- By default BPDU is blocked on a VPLS instance. L2 Loop is avoided by split horizon on PE. Split horizons does not allow any packets that arrived from a pseudo wire to be forwarded to another pseudo wire. Split horizon is on by default on Cisco IOS. Split horizon required fully meshed PEs that are on a VPLS instance.
- Another way to think about Split Horizon is it disables transitive relationship among PE. That means PE1 and connection to PE2, and PE2 is connected to PE3, however it DOES NOT mean PE1 and connected to PE3.
- Because Split-Horizon avoidance loop, we do not need to run STP on VPLS core.
- In cases where fully meshed PEs cannot be deployed, and you were forced to disable Split Horizon, BPDU need to be transport across the VPLS instance so that STP can detect and avoid loop
- On H-VPLS, where VPLS is used the access layer, N-PE needs to have split-horizon turn off because N-PE will receive packets from pseudo wire from the access VPLS and forward the packets to another pseudo wire on the core VPLS.