Provider edges, core routers and forwarding classes
A customer edge (CE) router connects to a provider edge (PE) router. The ingress PE classifies traffic into a forwarding equivalence class (FEC), such as packets bound for a particular egress prefix. A provider core (P) router can switch the resulting traffic by label without learning the customer’s individual routes.
The label information base (LIB) holds known label bindings. The label forwarding information base (LFIB) contains the forwarding actions used for installed labels, such as the outgoing interface, next hop and new label. The label-switched path (LSP) follows those actions from ingress to egress.
Push, swap and pop labels
The ingress pushes a label onto the packet. A transit router can swap the top label for a value its downstream router expects. The egress pops the label or receives the packet with that label already removed by the penultimate hop. With stacked labels, removing the top label can expose an inner service label.
For example, an ingress PE can push 16020, the first core router can swap it to 16030, and the next core router can pop it before forwarding to the egress PE. These values are examples. A label’s meaning is generally local to the router or label space that assigned it.
Uniform, pipe and short-pipe QoS
The MPLS Traffic Class (TC) field can carry information used for per-hop treatment. In the uniform model, treatment is shared across the tunnel boundary. A class change in the MPLS network can be mapped back to the exposed header when the label is removed.
In the pipe model, provider markings and the tunneled packet’s markings remain separate. The egress uses the tunnel classification for its forwarding treatment and preserves the inner marking. Short pipe also preserves the inner marking, but the egress uses the exposed packet’s classification for the outgoing treatment.
Try it in a lab
- On MPLS-capable lab routers, use two PEs and two P routers with an LSP for the egress PE’s loopback. Look up its FEC and compare the learned bindings with the installed forwarding actions.
- Send traffic over the LSP and inspect the label stack on each provider link. Match the observed labels to the ingress push, transit swap and any penultimate-hop pop shown in the forwarding table.
- On equipment supporting the QoS modes, send a known DSCP and use a different class in the provider core. Compare the exposed packet marking and the egress queue in uniform, pipe and short-pipe configurations.