The question indicates that a router performs a lookup in its FIB table for a packet and determines that the tunnel ID in the matching entry is 0, suggesting that the packet needs to be forwarded through a tunnel such as an MPLS tunnel. However, this is a misunderstanding of the FIB functionality.
FIB Table OverviewThe Forwarding Information Base (FIB) is used to make packet-forwarding decisions. Entries in the FIB include next-hop information, which can be directly linked to an interface or a tunnel.
If theTunnel IDis0, it indicates that the packet is forwarded via a normal routing path and not through a tunnel.
For MPLS or other tunnels, the Tunnel ID would have a non-zero value pointing to the associated tunnel.
MPLS Tunnel OperationWhen a router forwards packets through an MPLS tunnel, a label-switched path (LSP) is set up. The FIB would reflect specific tunnel identifiers for packets that need such encapsulation.
HCIP-Datacom-Core Reference
Routing Principles and MPLS​explain the forwarding mechanisms clearly, stating that if a packet is routed normally, the tunnel ID remains0.
The section on MPLS​clarifies the encapsulation process and the role of tunnel identifiers.
Hence, the claim in the question is incorrect. A Tunnel ID of0implies no tunneling is required, and normal IP forwarding occurs
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