Subject: [OSPF] ospfv2 NSSA address range question

From: sinbad <[email-masked]>
Date: Wed, 17 Oct 2007 12:46:10 +0530

RFC 3101 section 3.2 point (2)

Else the Type-7 LSA must be aggregated by the most specific

Type-7 address range that subsumes it.  If this Type-7 address

range has the same [address,mask] pair as the LSA's network

and no other translatable Type-7 LSA with a different network

best matches this range, then flag the LSA as not contained in

any explicitly configured Type-7 address range and process the

LSA as described in step (2).  Otherwise compute the path type

and metric for this Type-7 address range as described below.

The path type and metric of the Type-7 address range is

determined from the path types and metrics of those

translatable Type-7 LSAs that best match the range plus any

locally sourced Type-5 LSAs whose network has the same

[address,mask] pair.  If any of these LSAs have a path type of

2, the range's path type is 2, otherwise it is 1.  

-------------------------------------

"If the range's path type is 1 its metric is the highest cost amongst

these LSAs; if the range's path type is 2 its metric is the

highest Type-2 cost + 1 amongst these LSAs.  

(See Section 2.5 step (5).)  1 is added to the Type-2 cost to ensure that
the

translated Type-5 LSA does not appear closer on the NSSA

border than a translatable Type-7 LSA whose network has the

same [address,mask] pair and Type-2 cost."

-------------------------------------

The above quoted text is not very clear why + 1 is added to the cost ... any
one pls shed some light on this.

thanks

sinbad

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From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Wed, 17 Oct 2007 06:23:03 -0700 (PDT)

Sinbad,

Regarding,

  ...If the
  range's path type is 1 its metric is the highest cost amongst
  these LSAs; if the range's path type is 2 its metric is the
  highest Type-2 cost + 1 amongst these LSAs.  (See Section 2.5
  step (5).)  1 is added to the Type-2 cost to ensure that the
  translated Type-5 LSA does not appear closer on the NSSA
  border than a translatable Type-7 LSA whose network has the
  same [address,mask] pair and Type-2 cost.

I puzzled over a similar statement in RFC 1587 and added
the last sentence based on this subtle occurrence. If a
Type-7 LSA exists with the same [address,mask] pair as the
Type-7 range's [address,mask] pair, and other Type-7 LSAs
exist that are strictly subsumed by the address mask pair,
then adding +1 ensures that the Type-7 path is preferred.

Note normally a static discard route would be generated by
a Type-7 address range to avoid routing loops. Here this
Type-7 LSA's path should be administratively preferred over
the discard route.

Perhaps someone else remembers (or can think of) something more
pertinent.

Pat