NEXUS DC · MPLS TE

Nexus: RSVP-TE explicit path with a dynamic fallback

Every switch runs MPLS TE. Pin tunnel-te1 to the top path through P1 with a strict explicit path, and keep a dynamic path option as the fallback. The top links can reserve only 50 Mbps, so the tunnel first comes up on the fallback. Raise the RSVP bandwidth along the top path and watch the tunnel move back to its preferred path option.

Nexus DC track · Cisco Nexus (NX-OS-style) · 6 objectives

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Objectives

The simulator ticks each one as soon as the network meets it.

On PE1, under mpls traffic-eng configuration, create explicit path TOP with strict next-addresses 10.0.12.2 (P1) and 10.0.25.2 (PE2).
Build tunnel-te 1 to 5.5.5.5 for 100000 kbps with autoroute announce, path-option 1 explicit name TOP and path-option 2 dynamic.
Check show mpls traffic-eng tunnels: path option 1 is refused for bandwidth, so the tunnel is up on path option 2 through P2 and P3.
Raise the RSVP bandwidth to 1000000 kbps on the top path: PE1 Ethernet1/1, both P1 interfaces and PE2 Ethernet1/1.
tunnel-te1 re-optimises onto path option 1 through P1.
Ping 5.5.5.5 from PE1 through the tunnel.

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Before you start

Path options are tried in order, lowest number first; the first one that CSPF and RSVP accept carries the tunnel. A strict hop must be directly connected to the previous hop, and every link on the path must still have the requested bandwidth. When a better (lower-numbered) path option becomes possible, the head end re-optimises onto it.

Concept

Explicit paths and path options

A tunnel lists path options in order. A dynamic option lets CSPF choose; an explicit option names the hops the tunnel must take. Strict hops must be directly connected to the previous hop; loose hops can be reached over any TE path. The first option that can be signalled carries the tunnel.

Concept

CSPF: the shortest path that meets the constraints

An IGP always takes the lowest-cost path. MPLS traffic engineering lets the head end choose a different one: IS-IS floods every TE link with its TE metric and reservable bandwidth, and CSPF (constrained shortest path first) runs Dijkstra only over the links that meet the tunnel’s constraints.