UDLR Routing using GRE Tunnels
                    ------------------------------

Purpose of feature:

  This feature should be used with extreme caution since it does not scale
  well to large number of tunnels. This feature is used primarily to deploy
  a point-to-point link over a satellite network using the link as a 
  router-to-router transit subnet and not a general purpose multi-access
  transit subnet. 

  This feature allows IP (and any associated protocols that use IP) to
  believe the unidirectional link is logically bidirectional. Therefore,
  all routing protoocls (both unicast and multicast) can work over this
  link without modification.

  This feature supports HDLC and PPP encapsulation so you can get link
  quality information for this logical bidirectional link.

Using the GRE tunnel:

  When a GRE tunnel is used to support unidirectional links, it cannot be
  used as a general purpose tunnel interface. That is, in the upstream
  router (where "tunnel udlr receive-only <interface>" is configured), no 
  packets can be sent on the tunnel. The tunnel is used as a unidirectional 
  path from the downstream router to the upstream router solely to deliver 
  control packets. 

  The tunnel does not require an "ip address" or "ip unnumbered" command. 
  That is, you don't have to assign an IP address to the tunnel (but you do 
  have to configure the tunnel endpoint addresses).

Interface subcommands:

  [no] tunnel udlr receive-only <interface>
      Used in a router with a unidirectional interface with send-only
      capabilities. When you configure a GRE tunnel and use this command,
      specify <interface> as the unidirectional send-only interface. When
      packets are received over the tunnel, the upper layer protocols will
      think the packet is received over the unidirectional send-only
      interface.

  [no] tunnel udlr send-only <interface>
      Used in a router with a unidirectional interface with receive-only
      capabilities. When you configure a GRE tunnel and use this command,
      specify <interface> as the unidirectional receive-only interface. When
      packets are sent by upper layer protocols over <interface>, they
      will be redirected and sent over this GRE tunnel.

Configuration example:

  Upstream router is configured with OSPF and PIM:

      ip multicast-routing
     
      !
      ! Serial0 has send-only capability
      !
      interface serial0
      encapsulation hdlc
      ip address 10.1.0.1 255.255.0.0
      ip pim sparse-dense-mode
  
      ! 
      ! Configure tunnel as receive-only UDLR tunnel.
      !
      interface tunnel0
      tunnel source ethernet0
      tunnel destination <downstream-router>
      tunndl udlr receive-only serial0
  
      !
      ! Configure OSPF.
      !
      router ospf <pid>
      network 10.0.0.0 0.255.255.255 area 0

  Downstream router is configured with OSPF and PIM:

      ip multicast-routing

      !
      ! Serial1 has receive-only capability
      !
      interface serial1
      encapsulation hdlc
      ip address 10.1.0.2 255.255.0.0
      ip pim sparse-dense-mode
  
      ! 
      ! Configure tunnel as send-only UDLR tunnel.
      !
      interface tunnel0
      tunnel source ethernet0
      tunnel destination <upstream-router>
      tunndl udlr send-only serial1
  
      !
      ! Configure OSPF.
      !
      router ospf <pid>
      network 10.0.0.0 0.255.255.255 area 0

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