Gigabit Interface Subinterfaces

Answered Question
Dec 16th, 2009

When is there a performance hit when you subinterface a gigabit ethernet port?

We have a remote site with approxy 100-150 nodes. There is no layer 3 switch currently with which to implement VLANs. How bad of an idea is it to subinterface the router's inside interface to trunk down to the switch? There is a large amount of traffic traversing this router. While some traffic stays local for file storage and a few apps, a lot goes back to the hub site then out to the Internet. I was worried that the "hairpinning" might cause a performance hit. Currently there is no UC, but in the future there may be.

Thanks.

I have this problem too.
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Correct Answer by Jon Marshall about 6 years 12 months ago

b.gamble wrote:

Thanks for the responses. Do either of you guys have any documentation to support?

I missed the 100-150 users but Paolo makes a very valid point. The 2821 supports 87Mb CEF throughput so if you have a large amount of traffic traversing the router then it will not be the subinterfaces so much as the actual capability of the router to forward packets.

As Paolo says, you would be much better off with a L3 switch such as the 3560/3750 which has much greater throughput and use the 2821 for WAN connectivity only.

Jon

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b.gamble Wed, 12/16/2009 - 06:40

Router is currently a 2821. It might be a 3945 in the future. How much does processor speed and platform come into play when subinterfacing?

The switch in use is a 2960.

Jon Marshall Wed, 12/16/2009 - 07:51

p.bevilacqua wrote:

With 100-150 users, you must use a layer 3 switch for inter-vlan routing.

Paolo

Recommending a switch and not a router - never thought i'd see the day

Jon

Paolo Bevilacqua Wed, 12/16/2009 - 13:33

No, why ?

A layer 3 switch is a bad idea only when one expect it to do things it's not designed for.

Otherwise, a necessity.

Jon Marshall Wed, 12/16/2009 - 13:41

p.bevilacqua wrote:

No, why ?

A layer 3 switch is a bad idea only when one expect it to do things it's not designed for.

Otherwise, a necessity.

Paolo

I was just joking, after some of our conversations in previous threads

I think you were spot on with the advice.

Jon

Jon Marshall Wed, 12/16/2009 - 07:11

b.gamble wrote:

Router is currently a 2821. It might be a 3945 in the future. How much does processor speed and platform come into play when subinterfacing?

The switch in use is a 2960.

It is not so much processor speed rather the fact that you are limiting the amount of bandwidth per vlan because they are subinterfaced. If this is not an issue for you then it should not be a problem. The throughput of the router should be the same whether it is moving packets between subinterfaces or between 2 physical interfaces on the same router.

Jon

b.gamble Wed, 12/16/2009 - 07:29

Thanks for the responses. Do either of you guys have any documentation to support?

Correct Answer
Jon Marshall Wed, 12/16/2009 - 07:50

b.gamble wrote:

Thanks for the responses. Do either of you guys have any documentation to support?

I missed the 100-150 users but Paolo makes a very valid point. The 2821 supports 87Mb CEF throughput so if you have a large amount of traffic traversing the router then it will not be the subinterfaces so much as the actual capability of the router to forward packets.

As Paolo says, you would be much better off with a L3 switch such as the 3560/3750 which has much greater throughput and use the 2821 for WAN connectivity only.

Jon

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