Feb
05

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.

Petr Lapukhov, 4xCCIE/CCDE
About Petr Lapukhov, 4xCCIE/CCDE

Petr Lapukhov has more than 12 years of experience working with Cisco Systems products. Not only is he the only person in the world to have earned four CCIEs (Routing & Switching, Security, Service Provider, and Voice) in just two years, he also passed every exam the first time. He shares his knowledge and experience with INE’s students through our various products and programs. Petr works with all of the technologies covered within his four CCIE tracks on a daily basis, staying current with any changes in the industry. He has also received his Cisco Certified Design Expert (CCDE) certification, joining a small group of distinguished individuals who have achieved this status. Petr is a contributor to INE’s blog and our INE IEOC Community Forum. You may contact Petr Lapukhov at petr@ine.com.

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