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34 lines
1.6 KiB
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.. _tunnels:
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Tunnels
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---------
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Tunnels share a similar property to recursive routes in that after applying the
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tunnel encapsulation, a new packet must be forwarded, i.e. forwarding is
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recursive. However, as with recursive routes the tunnel's destination is known
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beforehand, so the recursive switch can be avoided if the packet can follow the
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already constructed data-plane graph for the tunnel's destination. This process
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of joining to DP graphs together is termed *stacking*.
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.. figure:: /_images/fib20fig2.png
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Figure 11: Tunnel control plane object diagram
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Figure 11 shows the control plane object graph for a route via a tunnel. The two
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sub-graphs for the route via the tunnel and the route for the tunnel's
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destination are shown to the right and left respectively. The red line shows the
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relationship form by stacking the two sub-graphs. The adjacency on the tunnel
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interface is termed a 'mid-chain' this it is now present in the middle of the
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graph/chain rather than its usual terminal location.
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The mid-chain adjacency is contributed by the gre_tunnel_t , which also becomes
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part of the FIB control-plane graph. Consequently it will be visited by a
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back-walk when the forwarding information for the tunnel's destination changes.
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This will trigger it to restack the mid-chain adjacency on the new
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*load_balance_t* contributed by the parent *fib_entry_t*.
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If the back-walk indicates that there is no route to the tunnel, or that the
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route does not meet resolution constraints, then the tunnel can be marked as
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down, and fast convergence can be triggered in the same way as for physical
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interfaces (see section ...).
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