Objectives
The simulator ticks each one as soon as the network meets it.
Find why PE1 has no SR path to 5.5.5.5 (show isis segment-routing label table, show mpls forwarding).
Fix the SID conflict: P3’s Loopback0 must use index 4, not 5 (PE2 owns index 5).
Fix the next fault: P1 does not run segment routing in IS-IS.
PE1’s LSP to 5.5.5.5 runs on segment routing again.
Ping 5.5.5.5 from PE1.
Before you start
Two routers that claim the same SID index cause a conflict: the SID is ignored for both prefixes. A router without SR in its IGP is not an SR node, so the routers before it have no label to send it. show isis segment-routing label table and the label tables show both.
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.
