2eW4ZRB

Figure 4: expand-loose-hop Option
We’ve just said that, when it is not possible to compute a path to the egress using the TED, a
loose hop to the ASBR is added to the ERO—but how can the Path message span multiple
ASs? Figure 4 shows how this task is accomplished thanks to the expand-loose-hop
option. It gives a transit router the ability to add information and modify the existing ERO of a
transit RSVP Path message. When an ASBR configured with expand-loose-hop finds its
router-id, as loose next hop, in the transit RSVP Path message, it will search in its TED to
compute a path toward the ASBR in the remote AS. The local ASBR will expand the ERO in
the Path message by including the IP address of the physical inter-AS link on the other
ASBR, and the remote ASBR router-id as the loose next hop. This option is mandatory on all
the ABR/ASBR routers; otherwise it will be impossible to cross the AS border and different
IGP areas.
Configuring it is easy:
user@mx80-ASBR# show protocols
mpls {
expand-loose-hop;
}
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