subinterfaces on router

Unanswered Question
Aug 11th, 2007

Hi all,

It may be a simple question to all, but I want know that we should create first sub interfaces on router for vlan's am I right? Those sub interfaces we can use for vlans in switch right? after that we should encapsulate and assign IP for sub int. but the thing is how we can create sub interfaces on router like for (f0/0)what are those commands and steps.

Please any body can expalin clearly it would be more appreciate.



I have this problem too.
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Jon Marshall Sat, 08/11/2007 - 09:00

Hi Naidu

int fa0/0.10

ip address

encapsulation dot1q 10 <--- this must match vlan number

no shut

int fa0/0.20

ip address

encapsulaton dot1q 20

no shut


You need to make the port on the switch that the router connects to a trunk with 802.1q encapsulation.



ilnaiduccna Sun, 08/12/2007 - 05:47

Hello Jon,

Thank you 4 ur early reply,

1. As you told we can create like f0/0.10, f0/0.20 like how many we can create in one router and what is the difference between dot1q & 802.1q

2. In my company 2950 Switches connects to 2811 Routers f0/0, In the router 5 vlans has been created in this scenario up to how many subinterfaces I can create & the IP address for the subint should be in Class C? like like that.



glen.grant Sun, 08/12/2007 - 08:26

I don't think there is a specified limit you just have to use common sense ,when you trunk everything that needs to be routed travels up and down that same link so traffic utlization has to be figured in . Also a router is not a switch so its thruput will not be as high as a L3 switch would be .

moses12315 Sun, 08/12/2007 - 09:41

Do not forget to remove the ip address from the interface (etc f0/0) . I had that problem recently believe me and i spent many hours until i solve it .


Richard Burts Sun, 08/12/2007 - 15:12


There is in fact a limit on how many subinterfaces you can create (actually the limit applies on the total number of interfaces - physical, and subinterfaces, and logical (like loopbacks)). The limit is the number of IDBs (Interface Descriptor Block) supported on your platform and version of code. The number of IDBs is typically in the hundreds and up. So I do not believe that this will be much of a problem for you. If you want to know what your limit is, there is a show command that you can use to get the particular number for your platform and code version.




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