2851 Router Int config

Unanswered Question
May 15th, 2009
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I inherited a nework that has a 2851 router with the following config:

interface GigabitEthernet1/0

ip address

ip virtual-reassembly


interface GigabitEthernet1/0.1

encapsulation dot1Q 1 native

ip address


interface GigabitEthernet1/0.25

encapsulation dot1Q 25

ip address

I am not clear on why the physical int is configured with an IP address since there is two logical interfaces configured under it.

Also, this is a L2/L3 router and I am not sure how to access the L2 switch part of it.

Any help would be greatly apprecaited.

Thanks in advance,


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Giuseppe Larosa Sun, 05/17/2009 - 11:31
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Hello Sadik,

you are right the IP config under the main interface should be moved to a new subinterface.

You can face problems because vlan1 (as a L2 broadcast domain ) is associated to two different L3 interfaces.

But to understand what to move you need to get a complete picture of the network.

A second ip subnet can be mapped to a broadcast domain using secondary ip address

ip address x.x.x.x

ip address y.y.y.y secondary

About the switch module:

if you post

sh ver

sh inventory

you can get help on this: depending on the type of modules they have a separate telnet session.

if with sh ip int br

you see multiple FE ports you can manage them from the router otherwise you can telnet to it (this could explain why two ip subnets are with vlan1 ..)

Hope to help



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