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configuring ip routing on a lan subinterface is only allowed if that subinterface is already configured as part of an

Posted on 2003-03-30
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Last Modified: 2012-06-21
I have a Cisco 1720 router that I am trying to configure for ISL VLAN routing.  I have a Cisco 3524xl switch that has been configured for the port of the router in switchport mode trunk.

However when I try to use a sub-interface on the 1 fast ethernet 0 port and enable isl encapsulation it gives me the error in the subject line.

it will let me add a secondary address but that does not do much good on creating sub interfaces to simulate two.

Any ideas?

This is ios 12.13b ip\fw\ids plus 3des

Thanks,

Troy
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Question by:tdampier
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by:pedrow
ID: 8236589
did you create the vlan on the switch yet? The trunk configured?

once the vlans are created on the switch, the eth interface configs should look something like this:

!
interface FastEthernet0/0
 description FE trunk to switch
 no ip address
 duplex auto
 speed auto
!
interface FastEthernet0/0.2
 description Management VLAN
 encapsulation isl 2
 ip address 172.16.1.1 255.255.255.0
 no ip redirects
!
interface FastEthernet0/0.3
 description Management VLAN
 encapsulation isl 3
 ip address 172.16.2.1 255.255.255.0
 no ip redirects
!

note that the FE trunk has no ip address...only the subifs.
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Author Comment

by:tdampier
ID: 8236612
that is the configuration I have tried.  Unfortunately, it will not allow me to type in the encapsulation isl ?vlan number command it is not an option.  It shows it then will not execute the command.
 
What does the switch configuration look like?  

I have VTP enabled v2 and I have added VLAN 2 and VLAN 3 but where do I place the VLAN 1 ip address on the example above?

Thanks for the quick reply.
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Expert Comment

by:pedrow
ID: 8236690
What kind of switch?

if it's one of the IOS based kind the port config will look something like:
!
interface FastEthernet0/1
 switchport mode trunk
!

the VLAN1 question, i'll answer like this:

I assume you want the vlan1 for the management interface of the switch...Why don't you assign a different vlan number and  use that for your switch management interface. Why you ask? because vlan1 is the default vlan. i.e. if someone mistakenly forgets to set the port's vlan properly, they will by default  be on your switch's management interface subnet, which might be a security issue.

So, that being said, let me call VLAN2 your management LAN :)

feel free to substitute vlan2 for vlan1 if that suits your needs better.

So, the router's vlan2 interface will still look like this:
!
interface FastEthernet0/0.2
description Management VLAN
encapsulation isl 2
ip address 172.16.1.1 255.255.255.0
no ip redirects
!

on the switch:

interface VLAN2
 description Switch's management interface
 ip address 172.16.1.2 255.255.255.0
 no ip directed-broadcast
 no ip route-cache

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Expert Comment

by:pedrow
ID: 8236692
sorry...you *said* what kind of switch...nm ;)
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epylko earned 200 total points
ID: 8242840
What version of IOS are you running on the 1720?  I just checked the lastest 12.2T train of IP/Plus and it doesn't offer the ability to subinterface the Fast Ethernet port.

This document: http://www.cisco.com/en/US/partner/tech/tk389/tk390/technologies_configuration_example09186a00800949fd.shtml looks like it was updated recently and doesn't list the 1720 as a device capable of ISL or dot1Q trunking.

So, I think you're out of luck...

-Eric
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Author Comment

by:tdampier
ID: 8242933
I am not using 12.2.T I am using 12.2.16.  I gave up on the other OS because I wanted the other feature enhancements.

What feature set are you looking for to create sub-interfaces?

thanks,

Troy
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Author Comment

by:tdampier
ID: 8242966
However, you are correct it does require a 2600 or above to for the features set IEEE 802.1Q Trunking :)  So you gave me my answer.  Good thing I ordered that second interface :)

Thanks,

Troy
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Author Comment

by:tdampier
ID: 8242972
Should have thought of this myself :)
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Expert Comment

by:pedrow
ID: 8243493

I'd say that the 1700 probably does support subinterfaces, just not isl trunking. The following link looks like 802.1q might be your best bet and the 3500 supports it.

http://www.cisco.com/en/US/products/hw/routers/ps221/products_configuration_guide_chapter09186a008007cd23.html

hope this helps.
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