MPLS and VPNs Flashcards

What does MPLS stand for and what is its primary purpose?

Answer: Multi-Protocol Label Switching — a technique that forwards packets using short fixed-length labels instead of IP routing lookups

MPLS is a packet-forwarding technique that adds short labels to packets at the network edge. Core routers forward packets based on labels rather than performing full IP routing table lookups, enabling speed, traffic engineering, and VPN services.

What is an LSP (Label Switched Path) in MPLS?

Answer: A unidirectional predetermined path through an MPLS network along which labeled packets are forwarded using swapped labels at each hop

An LSP is a unidirectional path from an ingress LSR to an egress LSR through one or more transit LSRs. Each LSR along the path has a label binding that maps the incoming label to an outgoing label and interface.

What is the purpose of a BGP L3VPN (RFC 4364)?

Answer: To provide IP VPN service over an MPLS backbone, separating customer routing with VRFs and using MP-BGP to distribute customer routes between PE routers

BGP L3VPN (formerly RFC 2547) uses MPLS labels and MP-BGP with VPNv4 address family to create separate per-customer routing domains (VRFs) connected across an MPLS service provider network.

In Junos MPLS, what is the difference between LDP and RSVP-TE?

Answer: LDP creates hop-by-hop LSPs following IGP paths without traffic engineering; RSVP-TE creates explicitly routed, bandwidth-reserved LSPs for traffic engineering

LDP distributes labels based on IGP routing and creates LSPs that follow the IGP shortest path. RSVP-TE allows explicit path specification, bandwidth reservation, and constraint-based routing for traffic engineering applications.

What is a Route Distinguisher (RD) in the context of Junos L3VPN?

Answer: A 64-bit value prepended to customer IPv4 prefixes to create unique VPNv4 routes in MP-BGP, allowing the same IP prefix to exist in multiple VRFs

The Route Distinguisher makes customer IPv4 routes globally unique in the BGP table. The same prefix (e.g., 10.0.0.0/8) can exist in multiple customer VRFs without conflict because each VRF has a different RD prepended.

What is the 'penultimate hop popping' (PHP) mechanism in Junos MPLS?

Answer: The second-to-last LSR pops the label before the egress LSR, so the egress processes the IP packet directly without needing to look up the MPLS label

PHP allows the penultimate (second-to-last) LSR to remove the MPLS label before forwarding to the egress LSR. This saves one label lookup on the egress LSR, which only needs to do an IP lookup.