Objectives
The simulator ticks each one as soon as the network meets it.
Check PE1’s forwarding table: LDP labels and SR labels (16005 for PE2) both exist, and the LSP to 5.5.5.5 uses LDP (show mpls forwarding).
Prefer SR on PE1: under router isis CORE → address-family ipv4 unicast, segment-routing mpls sr-prefer.
PE1’s LSP to 5.5.5.5 now uses the SR label 16005.
Build tunnel-te1 to 5.5.5.5 (100000 kbps, path-option 1 dynamic, autoroute announce). It comes up through P2 and P3.
Ping 5.5.5.5 from PE1: the echo now rides tunnel-te1.
Before you start
With both LDP and SR, IOS and IOS XR use the LDP label until SR is preferred (sr-label-preferred / sr-prefer). An RSVP-TE tunnel with autoroute announce changes the routing table itself: routes through the tail end point to the tunnel, whatever label protocol is underneath.
Concept
LDP, segment routing and RSVP-TE in one core
A core can run several label protocols at once, for example during a migration from LDP to segment routing. Each one builds its own labels toward the same loopbacks, and the head end has to pick. LDP wins over SR unless SR is preferred. An RSVP-TE tunnel with autoroute announce wins over both, because it changes the routing table itself.
Concept
Segment routing: one label per destination, everywhere
With segment routing the IGP itself distributes labels. Each router gives its loopback a prefix-SID, an index into the SR global block (SRGB, 16000–23999 by default). Every SR router then reaches that loopback with the same label, 16000 + index. No LDP sessions are needed.
