MPLS
Multiprotocol Label Switching is a forwarding system that carries packets across a network by using short labels instead of repeating a full network-layer lookup at every hop. Operators use it to build controlled transport paths and support services such as VPNs, traffic engineering, and fast recovery.
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Don't Panic
Don't Panic - MPLS
MPLS is the subject of this course. IP forwarding asks each router to examine a destination address and choose a next hop. Multiprotocol Label Switching, or MPLS, adds another forwarding method.
The useful unit of work is a closed loop: clarify the goal and boundaries, gather the inputs the practice requires, make the decision or change, record evidence, and return with owners for the next cycle. Skipping any link leaves teams busy without durable results.
Tooling supports the loop; it does not replace it. Choose tools after the boundary and evidence model are clear. Comparing products without that model produces feature matrices that do not change how the work runs.
Common failure modes include undefined ownership, metrics that count activity instead of outcomes, and irreversible steps taken without a review path. Treat those as design defects in the practice, not as individual heroics to compensate later.
Operators should be able to explain which signals would change a decision this week. If no signal can change the plan, the practice has become ritual. Keep the feedback path short enough that evidence still influences the next cycle.
Name the owners for each stage of the loop before the work scales. Unowned stages become permanent exceptions. Record decisions with enough context that a future operator can tell why a tradeoff was accepted. Prefer fewer, sharper metrics that change behavior over broad dashboards that only describe activity after the fact.
Read the Intro for the core model. Use the Cheatsheet when you need the operating map. Updates tracks official guidance when this course configures an update source; otherwise the practice is settled without a live feed.
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Sources
- https://www.rfc-editor.org/rfc/rfc3031.html
Supports
- Labels as short, fixed-length, locally significant FEC identifiers
- Forwarding equivalence classes and label switching routers
- Label stacks, label switched paths, ingress, transit, and egress roles
- FTN, ILM, NHLFE, label swapping, and next-hop behavior
- Push, swap, pop, label hierarchy, and penultimate hop popping
- MPLS architecture allowing multiple label-distribution protocols
- https://www.rfc-editor.org/rfc/rfc3032.html
Supports
- Label-stack placement between data-link and network-layer headers
- 20-bit label, 3-bit class field, 1-bit bottom-of-stack field, and 8-bit TTL
- Four-byte size of each label-stack entry
- Stack order and bottom-of-stack processing
- IPv4 and IPv6 explicit-null behavior
- Implicit null as a distributed value that is not transmitted
- https://www.rfc-editor.org/rfc/rfc5462.html
Supports
- Rename of the three-bit experimental field to Traffic Class
- Traffic Class as the current name across MPLS documents
- Updated label-stack entry diagram and traffic-class use
- https://www.iana.org/assignments/mpls-label-values/mpls-label-values.xhtml
Supports
- Current base and extended special-purpose label assignments
- Values for explicit null, implicit null, router alert, generic associated channel, and extension labels
- Current references and allocation status for special-purpose labels
- https://www.rfc-editor.org/rfc/rfc5036.html
Supports
- LDP peers and bidirectional sessions
- Discovery, session, advertisement, and notification message categories
- Exchange of label-to-FEC mapping information
- Downstream unsolicited and downstream-on-demand distribution
- Label retention and transport behavior
- https://www.rfc-editor.org/rfc/rfc3209.html
Supports
- Signaling of MPLS LSP tunnels
- Explicit routes independent of conventional IP forwarding
- Ingress control of an explicitly routed path
- Policy and resource considerations for traffic engineering
- https://www.rfc-editor.org/rfc/rfc4090.html
Supports
- Local repair with pre-established backup LSP tunnels
- One-to-one detours and facility bypass tunnels
- Points of local repair and redirection near a failure
- Label stacking for shared facility protection
- https://www.rfc-editor.org/rfc/rfc4364.html
Supports
- Separation of VPN routes and overlapping address spaces
- BGP distribution of MPLS labels with VPN routes
- Outer transport and inner VPN route label roles
- Egress provider-edge handling based on the VPN route label
- Provider-core transport beneath a VPN service
- https://www.rfc-editor.org/rfc/rfc3985.html
Supports
- Pseudowires emulating essential attributes of a service
- Ethernet, circuit, and other service carriage over packet-switched networks
- IP or MPLS tunnels beneath pseudowire service encapsulation
- Attachment-circuit, service, and packet-switched-tunnel layering
- https://www.rfc-editor.org/rfc/rfc6790.html
Supports
- Per-flow consistency as a load-balancing requirement
- Difficulty finding flow keys beneath MPLS encapsulation
- Ingress generation of entropy information
- Entropy Label Indicator value 7 immediately preceding each entropy label
- Two-entry stack-depth cost of the indicator and entropy label pair
- Transit-LSR load balancing without deep payload inspection
- https://www.rfc-editor.org/rfc/rfc8402.html
Supports
- Segments as ordered topological or service instructions
- Per-flow state at ingress nodes
- MPLS labels encoding segments
- MPLS label stacks encoding ordered segment lists
- https://www.rfc-editor.org/rfc/rfc8660.html
Supports
- SR-MPLS forwarding-plane behavior
- Segment identifiers represented by MPLS labels
- Segment-routing headers represented by MPLS label stacks
- Control-plane and forwarding-plane relationship with MPLS
- https://www.rfc-editor.org/rfc/rfc8029.html
Supports
- MPLS LSP ping and traceroute for FEC data-plane validation
- Distinction between ordinary IP reachability and MPLS FEC testing
- Control-plane rate limiting and source filtering for LSP OAM
- OAM topology-disclosure and denial-of-service considerations
- https://www.rfc-editor.org/rfc/rfc5920.html
Supports
- Threats and controls for MPLS control, management, and data planes
- Isolation of trusted domains and boundary protection
- LDP and RSVP control-plane protections
- Cryptographic confidentiality and integrity as separate protections
- CE-to-CE, PE-to-PE, and access-link protection boundaries
