Switching and aggregation
This domain tests Layer 2 transport and the service constructs Nokia uses to aggregate it: VLANs, STP variants, LAG/MC-LAG, and Ethernet services (Epipe, VPLS, IES, VPRN) built from SAPs and SDPs.
Core rules
- 802.1Q tags carry a VLAN ID; QinQ (802.1ad) stacks an outer S-VLAN for provider aggregation.
- STP elects one root by lowest bridge ID (priority, then MAC). RSTP converges faster with edge and point-to-point roles; MSTP maps VLANs to instances.
- LAG bundles links with LACP; MC-LAG stretches a bundle across two PE routers for active/active access.
- A SAP is the attachment on a port (e.g.
1/1/1:100); an SDP is the tunnel (GRE, LDP, RSVP, SR) to a far PE. Service MTU must be at least as large as the largest frame. - Epipe is point-to-point; VPLS is multipoint bridged; IES and VPRN are IP services.
Worked example
Two access switches dual-home to PE-1 and PE-2. Each PE has a LAG to the switch, and the two LAGs form an MC-LAG with LACP. The customer VLAN 100 is mapped to a SAP on each PE and connected by a VPLS with an SDP between PEs. Failure of one PE link leaves the bundle up, and the VPLS floods through the SDP, so no STP instance is needed on the customer side.
Traps
- Service MTU lower than the port MTU causing large frames to drop at the SAP.
- Mismatched native VLAN or tag mode (access vs. tagged) on a SAP.
- Forgetting that MC-LAG requires LACP and consistent configuration on both peers.
- STP still running where the design expects a loop-free LAG/VPLS topology, blocking links unexpectedly.