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Cisco Catalyst VLAN Trunkinq Question

Posted on 2007-03-29
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Last Modified: 2010-04-12
Here is a slice of config from a 2950XL

!
interface FastEthernet0/1
switchport trunk encapsulation dot1q
 switchport mode trunk
!

OK I know what thats doing - but! If I try to do the same on a 2950T or 2950G then "switchport trunk encapsulation dot1q" is unrecognised

Fair enough - but if I want to replace a 2950XL with one of these other switches is it going to work?
The reason Im asking is,

There is a PIX firewall in Fe0/1 of the switch configured thus.....................

interface Ethernet2
 no nameif
 no security-level
 no ip address
!
interface Ethernet2.2
 vlan 2
 nameif dmz1
 security-level 50
 ip address 172.31.5.1 255.255.255.0
!
interface Ethernet2.3
 vlan 3
 nameif dmz2
 security-level 55
 ip address 172.31.4.1 255.255.255.0


So will one othe the other switches work?
what commands need to be input for FE0/1 to trunk traffic from other VLANS on THIS switch to the ethernet card of the PIX?
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Question by:Pete Long
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LVL 79

Expert Comment

by:lrmoore
ID: 18818143
The 2900XL supports both ISL and dot1q trunking.
The 2950 series supports only dot1q and therefore there is no option to specify which encapsulation
Yes, it will work if you simply specify mode trunk on the interface

 interface fast 0/1
  switchport mode trunk

Done.
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Author Comment

by:Pete Long
ID: 18822047
Great - I thought as much but I couldnt find any proof.

Is that because Cisco is turning its back on ISL ? (I seem to remember ISL is a Cisco standard)

on the 2950XL did you "HAVE TO" specify? or did it default to ISL? - was there any benefit to manually setting dot1q, or it that just a quirk of the old switch?

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Accepted Solution

by:
lrmoore earned 500 total points
ID: 18822666
Yes, Cisco has abandoned proprietary ISL in favor of the industry standard dot1q.
I can't say that I know what the default was on the old XL switches..but I found this tidbit in 2900XL documentation, which says ISL is the default:

"100BASE-T and Gigabit Ethernet trunks use Cisco Inter-Switch Link (ISL), the default protocol, or industry-standard IEEE 802.1Q to carry traffic for multiple VLANs over a single link."
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by:Pete Long
ID: 18823894
Sorted

Cheers Again - Im still playing with the ASA so Expect some more questions soon - Have a good Weekend m8
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