Fixed Networks
Fixed networks in the SRA Composite exam test how Nokia SR OS builds and operates subscriber-facing services: BNG subscriber management, Layer 2 and Layer 3 service delivery, QoS, and resilient wholesale access. Questions often ask you to place a construct (SAP, SDP, Epipe, VPLS, VPRN) in the right layer and to trace a subscriber session end to end.
Subscriber edge and service constructs
- A BNG terminates PPPoE or IPoE sessions, interacts with RADIUS for authentication and authorization, uses DHCP for address assignment, and applies subscriber profiles and QoS.
- SAPs attach services to access ports; SDPs carry traffic between service instances across the MPLS core.
- Epipe is point-to-point, VPLS is multipoint Layer 2, VPRN is multipoint Layer 3, and IES provides direct IP access.
- MC-LAG and multi-chassis redundancy protect access links and BNG pairs.
Worked example
A residential subscriber on an IPoE session receives no address. The SAP is up and the VPRN is bound to the correct subscriber interface. Check the DHCP relay configuration: the relay must point to the DHCP server, include Option 82 if the server uses it for subscriber identification, and match the subscriber's routing instance. If RADIUS returns a wrong IP pool or no pool, address assignment fails even though DHCP is reachable.
Traps
Do not confuse a SAP (local attachment) with an SDP (core transport). Do not assume VPLS provides IP routing; it does not. In wholesale models, verify the access versus retail responsibility boundary before choosing a service type.