Ethernet-Tree (E-Tree) Support in Virtual Private LAN Service (VPLS)
Extracted elements (25)
A single T-VSI with a dual-VLAN interface was chosen over two VSIs (one per root/leaf VLAN) because the two-VSI model requires two VPLS instances and two sets of PWs per E-Tree service, which scales poorly in large MPLS/VPLS networks and requires the two VSIs to share MAC addresses.
The solution reuses the IEEE 802.1Q two-VLAN mechanism (one Root VLAN, one Leaf VLAN) rather than developing a new mechanism such as using PW control words or separate PWs to indicate root/leaf attributes. A new mechanism would introduce complexity for interworking with 802.1Q-based Ethernet domains.
A T-VSI PE can interwork with a traditional VPLS PE (VSI without E-Tree support) using Compatible Mode. In this mode, the legacy PE can only be attached with root nodes; a raw PW SHOULD be used, and the T-VSI PE strips the E-Tree VLAN on egress and restores the root VLAN on ingress.
Ethernet OAM frames for E-Tree SHALL undergo the same VLAN mapping as data traffic. Root VLAN SHOULD be applied to segment OAM frames to prevent them from being filtered by the leaf-filtering mechanism.
For S-VLAN tagged ports with many multiplexed customer VLANs, the PBB VPLS PE model (IEEE 802.1ah bridge with root/leaf B-VLAN or dual I-SID tagging) MAY be used, providing both VLAN scalability and MAC address scalability without further changes to PW processing.
A PE MUST send a Label Mapping message with an updated E-Tree sub-TLV to all other PEs when its role changes from leaf-only to not leaf-only (i.e., when a root node is added), ensuring peers can update their Optimized-Mode state.
A PE that wishes to support an E-Tree service MUST include an E-Tree sub-TLV in its PW Label Mapping message, providing its local root VLAN ID and leaf VLAN ID. A PE capable of VLAN mapping MUST set the V bit to 1.
For an untagged or VLAN-unaware AC port where the bridge module is a C-VLAN bridge, Ethernet frames received from root ACs MUST be tagged with a root C-VLAN. When the bridge module is an 802.1ad bridge, frames received from root ACs MUST be tagged with a root S-VLAN.
If a PE is in Compatible Mode for a PW (connecting to a legacy non-E-Tree VSI), it MUST remove the root or leaf VLAN from frames upon transmit, and add a VLAN tag with the local root VLAN value to received frames. A raw PW (type 0x0005) SHOULD be used in this mode.
If a PE is in Optimized Mode for a PW (peer PE has only leaf nodes), upon transmit the PE SHOULD drop any frame whose VLAN ID matches the local leaf VLAN ID, avoiding unnecessary leaf-to-leaf traffic over the PW.
If a PE is in VLAN Mapping Mode for a PW, it MUST map the VLAN in each frame: upon transmit, map local VLAN to the remote VLAN; upon receive, map the remote VLAN to the local VLAN before forwarding or dropping in the egress bridge module.
If both the local PE and its peer have the P bit set (both are leaf-only), the local PE MUST send a Label Release message with status code 'Leaf-to-Leaf PW released' and exit the signaling process, preventing a useless leaf-to-leaf PW from being established.
In tagged PW mode, two service-delimiting VLANs MUST be allocated in the VPLS domain for an E-Tree service — one Root VLAN and one Leaf VLAN.
PEs with both leaf and root nodes MUST support BGP E-Tree signaling and MUST attach an E-Tree Extended Community (with local Root and Leaf VLAN IDs) to BGP UPDATE messages if participating in an E-Tree service. If some PEs don't support VLAN mapping, global VLANs MUST be provisioned.
T-VSIs in a VPLS MUST be interconnected with a bidirectional Ethernet PW. The Ethernet PW SHOULD work in tagged mode (PW type 0x0004), in which case a VLAN tag MUST be carried in each frame on the PW to indicate whether the frame originated from a root or leaf.
When both peer PEs have the V bit set and VLAN IDs differ, the PE with the minimum IP address MUST set VLAN-Mapping-Mode to TRUE, establishing which peer performs the VLAN translation.
A Tree VSI (T-VSI) is a single VSI with E-Tree support that connects to a bridge module via a dual-VLAN virtual interface, carrying both root VLAN and leaf VLAN traffic on the same T-VSI. This enables a single VPLS instance and one set of PWs per E-Tree service, sharing a single FIB with shared VLAN learning.
E-Tree (Ethernet Tree) is a Rooted-Multipoint EVC service where frames from a root may be received by any other root or leaf, but frames from a leaf may be received by any root and must not be received by another leaf. The service may include multiple roots and multiple leaves.
The outermost VLAN in the resulting Ethernet header is used to indicate the E-Tree attribute of a frame: a Root VLAN ID marks frames originating from root ACs, and a Leaf VLAN ID marks frames from leaf ACs. These VLANs may be C-VLAN, S-VLAN, or B-VLAN depending on the bridge type in use.
VLAN mapping may be performed globally (two VLANs shared across the VPLS domain, no per-PW mapping needed) or locally (two VLANs provisioned independently per PE, requiring VLAN mapping on the PW). The local method is more scalable in VLAN use but requires mapping negotiation via signaling.
IANA allocated BGP Extended Community sub-type value 0x0b ('E-Tree Info') under type 0x80 in the 'Generic Transitive Experimental Use Extended Community Sub-Types' registry within the BGP Extended Community registry.
IANA allocated Pseudowire Interface Parameters Sub-TLV type 0x1A (length 8) for 'E-Tree' in the 'Pseudowire Interface Parameters Sub-TLV type Registry'; LDP status codes 0x20000003 ('E-Tree VLAN mapping not supported', E=1) and 0x20000004 ('Leaf-to-Leaf PW released', E=0) in the 'Status Code Name Space' registry.
The solution prevents leaf-to-leaf communication in the VPLS data plane, enhancing security for customers by blocking network attacks from one leaf to another. However, if a PE is compromised or misconfigured and treats a leaf node as a root, a leaf may receive traffic from other leaves inappropriately. LDP authenticity and integrity per RFC 5036 must be considered.
The BGP E-Tree Extended Community (type 0x80, sub-type 0x0b) is 8 octets: a 2-octet type field, two 2-octet fields each carrying a 4-bit MBZ and a 12-bit VLAN ID (Root then Leaf), and a 2-octet field with 14 reserved bits plus a P bit (leaf-only) and V bit (VLAN mapping capability).
The E-Tree LDP Interface Parameter Sub-TLV (type 0x1A, length 8 octets) carries: a Reserved field (must be zero), a P bit (leaf-only flag), a V bit (VLAN mapping capability), 4-bit MBZ fields, a 12-bit Root VLAN ID, and a 12-bit Leaf VLAN ID.