MPLS and VPNs Flashcards

In Junos L3VPN, what is the function of a Route Target (RT)?

Answer: Controls which VRFs import which VPNv4 routes, implementing the VPN topology (full-mesh, hub-spoke, extranet)

Route Targets are BGP extended communities that control VRF route import and export. A VRF exports routes with certain RT values; other VRFs import routes matching specific RT values, defining VPN connectivity topology.

What is VPLS (Virtual Private LAN Service) in Junos?

Answer: A Layer 2 VPN service that provides multipoint Ethernet connectivity across an MPLS network, emulating a LAN switch for geographically dispersed sites

VPLS emulates a LAN (with MAC learning, flooding, and bridging) across an MPLS backbone, allowing multiple customer sites to appear as if they are connected to the same Ethernet switch.

How does Junos implement MPLS traffic engineering (TE)?

Answer: Using RSVP-TE to signal LSPs with explicit routes and bandwidth reservations, with OSPF or IS-IS TE extensions advertising link bandwidth

Junos MPLS TE uses RSVP-TE for LSP signaling with constraint-based routing. OSPF-TE or IS-IS-TE extensions flood link attributes (bandwidth, TE metric, SRLG) that CSPF uses to compute explicit paths.

What is 'fast reroute' (FRR) in Junos MPLS?

Answer: A protection mechanism that pre-computes backup paths for LSPs, enabling sub-50ms traffic rerouting upon link or node failure without waiting for IGP reconvergence

MPLS FRR pre-establishes detour or bypass LSPs before failures occur. When a protected link or node fails, traffic is immediately switched to the pre-computed backup path, typically within 50ms.

In Junos, what command shows the MPLS label forwarding table?

Answer: show route table mpls.0

'show route table mpls.0' displays the MPLS label table (mpls.0 routing table) showing incoming labels, operations (pop/swap/push), and next-hops for each MPLS label binding.

What is Segment Routing (SR-MPLS) and how does it differ from LDP-based MPLS?

Answer: SR-MPLS encodes the entire path in the packet header as a stack of node/adjacency SIDs, eliminating per-flow state in the network core and simplifying operations

Segment Routing places the path information (as a stack of Segment IDs) in the packet at the ingress, eliminating the need for RSVP signaling and per-LSP state in core routers, while retaining MPLS forwarding.