H3C Technologies H3C Intelligent Management Center - Manual (Page 188)

IGP Shortcut—Interior Gateway Protocol (IGP) shortcut is an extra step in the intra-area (or
intra-level) best route selection process in OSPF or IS-IS. Without the autoroute-enabled MPLS TE
tunnels, the Shortest Path First (SPF) algorithm builds the shortest path tree to compute the minimum
cost to all other routers in the area, and then selects the best routes based on the minimum cost. The
IGP Shortcut feature modifies the shortest path tree on the tunnel head-end LSR before the best
routes are selected by inserting the MPLS TE tunnel in the SPF tree. The IGP Shortcut feature ensures
that the tunnel is used to transport all the traffic from the head-end LSR to all destinations behind the
tailed LSR. The path toward the tunnel tail-end LSR is replaced by a direct link (MPLS TE tunnel) in
the SPF tree.
The modification of the SPF tree is local to the head-end router and does not affect the topology
database or the SPF trees built by other routers. Therefore, the MPLS TE tunnel created by IGP
Shortcut is not seen by other routers, which means that other routes cannot use this tunnel to
compute the shortest path for routing traffic throughout the network.
Forwarding Adjacency—The Forwarding Adjacency feature allows a network administrator to
handle a TE, label-switched path (LSP) tunnel as a link in an IGP network based on the SPF
algorithm. A Forwarding Adjacency can be created between routers regardless of their location in
the network. The routers can be located multiple hops from each other.
OSPF includes MPLS TE tunnels in the OSPF link-state database in the same way that other links
appear for purposes of routing and forwarding traffic. When an MPLS TE tunnel is configured
between networking devices, that link is considered a Forwarding Adjacency. You can assign a
cost to the tunnel to indicate the preference of the link.
The benefit of the Forwarding Adjacency feature is that TE tunnel interfaces are advertized in the
IGP network just like any other links. Routers can then use these advertisements in their IGPs to
compute the SPF even if they are not the head-end of any TE tunnels. This means that routers outside
of the TE domain can see the TE tunnel and use it to compute the shortest path for routing traffic
throughout the network. However, using the MPLS TE Forwarding Adjacency feature increases the
size of the IGP database by advertising a TE tunnel as a link. Moreover, MPLS TE Forwarding
Adjacency tunnels must be configured bidirectionally.
IGP shortcut and Forwarding Adjacency—IGP Shortcut and Forwarding Adjacency are different.
In the Forwarding Adjacency approach, routes with TE tunnel interfaces as outgoing interfaces are
advertised to neighboring routers but not in the IGP Shortcut approach. Therefore, TE tunnels are
visible to other routers in the Forwarding Adjacency approach but not in the IGP Shortcut
approach.
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