Layer 2 Bridging, VLANs, and STP for JNCIS-SP

Wed, 15 Apr 2026 00:00:00 +0000

Layer 2 Bridging and VLANs

Service provider networks often need to deliver Layer 2 connectivity between geographically separated customer sites. Junos implements this using bridge domains, which define the L2 forwarding boundaries, and 802.1ad (Q-in-Q) to tunnel customer VLAN spaces across the provider network without overlap.

Terms

Bridge Domain — a Layer 2 forwarding domain. Analogous to a VLAN. Defines which interfaces share the same broadcast domain and MAC table.
EVC (Ethernet Virtual Connection) — the L2 service sold by the SP to the customer. Defines the endpoints of a Layer 2 circuit.
C-Tag (Customer Tag) — the inner 802.1q tag. Any VLAN 1–4094 from the customer’s space.
S-Tag (Service Tag) — the outer 802.1ad tag. Assigned by the SP to identify the customer. Encapsulates all of that customer’s C-Tags.
PBN (Provider Bridge Network) — the entire SP Layer 2 fabric.
PEB (Provider Edge Bridge) — the SP edge device. Pushes/pops S-Tags on customer-facing ports.
S-VLAN Bridge — an interior SP device that only examines and switches based on the S-Tag.
Customer ports — PEB ports facing the customer. S-Tags are applied or removed here.
Network ports — interior SP ports that carry double-tagged frames without modification.
IRB (Integrated Routing and Bridging) — a logical interface that gives a bridge domain an IP address, enabling the router to act as the default gateway for hosts in that domain.

802.1q

The standard VLAN tagging protocol. Inserts a 4-byte tag into the Ethernet frame.

Vlan on TTL Expired

Layer 2 Bridging, VLANs, and STP for JNCIS-SP

Layer 2 Bridging and VLANs

Terms

802.1q