MPLS flashcards
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33 cards. Read the term and the definition here. The deck at the top of this page is optional.
- MPLS shim header
- A 4-byte header inserted between the Layer 2 and Layer 3 headers, carrying a 20-bit label, 3-bit EXP/TC, 1-bit bottom-of-stack, and 8-bit TTL.
- Implicit null label
- Reserved label value 3. Advertising it tells the penultimate hop to pop the label so the egress receives a native IP packet and performs an IP lookup instead of a label lookup.
- Explicit null label
- Reserved label values 0 (IPv4) and 2 (IPv6). The egress keeps a label in the packet, allowing label-based QoS, OAM, or VPN context rather than plain IP forwarding.
- LIB versus LFIB
- The LIB stores all label bindings learned locally and from peers. The LFIB contains only the selected bindings actually used to forward labelled packets.
- LDP discovery and session
- LDP sends UDP hellos to 224.0.0.2 on port 646 for neighbor discovery, then forms a TCP session on port 646 to exchange label mappings.
- Downstream unsolicited mode
- A label distribution mode where an LSR advertises label bindings to peers without waiting for a request. LDP uses this mode by default.
- RSVP-TE Path message
- A downstream message carrying the session, sender template, explicit route object, and Tspec. It requests bandwidth and a label along the specified path.
- CSPF computation
- Constrained shortest path first prunes links failing bandwidth, affinity, or admin-group constraints from the traffic engineering database, then runs shortest path on the remainder.
- Penultimate hop popping
- The penultimate router removes the transport label so the egress receives an unlabelled IP packet. It is requested by the egress advertising implicit null.
- Bottom-of-stack bit
- A one-bit field in the MPLS shim header. A value of 1 marks the last label in the stack, telling the router that the payload follows.
- Entropy label
- An optional label carrying additional hashing bits so that ECMP in the core can spread traffic when the label stack alone provides too little entropy.
- MPLS TTL pipe mode
- A mode where the ingress sets the label TTL to 255 and does not propagate the IP TTL across the core, hiding the core topology from traceroute.
- Route distinguisher
- An 8-byte value prepended to an IPv4 prefix to create a unique VPN-IPv4 NLRI. It provides uniqueness only and is not used for forwarding decisions.
- Route target
- An extended BGP community attached to a VPN route. The export RT is set by the originating PE; the receiving PE imports routes whose RT matches its VRF policy.
- VPNv4 address family
- MP-BGP AFI 1, SAFI 128. It carries the route distinguisher, IPv4 prefix, and VPN label between provider edge routers.
- VRF definition
- A per-customer routing and forwarding table containing a route distinguisher, import and export route targets, and the interfaces assigned to it.
- Pseudowire VC label
- The inner label identifying a pseudowire instance. It sits below the transport label and is used by the egress PE to map the frame to the correct attachment circuit.
- Control word
- An optional 4-byte field between the VC label and the Layer 2 frame. It preserves sequencing, pads small frames, and carries original control bits.
- BGP auto-discovery
- A mechanism using BGP NLRI (AFI 25, SAFI 65) to discover pseudowire endpoints and their attributes, followed by LDP FEC 129 to signal the pseudowire.
- VPLS split horizon
- A loop-prevention rule: a frame received on a pseudowire is forwarded only to attachment circuits, never back out to another pseudowire in the same VPLS instance.
- TE affinity
- A 32-bit link attribute matched against a tunnel's affinity value and mask. A link is usable when (link_affinity AND mask) equals (tunnel_affinity AND mask).
- TE setup and hold priority
- Priority values 0 to 7 where a lower number means higher priority. A new tunnel can preempt an existing one if its setup priority beats the existing hold priority.
- DiffServ tunnel mode
- A QoS mode where the ingress copies the IP DSCP into the EXP/TC bits, the core queues on EXP, and the egress copies EXP back into DSCP.
- Facility backup FRR
- A fast reroute mode where one backup tunnel protects all label-switched paths crossing a link. The point of local repair pushes the backup label.
- Point of local repair
- The router adjacent to a protected link or node that redirects traffic onto a pre-signalled backup tunnel when the failure is detected.
- Merge point
- The router where a backup tunnel rejoins the primary label-switched path. It pops the backup label and continues forwarding along the original path.
- Loop-free alternate
- A pre-computed backup next hop whose path to the destination does not loop back through the computing router, used for fast LDP or IP convergence.
- MPLS LSP ping
- An OAM tool that sends a labelled echo request along a label-switched path to verify forwarding and detect broken mappings, using the command ping mpls ipv4.
- LDP-IGP synchronization
- A feature that delays the IGP from advertising a link until LDP has exchanged labels over it, preventing blackholes during session bring-up.
- Segment routing global block
- A label range reserved for segment routing. A node SID equals the SRGB base plus the prefix index, giving the same SID across the domain.
- Adjacency SID
- A segment routing SID bound to a specific link. It is locally significant and forces traffic onto that link, unlike a node SID bound to a prefix.
- SR-TE policy
- An ordered list of SIDs plus candidate paths installed at the headend. The headend pushes the SID list, and the core holds no per-path state.
- TI-LFA
- Topology-independent loop-free alternate uses segment routing to encode the post-convergence path as a SID list, giving full coverage without LFA topology constraints.