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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 an 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
As shown in Figure 96, in MPLS access mode, the edge domain is an MPLS network. The UPE
creates a U-PW with NPE 1. The UPE does not create PWs to any remote PEs. After the UPE
receives a packet from a CE, it adds the label assigned to the U-PW into the packet and then
forwards the packet to NPE 1 through a public tunnel. NPE 1 maps the packet to the VSI that
corresponds to the PW label, and searches the MAC address table of the VSI to forward the packet.
U-PW created on the NPE must have split horizon disabled because the NPE needs to forward
packets between U-PW and N-PW.
CE 3
NPE 3NPE 1
N-PW
CE 1
CE 2
NPE 2
UPE
U-PW
N-PWN-PW
Backbone
domain
Edge domain
(MPLS)
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