Within the scope of Metro Ethernet services, it is often beneficial to provide customers “point-to-point” VLAN service, where VLAN (multipoint service in essence) is effectively set up to emulate ethernet “pseudowire”, by disabling MAC-address learning. The benefit comes from saving metro switches CAM tables address space, thus improving overall scalability (which is far from perfect with Ethernet). There is special command, mac address-table learning available on Cisco Metro swtiches (e.g. ME 3400) which allows to disable MAC-address learning per specific VLAN. However, many commonly used switches does not have this feature implemented. Still, there is a way to disable MAC-address learning on a group of ports, by using RSPAN VLAN feature. By it’s functional design, RSPAN VLAN does not learn MAC addresses. However, we are not allowed to assign this type of VLAN directy to switch access ports. Still, we may overcome this issue by configuring switchports as trunk with a single allowed VLAN (RSPAN VLAN) which is also configured as native:
vtp mode transparent ! vlan 555 remote-span ! interface range Fa 0/1 - 3 switchport trunk encapsulation dot1q switchport mode trunk switchport trunk allowed vlan 555 switchport trunk native vlan 555
This configuration is applicable to any switch that supports RSPAN functionality. Specifically, it was verified on Catalyst 3550 series.
It’s a nice idea!
Wouldn’t that cause constant flooding?
to: dknov
Well, just as with any “classic” hub, all ports get flooded with frames received on any other ports in the same VLAN. Nice to have, if you like to know what your peers are doing
But seriously, the only real benefit is getting rid of mac-address learning and CAM table blowup. This kind of service applies perfectly to “port-to-port” services, where mac-address learning is unnecessary.
what about the cisco 3500XL layer 2 switch? it is the same configuration i need to perform? I really appreciate if you can answer this for me. thanks.
To: JOHN
This method would work with any switch supporting RSPAN VLANs. AFAIK 3500XL/2900XL do not support RSPAN functionality, so you won’t be able to use this trick there.