H3C Technologies H3C S10500 Series Switches - Manual (Page 105)

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Two approaches are available to automatic route advertisement: IGP shortcut and forwarding
adjacency.
OSPF and IS-IS support both approaches where TE tunnels are considered point-to-point links and TE
tunnel interfaces can be set as outgoing interfaces.
IGP shortcut, also known as autoroute announce, considers a TE tunnel as a logical interface directly
connected to the destination when computing IGP routes on the ingress of the TE tunnel.
IGP shortcut and forwarding adjacency are different in that in the forwarding adjacency approach,
routes with TE tunnel interfaces as outgoing interfaces are advertised to neighboring devices but not in
the IGP shortcut approach. Therefore, TE tunnels are visible to other devices in the forwarding adjacency
approach but not in the IGP shortcut approach.
Figure 28 IGP shortcut and forwarding adjacency
Router D
Router A
Router B
Router C
Router E
10
20
10
10
10
20
A TE tunnel is present between Router D and Router C. With IGP shortcut enabled, the ingress node
Router D can use this tunnel when calculating IGP routes. This tunnel, however, is invisible to Router A;
therefore, Router A cannot use this tunnel to reach Router C. With forwarding adjacency enabled, Router
A can know the presence of the TE tunnel and forward traffic to Router C to Router D though this tunnel.
The configuration of IGP shortcut and forwarding adjacency is broken down into tunnel configuration
and IGP configuration. When making tunnel configuration on a TE tunnel interface, take the following
issues into consideration:
• The tunnel destination address must be in the same area where the tunnel interface is located.
• The tunnel destination address must be reachable through intra-area routing.
CR-LSP backup
CR-LSP backup provides end-to-end path protection for the entire LSP without time limitation. This is
different from FRR which provides quick but temporary per-link or per-node protection on an LSP.
In the same TE tunnel, the LSP used to back up a primary LSP is called a secondary LSP. When the ingress
of a TE tunnel detects that the primary LSP is unavailable, it switches traffic to the secondary LSP and after
the primary LSP becomes available, switches traffic back. This is how LSP path protection is achieved.
The following approaches are available for CR-LSP backup:
• Hot backup where a secondary CR-LSP is created immediately after a primary CR-LSP is created.
MPLS TE switches traffic to the secondary CR-LSP after the primary CR-LSP fails.
• Standard backup where a secondary CR-LSP is created to take over after the primary CR-LSP fails.