Basic UDLR Tunnel Operation
Traffic from the downstream network (B or C)
destined for A’s unidirectional interface
will traverse the unidirectional tunnel.
Packets are encapsulated by B in a GRE
IP packet and forwarded to upstream router A.
A decapsulates the GRE packet, and places
the original packet on the input queue belonging
to A’s unidirectional link.
Access filters limit traffic on the upstream
and downstream tunnel interfaces
IP, ARP, NHRP, and CLNS (for IS-IS IP routing)
Upstream router A’s routing protocol must never
advertise the tunnel subnet over the
unidirectional link interface - this will cause
a recursive tunnel in downstream routers.