3750 Inter-vlan switching (joining vlans)

Answered Question

Scenario:


3rd Party Carrier Switch 3750 - bringing back 5 differnet Vlans (layer2. ex 201,202,203,etc ) to the differnet locations of the same customer.


What i need to do is join all vlans together on a unique seperate layer2 vlan (ex.901)without routing?


I have the option of physically cross-patching the ports together but i didn't want to eat up 10 ports in the 3rd Party Carrier switch.


Is this possible? Any ideas would be appreciated. I look into several things like bridge-groups but couldn't find a clear example.


Thanks

Correct Answer by Joseph W. Doherty about 8 years 7 months ago

Like Rick, can't see an easy way to accomplish what you desire if all 5 VLANs are all tagged and crossing a trunk.


If you have control over the far side, if you can send one of the VLANs native (i.e. untagged), you might convince the receiving switch that two VLANs are the same VLAN; saving one cross patch.


Unsure whether a router will really assist assuming you really want to L2 bridge all 5 VLANs.


If you want to avoid using your 3750 ports, you might consider using the least expensive pure L2 switches to bridge the traffic together.

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Richard Burts Mon, 11/17/2008 - 07:52
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Brent


If you have incoming 5 separate VLANs (201, 202, 203, etc) and you want to join them into VLAN 901 I am not aware of a way to do that without routing.


HTH


Rick

Correct Answer
Joseph W. Doherty Mon, 11/17/2008 - 10:07
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Like Rick, can't see an easy way to accomplish what you desire if all 5 VLANs are all tagged and crossing a trunk.


If you have control over the far side, if you can send one of the VLANs native (i.e. untagged), you might convince the receiving switch that two VLANs are the same VLAN; saving one cross patch.


Unsure whether a router will really assist assuming you really want to L2 bridge all 5 VLANs.


If you want to avoid using your 3750 ports, you might consider using the least expensive pure L2 switches to bridge the traffic together.

Giuseppe Larosa Mon, 11/17/2008 - 10:34
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Hello Brent,

it is not possible but close to be.


I did it on a C3550 4 years ago

the feature to be used is called fallback bridging.


But unfortunately it would work only for non IPv4 packets


see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_46_se/configuration/guide/swfallbk.html


This chapter describes how to configure fallback bridging (VLAN bridging) on the Catalyst 3750 switch. With fallback bridging, you can forward >>non-IP packets<< that the switch does not route between VLAN bridge domains and routed ports.


To use this feature, the stack master must be running the IP services image (formerly known as the enhanced multilayer image [EMI]). Unless otherwise noted, the term switch refers to a standalone switch and to a switch stack.


A c6500 would be able to do this also with IPv4 packets but not this kind of device.


In my tests I used IPv6 packets generated with a traffic generator so for me it was enough.


Hope to help

Giuseppe


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