switchport mode multi

Unanswered Question
May 23rd, 2007

Hi,


What happen if I connect one switch port configured with "switchport mode multi" to another switch? broadcast strom? like spanning-tree loop? Can you give me some comment?


Best Regards,


Jackson Ku

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bjornarsb Wed, 05/23/2007 - 06:04

Hi,


mode multi ??, what type of switch is this ?


Regards,

Bjornarsb

mheusinger Wed, 05/23/2007 - 06:15

Hi,


afaik this was a "special" feature on 2900XL and 3500XL switches to connect routers and servers, but NOT switches or Hubs. Have a look at "Overlapping VLANs and Multi-VLAN Ports"

http://www.cisco.com/en/US/docs/switches/lan/catalyst2900xl_3500xl/release12.0_5_wc5/swg/swvlan.html#wp1028684


You should not connect such a port to another switch. Basically per-VLAN spanning tree will not work properly and loops could result (assuming redundant topology).


Hope this helps!


Regards, Martin

jackson.ku Wed, 05/23/2007 - 07:19

Hi,


Can you explain "Basically per-VLAN spanning tree will not work properly and loops could result"? I do not clearly understand it.


Best Regards,


Jackson Ku

ankbhasi Wed, 05/23/2007 - 08:00

Hi Jack,


A multi-VLAN port functions normally in all its VLANs. For example, when an unknown MAC address is received on a multi-VLAN port, it is learned by all the port in those VLANs.


Multi-VLAN ports also respond to the STP messages generated by different instances of STP in each VLAN. Because the multi-VLAN port is a member of more than one VLAN, flooded traffic received from the multi-VLAN port is forwarded to ports in all VLANs assigned to the multi-VLAN port.


So in short there can be multiple unpredictable results so I will recommend you not to connect it to switches or hub.


HTH


Ankur


*Pls rate all helpfull post

ankbhasi Wed, 05/23/2007 - 06:26

Hi Jack,


Cisco will never recommend you to connect multi mode switchport to any other switches or hubs, it is recommended to connect these switch to routers or servers only.


It may result in broadcast stroms when loops are created and may result in loss of connectivity also.


Have a look at this link and there is a caution mentioned for multi vlan switchport usage


http://www.cisco.com/en/US/docs/switches/lan/catalyst2900xl_3500xl/release12.0_5_wc5/cli/clicmds.html#wp1057945


HTH


Ankur


*Pls rate all helpfull post

Wilson Samuel Wed, 05/23/2007 - 06:33

I practically ran into a big issue when I used mode multi.


Infact it was used on 2900 Series Switches long back whenever we wanted a port to support multiple VLANs and ofcourse in those days the L-3 Switches were quite rare and expensive.


These days I guess with 3560s around we can easily do the same stuff with L-3 routing.


Kind Regards,

Wilson Samuel

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