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TOPIC: Traffic Flow between Trunk and Access Links

Traffic Flow between Trunk and Access Links 10 years 7 months ago #13119

  • drizzle
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Scenario:
1 x 24 port gigabit layer two switch
1 x 2621 with 1 x 100 megabit ethernet port

Port 1/1 on the switch is set up as a trunk. Port 1/2 is on vlan2 and port 1/3 is on vlan3. Now, both computers hooked up to port 1/2 and 1/3 have gigabit nics and are capable of communicating with the switch at gigabit speeds. For computer A to talk to computer B, the 2621 is required to route the traffic since they are on different vlans (and networks).

My question is this, in the above scenario, are the packets from computer A actually sent to the switch, out through the trunk link so the router can do its job and then back through the same trunk link, back to the switch and finally to computer B?

And if that is indeed how the data is sent, then the maximum throughput between the two VLAN's is 100 mbps.

And if that is the case, then I guess it would be stupid to even contemplate setting this up until I get a layer 3 switch or router that can handle decent throughput.
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Re: Traffic Flow between Trunk and Access Links 10 years 7 months ago #13132

  • havohej
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Hi.

the scenario is called a router on a stick.

of course the throughput reduces when you route traffic between vlans, becasue the routing device only suports 100 mbps, and all other ports if connected to giga nic, wokr in giga speed.

the path of traffic sent from device A in vlan 1 to device B in vlan 2 exits the giga nic A enters the gigabitport that belong to vlan 1 in the switch, then it goes to the router tagged as 8021q frame, the router performs the routing lookup via software looks a match and then routes to the appropiate subinterface that is of course attached to the same physical interface, the frame returns again to the switch, and by the cam entrys in the switch it goes to the appropiate vlan 2 port.

almost all switches have autosensing ports by default, and them perform assymetric switching, using some space of the memory for buffering when there is a flow between different port speeds.
It does it by sending a low speed device traffic originated by a high speed device, and for not overflowing the queues of the low speed device, because it can not process the frames as fast as the high speed device is sending it.

I have the same topology you mention, in a big lan consisting of about 700 nodes, and routing the vlans between them.

Of course the router I am using is a superior than the 2600, and connecting it to the switch, I have some multimeadia apllications and large graphic file traffic between vlans, but It doesnt affect the throughput.

You can also use a multilayer switch but it is a more expensive device, but if youy budget can, try it, when doing intervlan routing in the multilayer switch it performs really faster than with an external router using CEF technology as it uses some tables for routing at the wire speed in hardware, and not doing in software like the external router does.

and the intervlan routing travels within the backplane of the switch, not traversing an external wire.
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Re: Traffic Flow between Trunk and Access Links 10 years 7 months ago #13141

  • drizzle
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That confirms what I was thinking. Thanks for the info. I am actually working on a 6509 right now. Its a big step up from what I am used to. 112 Gbit ports and 2 10 Gbit... all layer 3 capable.
It is a great learning experience though.
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