To add a bit to this thread.

 A discard entry is a routing table entry, forwarding
 along which results in a packet drop. I.e., if the forwarding
 mechanism inside a router finds such an entry as the best match
 for the destination address, the packet should be dropped.

 The reasoning behind this entries is to prevent routing loops
 when the ABR does not have a specific route to the destination
 address, but has a less-specific or the default route pointing
 back to the previous node (in the simple case of a two-hop loop).

--
Alex Zinin
Wednesday, July 18, 2001, 6:42:38 AM, Xie, Feng wrote:
> Hi, Jina:
>     When a net range is configured for an area, it is possible that some
> prefixes inside the net range do not exist in the area. In other words, the
> ABR does not know how to deliever packets to those prefixes. To prevent
> routing loops, a discard route is configured for each netrange. When a
> packet destined for a prefix in the net range is received by an ABR, the ABR
> checks its routing table to find a route. If the destination prefix is
> inside the area, a more specific match than the discard route will be found
> and the packet will be delivered. If the destination prefix is not inside
> the area, the discard route will be found and the packet will be discarded.
>     Depending on the implementation, a flag can be used to distinguish
> discard routes with normal routes.
> Feng Xie
> -----Original Message-----
> From: Wu Jina [mailto:jinawucnYAHOO.COM.CN]
> Sent: Wednesday, July 18, 2001 7:38 AM
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: About routing table loopup.
> Hello, everyone.
> Now, I am reading the RFC2328, but I found one question, can your help me?
> In Section 11.1:
> ....
> Before the lookup begins, "discard" routing table entries should
> be inserted into the routing table for each of the router's active area
> address ranges (see Section 3.5). (An area range is considered "active"
> if the range contains one or more networks reachable by intra-area
> paths.) The destination of a "discard" entry is the set of addresses
> described by its associated active area address range, and the path type
> of each "discard" entry is set to "inter-area".[10]
> ...
> I don't know what is the "discard route", why need this step?
> Thanks.
> Jina.

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