hwics and vlans

Answered Question
Aug 24th, 2010
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   I have a 3845 with a 4 port hwic  installed with 2 vlans running to a switch downstream .   They are adding a new "gig" hwic to this 3845 and adding a new Cat 4500 below this .  Will I be able to trunk the existing vlans to the new switch or are those restricted to that 4 port fast ether hwic ?  I'm guessing no you can;t do that because its not part of that little 4 port switch .  Under a tight schedule thats why I am asking here  versus trying to set it up in the lab.

Correct Answer by Edison Ortiz about 6 years 10 months ago

Glen, thanks for providing the part number and you are correct. This particular HWIC will behave like a regular routed port that comes with the router. You need to look into the HWIC EtherSwitch family for trunking support without using subinterfaces.


If you look at this list, you can see its category

http://www.cisco.com/en/US/products/ps5855/products_relevant_interfaces_and_modules.html


Regards,


Edison

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Edison Ortiz Tue, 08/24/2010 - 06:11
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Yes, you can trunk from the 3845 HWIC to the 4500.


Regards,


Edison

glen.grant Tue, 08/24/2010 - 06:19
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  Hi Edison ,  so I should be able trunk those existing vlans that were used on the the 4 port hwic  on the new gig hwic ?   You would then just set it up as a regular trunk port on the 3845 and not use  any subinterfaces correct ?     Just had a concern that you could not do this because  on other  hwic switchcards you have to stack them together to be able to use the same vlans .

Edison Ortiz Tue, 08/24/2010 - 06:23
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Hi Glen,


If you can provide the HWIC model in question I can be more certain of its capabilities but generic HWIC modules on 3800 ISR routers support trunking the same way a regular switch would.


There is a new generation of HWICs where they behave like a 3750 switch. You need to configure everything separately from the 3800 CLI. They are two separate entities. With that said, even that generation of HWICs support trunking - you just need to configure it directly onto the device instead of the 3800 CLI.


Regards,


Edison

glen.grant Tue, 08/24/2010 - 06:31
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Hi Edison , the module is a  HWIC-1GE-SFP  .    I guess my concern with it being a

single interface it would not be able to access the vlan database like t

he 4 port hwic does  and the only way to trunk down to the new switch would be to use subinterfaces  and in which case you would have an issue where you a have a layer 3 SVI defined for the switch on the 4 port fast ether and then you could not do the same scenario for the gig hwic.

If the gig hwic can access that vlan database  and

trunk like a normal switch does it will probably be ok .  Yeah I know about those bigger 3750 type add in cards and is one of the reasons i had some concerns because I know you have to "stack" them together if you want to use them as one switch  and you don't have that option with a single interface gig hwic .


https://supportforums.cisco.com/thread/147407

Update:   I found another thread on this card and the user was trying the same thing. It appears that this card is strictly a routable port card and does not even take any switchport commands and thus you would have to use subinterfaces to trunk anything and would basically hose what we are trying to do , can you verify Edison ?   PS   I didn't design this    :-)

Correct Answer
Edison Ortiz Tue, 08/24/2010 - 07:48
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Glen, thanks for providing the part number and you are correct. This particular HWIC will behave like a regular routed port that comes with the router. You need to look into the HWIC EtherSwitch family for trunking support without using subinterfaces.


If you look at this list, you can see its category

http://www.cisco.com/en/US/products/ps5855/products_relevant_interfaces_and_modules.html


Regards,


Edison

glen.grant Tue, 08/24/2010 - 07:51
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  Thanks Edison that's what i was afraid of .  Better to find out ahead of time though, thanks again.

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