Objectives
The simulator ticks each one as soon as the network meets it.
On PE1, enable MPLS TE on both core interfaces (mpls traffic-eng → interface).
Flood TE information in IS-IS: under router isis CORE → address-family ipv4 unicast, mpls traffic-eng level-2-only and router-id Loopback0.
Create tunnel-te1 to 5.5.5.5, unnumbered to Loopback0, asking for 100000 kbps with path-option 1 dynamic.
tunnel-te1 comes up through P2 and P3, because the top path can reserve only 50000 kbps (show mpls traffic-eng tunnels).
Add autoroute announce: PE1’s route to 5.5.5.5 now points to tunnel-te1 (show route isis).
Ping 5.5.5.5 from PE1: the echo crosses the tunnel.
Before you start
CSPF runs on the head end over the TE database that IS-IS floods; a router or link that does not flood TE is invisible to it. The bandwidth request is a constraint, so CSPF skips links that cannot reserve it, even when the IGP prefers them. On IOS XR, a tunnel-te interface needs no tunnel mode: it is always an MPLS TE tunnel. Without autoroute announce the tunnel is up but carries nothing.
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.
Concept
RSVP-TE: signalling the tunnel hop by hop
Once CSPF has a path, RSVP-TE builds the label-switched path along it. The head end sends a Path message carrying the explicit route and the bandwidth; the tail end answers with a Resv message that reserves the bandwidth and hands out labels on the way back.
