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L3 Switches and Router on a stick

Posted on 2014-07-28
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Last Modified: 2014-08-04
If I understand L3 switches when configured with SVIs , improves the performance compared to the old method of using Router On Stick.

in the snapshot below, taken from Cisco website, PCs are connected to L2 Switches, assuming L2 Switches are configured with Separate Vlans, then L2 Switches  are trunked to L3 Switch.
 
In this case if foe instance a PC in Vlan 20 wants to talk to another PC in Vlan 30, the traffic will still have to go through the Trunk Link between L2 and L3 Switches , then it gets routed from Vlan 20 to Vlan 30 in L3 Switch, then will come back through the Trunk to L2 Switch Vlan 20 then to the PC.
I wonder if there is any better performance using L3 switch in this case than using Router on a Stick .

To my understanding, The only case where L3 is better in performance than the router on a stick, is when PCs are directly connected to L3 switch and Vlans are directly configured on that L3 Switch as well as the SVIs with IP Routing command.

I could be wrong, but please shade some light on this.

Thanks
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Question by:jskfan
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by:Don Johnston
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L3 switches are significantly faster routing traffic than a comparable router.  But in your example, the L2 to L3 trunk could still be a point of contention.
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by:jskfan
ID: 40224791
<<But in your example, the L2 to L3 trunk could still be a point of contention. >>>
Which topology do you suggest so that there will not be traffic congestion ?
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by:Don Johnston
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You would have to connect the hosts to an L3 switch. But that's isn't necessarily something you would have to do.  It would depend on the traffic flow.
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Author Comment

by:jskfan
ID: 40225542
I believe most of the medium to large environment plugs user PCs to L2 switches and even Trunk L2 switches to other L2 switches, then use L3 switches as backbone switches , that 's where they configure SVIs and routing between Vlans.
I could be wrong that's only my assumption...but if that is the case I do not think PCs get a better response time than if they were directly plugged to L3 Switches and routing is enabled right there on the L3 Switch.

**Second thought, I guess you still can plug PCs to L3 Switches, and configure a domain so that you will have the same Vlans  on other switches , and if one L3 switch does not have enough ports for PCs, other L3 switches still can be used....
example L3 Switch1 can have 30 ports in Vlan20 and 16 ports in Vlan40, and L3 switch2 can have 15 ports in Vlan40 (I said Vlan40 because Switch1 does not have enough ports left for PCs to put in Vlan40)...
But the question that will come up is, whether we can configure Interface Vlan40 on both Switch1 and Switch2 and assign it the same IP address....

Thanks
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by:Don Johnston
ID: 40226485
I believe most of the medium to large environment plugs user PCs to L2 switches and even Trunk L2 switches to other L2 switches, then use L3 switches as backbone switches , that 's where they configure SVIs and routing between Vlans.
This is correct.
I could be wrong that's only my assumption...but if that is the case I do not think PCs get a better response time than if they were directly plugged to L3 Switches and routing is enabled right there on the L3 Switch.
This is correct also.
But the question that will come up is, whether we can configure Interface Vlan40 on both Switch1 and Switch2 and assign it the same IP address....
No. You can't do that.  This is a situation where a FHRP such as HSRP, VRRP or GLBP would be used.
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by:jskfan
ID: 40227699
if you have no available ports in Switch1 for PCs to put in VLAN40, then you cannot create Vlan40 on Switch2 that has more empty ports ???

I though VTP is to have the same VLANs in multiple switches ?
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by:Don Johnston
ID: 40227729
if you have no available ports in Switch1 for PCs to put in VLAN40, then you cannot create Vlan40 on Switch2 that has more empty ports ???
I guess that I'm missing something. I don't understand the question.

I though VTP is to have the same VLANs in multiple switches ?
That is correct.
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Author Comment

by:jskfan
ID: 40227770
The question was about using L3 Switches to plug user PCs to.
Then if an L3 Switch has 24 ports, and just Vlan40 by itself has 60 PCs to plug to...
You may need about 3 L3 switches for to get 60 PCs to plug to Vlan40....
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Don Johnston earned 500 total points
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Correct. Or you could use a switch with more ports or you could use L2 switches.
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Author Closing Comment

by:jskfan
ID: 40239425
Thank you
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