segment routing could be considered as the successor of rsvp based
te, most of the features described for te could be
done with sr too. the routers within the core signal the te
attributes in isis or ospf as before, but
you assign an additional prefix sid to their loopbacks. the routers will allocate
an adjacency sid to all of it's neighbors, and will advertise the beginning of the
sr label block too.
from that point, you can calculate the label
of any given pe by simply adding the base advertised by the
next hop p to the prefix sid advertised by the target pe.
with this, you can send labeled traffic to any given pe
without involving any other protocols or consulting any other devices by adding the base of the
nexthop and the prefix sid of the target. you can construct loosely specified paths by stacking
these labels expressing the nodes that must be traversed. one specific
subcase of this is when you put a single label to address the remote
pe as an outer, transport label.
stacking the adjacency sids gives you rsvp like explicit routing capabilities.
the main advantage of using sr is that it have no state at the p routers.
the dataplane can use mpls or extension header before the ip header.
here are the captures for