Computer Networks · free preview
MPLS
Multiprotocol label switching grafts virtual-circuit forwarding onto the datagram Internet, edge routers classify each arriving packet into a forwarding equivalence class and attach a short fixed-length label carried in a shim header between link and network layers, interior routers forward by exact-match label swap without ever reading the IP address, the control plane remains ordinary IP routing so labels follow the same paths unless explicit routing chooses otherwise, and the technique's lasting applications are forwarding on devices that cannot do IP lookups, traffic engineering along precalculated paths, and virtual private networks that stack a second label to keep customers apart.
1. The Shim
The Shim
2. Bindings and Classes
Bindings and Classes
3. Same Routes, Cheaper Forwarding
Same Routes, Cheaper Forwarding
4. Explicit Routes and Private Networks
Explicit Routes and Private Networks
5. What the label replaces
Inside an MPLS domain, what does a label-switching router examine when forwarding a packet?
6. Where the shim sits
Where is an MPLS label carried in a packet?
7. Equivalence classes
MPLS forwarding tables are indexed by class rather than by address. What exactly is a forwarding equivalence class?
8. Engineering the path
An operator wants certain traffic to use an underutilized long path instead of the congested shortest one. How does MPLS make that possible?
9. The VPN stack
Two customers of one provider both number their sites with the same private address range. How does the provider's MPLS backbone keep them apart?
10. Why not for speed
What became of MPLS's original promise of faster forwarding than IP?
11. The classifier
At an MPLS domain's edge, which router performs the full IP lookup and pushes the first label onto a packet?
12. The shim width
An MPLS label shim header is how many bits long?
13. The swap
An interior label-switching router rewrites the incoming label to the outgoing label, an operation called what?
14. Outer and inner
In an MPLS VPN, the label that names the customer is the inner label, while the label that tunnels the packet across the backbone is the what?
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