374
Multicast and broadcast traffic forwarding and flooding
After a PE receives a multicast or broadcast packet from an AC, the PE floods the packet to all other ACs
and the PWs in the VSI bound to the AC.
After a PE receives a multicast or broadcast packet from a PW, the PE floods the packet to all ACs in the
VSI bound to the PW.
PW full mesh and split horizon
A loop prevention protocol such as STP is required in a Layer 2 network to avoid loops. However,
deploying a loop prevention protocol on PEs brings management and maintenance difficulties. Therefore,
VPLS uses the following methods to prevent loops:
•
Full mesh
—Every two PEs in a VPLS instance must establish a PW to create a full mesh of PWs
among PEs in the VPLS instance.
•
Split horizon
—A PE does not forward packets received from a PW to any other PW in the same VSI
but only forwards those packets to ACs.
H-VPLS
VPLS requires a full mesh of PWs among all PEs in a VPLS instance. In a large-scale network, however, a
full mesh of PWs causes very high PW signaling overhead and creates difficulties for network
management and expansion. Hierarchical VPLS (H-VPLS) reduces the number of PWs by dividing a VPLS
network into a backbone domain and edge domains.
Only static PWs and LDP PWs support H-VPLS.
In H-VPLS:
•
An edge domain provides access for a user network to the backbone domain.
•
The Network Provider Edge (NPE) devices are fully meshed in the backbone domain. A PW
between NPEs is referred to as an N-PW.
•
A User facing-Provider Edge (UPE) device only establishes a PW with the neighboring NPE. A PW
between a UPE and an NPE is referred to as a U-PW.
H-VPLS access modes
Two access modes are available in H-VPLS: MPLS access and Ethernet access.
Figure 96
H-VPLS using MPLS access
