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    Turning Switch into Hub
    05 February 08

    Turning Switch into Hub

    Posted byPetr Lapukhov
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    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.

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