Posts Tagged ‘mac’


A key to the mastery of a CCENT-level of networking knowledge is to intimately understand the use of Layer 2 and Layer 3 addressing when two hosts communicate on the network.

This blog post will detail how these addresses are used during the network communications between two host devices (Personal Computers, PCs). Here is the topology that will be used in this example:
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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
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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