In traditional physically looped L2 switched networks, the redundancy is removed by the STP algorithm. There we use the PVST or MST to tune that some link are not blocked for a specified VLAN. In a long run this will be a huge administrative burden.
To achieve forwarding over all links in the core, is to use some protocol which has the knowledge of the whole topology and the active links. Examples would be Fabric Path (FP) from Cisco, or the open standard Transparent Interconnection of Lots of Links (TRILL). We could also use the old good MPLS also to achieve this; specifically we will be encapsulating L2 Ethernet frames into MPLS
In the below topology we have H1 and H2 belonging to VLAN 2, which are supposed to communicate over the EoMPLS backbone. All of the packets send from VLAN 2 will be encapsulated inside the MPLS payload. At the end we have 2 logical wires or Pseudo-wire between the 2 switches.
For MPLS to work at all we have to take the following points into account:
- Ensure L3 reachability via static routing or dynamic routing.
- Enable MPLS on the interfaces.
- Loopback interfaces on routers to specify the MPLS Label Distribution Protocol (LDP) router-id.
- CEF switching must be enabled for MPLS to work (default enabled).