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Understanding FEC 129 BGP Autodiscovery for VPWS

The major functional components in a VPWS with FEC 129 are BGP, LDP, and the Layer 2 VPN module of Junos OS. BGP is responsible for distributing the local autodiscovery routes created on each PE device to all other PE devices. LDP is responsible for using the autodiscovery information provided by BGP to set up targeted LDP sessions over which to signal the pseudowires. The Layer 2 VPN is the glue that binds the BGP and LDP functionalities together.

Supported Standards in FEC 129 BGP Autodiscovery for VPWS

The relevant RFCs for this feature are as follows:

  • RFC 4447, Pseudowire Setup and Maintenance Using the Label Distribution Protocol (LDP)

  • RFC 6074, Provisioning, Auto-Discovery, and Signaling in Layer 2 Virtual Private Networks (L2VPNs)

Routes and Routing Table Interaction in FEC 129 BGP Autodiscovery for VPWS

BGP, LDP, and Layer 2 VPNs interact through different types of routes installed in the instance.l2vpn.0 table. Theroutes that are present in the table are autodiscovery routes and pseudowire routes.

  • Autodiscovery routes are used by BGP to allow autodiscovery of remote source access individual identifiers (SAIIs) (the sources of the point-to-point pseudowires) and PE device addresses. Autodiscovery routes are advertised when you configure the l2vpn auto-discovery-only address family.

    The format of the autodiscovery routes is a combination of the route distinguisher and the SAII. For example: 10.255.0.1:100:0.0.0.1/96 AD.

    Table 1 lists the route elements and the number of associated bytes allocated to each element.

    Table 1: Autodiscovery Route Format

    Route Element

    Bytes

    RD

    8 bytes

    SAII

    4 bytes

    The l2vpn-id of the FEC 129 VPWS instance is attached to the route in a BGP extended community. One autodiscovery route is advertised for each source attachment identifier (SAI) in the instance.

  • Pseudowire routes are installed by the Layer 2 VPN (local) and LDP (remote) to represent the bidirectional components of the pseudowire. For example: NoCtrlWord:5:100:200:2:0.0.0.1/176. The format of the routes is described in Table 2.

Table 2: Pseudowire Route Format

Field Name

Field Description

Pseudowire type + control word bit

2 bytes

Remote PE address

4 bytes

Attachment group identifier (AGI)

The AGI field of the pseudowire route is always set to the l2vpn-id of the instance.

8 bytes

SAII

4 bytes

Target attachment individual identifier (TAII)

4 bytes

Layer 2 VPN Behavior in FEC 129 BGP Autodiscovery for VPWS

A Layer 2 VPN installs a locally generated autodiscovery route into the instance.l2vpn.0 table for every SAII configured in an FEC 129 VPWS instance. The extended community containing the l2vpn-id is attached when the route is added to the instance.l2vpn.0 table.

For each autodiscovered SAII from a remote neighbor where the l2vpn-id matches the local l2vpn-id and the received SAII matches a locally configured TAII, the Layer 2 VPN obtains an MPLS label and generates a pseudowire route and adds it to the instance.l2vpn.0 table. The remote PE address is copied from the BGP protocol next hop for the autodiscovery route.

The Layer 2 VPN module of Junos OS is responsible for installing the forwarding routes into the mpls.0 table as usual.

BGP Autodiscovery Behavior in FEC 129 BGP Autodiscovery for VPWS

Local autodiscovery routes installed by the Layer 2 VPN in the instance.l2vpn.0 table are advertised by BGP to remote PE devices sl2vpn auto-discovery-only address family according to the instance and BGP export policies.

On the receiving side, BGP accepts autodiscovery routes from remote peers and installs them in the local bgp.l2vpn.0 table, if they are allowed by inbound policy. The route is installed, and a secondary route is imported into the instance.l2vpn.0 table when an import route target match between the route and instance is found.

LDP Signaling Behavior in VPWS in FEC 129 BGP Autodiscovery for VPWS

In the Junos OS implementation of LDP, the router monitors for routes from instance.l2vpn.0 for any instance configured for FEC 129 VPWS. These routes are identified by the instance-type l2vpn statement in the routing instance and the presence of the l2vpn-id statement.

When a BGP autodiscovery route is installed, LDP sets up a targeted session with the remote peer, where the peer address is identified as the protocol next hop of the BGP autodiscovery route.

When a pseudowire route is installed in the instance.l2vpn.0 table, LDP uses the parameters associated with the route to signal the creation of the pseudowire using FEC 129. Upon receiving an FEC 129 label mapping message from a remote peer, LDP installs the pseudowire route in the ldp.l2vpn.0 table.

Upon a successful l2vpn-id match with a configured FEC 129 VPWS instance, a secondary pseudowire route is imported to the instance.l2vpn.0 table. If an outgoing pseudowire has not already been set up when the incoming pseudowire signaling is received, LDP initiates the outgoing pseudowire creation as well.

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