This article explains the ERO expansion in L1 domain of ISIS-FR.
Below is the topology:
RSVP LSP is from R4 to R5 and Vice-versa
The computed path on RSVP ingress node R4 is
R4 -- > R0 -- > R6 -- > R2 -- > R5
R0-R6, R6-R2 are FR TE links
RSVP PATH msg signaled by R4 to R0
ERO : {R0, R6, R2, R5}
RSVP PATH msg sent by R0 to R7:
ERO: {R7, R6, R9, R2, R5}
FR hops indicated by the presence of FR TE links R0-R6 and R6-R2 are expanded on R0
R4 is unaware of the L1 underlying links.
The L1/L2 router has visibility to L2 router and the entire L1 domain. When this L1/L2 router received path message, it runs CSPF to expand hops in L1 domain to reach L2 router. Once it calculates a successful path, it appends the ERO