, so I guess that more the question, sorry for phrasing – user59792 Jul 15 '19 at 21:51
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    The delay is a number representing the serialization delay by default, but it is simply a number that can be changed, and it is really only used in in EIGRP calculations, nothing to do with QoS or the interface performance. Traffic can only be sent serially on a link. – Ron Maupin Jul 15 '19 at 21:54
  • much appreciated, thank you – user59792 Jul 15 '19 at 21:55

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