Switching, SAPs, and Aggregation for VPRN Access
What this domain tests
This domain tests how customer traffic reaches a VPRN through SR OS switching constructs: Ethernet ports, LAGs, SAP encapsulation, and aggregation. The exam expects you to map VLAN tags to the correct SAP and to keep Layer 2 aggregation from breaking VPRN interface state.
Core rules
- A SAP is the service access point where a VPRN IP interface attaches. It is defined on a port or LAG with an encapsulation: null, dot1q, or qinq.
- With dot1q, the SAP ID is the customer VLAN, e.g. sap 1/1/1:100. With qinq, outer and inner tags are both significant.
- A LAG aggregates links with LACP; a SAP on a LAG stays up while at least one member is active. MC-LAG extends this across peers.
- MTU must be consistent across the SAP, VPRN interface, and core transport, or large packets are dropped.
- Spoke SDPs can extend a VPRN attachment across the service core when the access is not local.
Worked example
A customer delivers double-tagged frames with outer VLAN 200 and inner VLAN 100 on port 1/1/1. You configure a qinq SAP that matches 200/100 and bind it to the VPRN interface. If you instead configure dot1q SAP 1/1/1:100, the outer tag 200 is not matched, so the frames are not classified into the VPRN. After moving the SAP to lag-1 for redundancy, the same encapsulation must be repeated on the LAG SAP.
Traps
Mixing a SAP on a physical port and a LAG that contains that port causes conflicts. A null-encapsulated SAP accepts untagged traffic only. Changing an access VLAN without changing the SAP silently black-holes the service.