4948 configuration for sub interfaces

Unanswered Question
Dec 19th, 2008


I have 4948 switch and my service provider provides my internet bandwidth, voice ,connectivity to different branch offices and live ips for FTP, WEB etc on dot1q tunnels.They provide different services on different vlans and told me to configure my interface as sub interface. I think 4948 doesnot support sub interfaces.I have created dot1 trunk on port which is connected to service provider.How can i do this configuration on my 4948 switch. Do'not know what to do next.


I have this problem too.
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Jon Marshall Fri, 12/19/2008 - 01:14


I answered this in your previous post. Just use Layer 3 SVI's on switch eg.

vlan 10 =

vlan 11 =

vlan 12 =

on the 4948

int vlan 10

ip address

no shut

int vlan 11

ip address

no shut

int vlan 12

ip address

no shut


itdsmartnet Fri, 12/19/2008 - 01:25

hi Jon

i have the following queries.

1) i have pool of 14 real ips for services such as FTP, MAIL NMS etc what ip should i assign to my SVI interface.

2) For WAN interface my service provider assign my ip with /30 mask for that vlan what ip should i assign for my SVI interface.

3) i have created a trunk port with dot1 q tunneling protocol. how can i route all my traffic to my trunk port. because my service provider port is connected to this port.

4) for all vlans my default gateway will be ip of SVI interface from there how my traffic route to the trunk port.


Jon Marshall Fri, 12/19/2008 - 01:38


1) You can't do NAT on a 4948 switch so this will have to be just another L3 SVI using the real IP addresses. Use the same IP address you were going to use on the subinterface.

2) Again use the same address that you have used on the corresponding subinterface.

3) Is it dot1q tunneling or it is just dot1q. My understanding from your original post was that this was just a trunk link carrying multiple vlans ?

Perhaps i am not understanding your setup correctly.

4) Again this may be to do with me not fully understanding your setup.

My understanding of you setup was that the provider was connecting to you with a dot1q trunk and they were sending multiple vlans down this trunk. If that is correct there is functionally no real difference between using a L3 SVI (ie. interface vlan) or subinterfaces to route between your vlans so whatever IP address etc. you were going to use on subinterfaces you just use on vlna interfaces.


itdsmartnet Fri, 12/19/2008 - 01:46

hi Jon

it is dot1q trunk port.

what ip should i use on subinterfaces e.g WAN side ip address assigned by service provider is what ip should be of that SVI.

Routeable ip assigned by my service provider for FTP and MAIL is 11.11.11.x/28 what ip should i assign to my SVI.

how my 11.11.11.x ip route to the internet.

what is the default gateway.


Jon Marshall Fri, 12/19/2008 - 01:57


These are questions your SP can answer. If your service provider told you to create subinterfaces to route between vlans then ask them what addressing to use because all you then do is instead of using the address on a subinterface you use it on a L3 SVI on the switch instead.

Or has the service provider not given you detailed instructions.

Where i am having a problem answering is that you say the SP has provided you with an 802.1q trunk connection. So whether you route between subinterfaces or L3 SVI's, it makes no difference.



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