Subject: Re: [OSPF] type-2 cost plus 1

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Mon, 03 Jan 2011 12:38:47 -0700 (PDT)

>In nssa rfc.

>3.2 Translating Type-7 LSAs into Type-5 LSAs

>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.

>1 is added to the type-2 cost. can anyone explain me with a topology ?

Note that the following example should apply during the NSSA Section 2.5
external route calculation when RFC1583Compatibility is set to "enabled" as
indicated below. 
                      Z1------------------------Z2
                      |            2             |
                      |                          |
                    2 |         Area 0           | 2
                      |                          |
                      |            1             |
                      |  +--------------------+  |
                      | /                      \ |
                     TR1------------------------BR2
                      |            4             |
                      |                          |
                      |  4      NSSA 1         4 |
                      |                          |
                      |                          |
                    ASBR1                      ASBR2

Suppose NSSA 1's ASBR1 advertises a Type7 LSA for prefix 192.168.1.0/24 with
a Type 2 external metric of 10; and NSSA 1's ASBR2 advertizes a Type7 LSA
for prefix 192.168.0.0/16 with a Type 2 external metric of 20. ASBR2's Type7
LSA serves as a backup path for 192.168.1.0/24 as well as the primary path
for everything else in 192.168.0.0/16.

Assume NSSA 1's translator, TR1, has a Type7 address range for the prefix
192.168.0.0/16. Hence it originates a single translated Type5 LSA
aggregation for both Type7 LSAs.  Since TR1 only sees ASBR2's type7 LSA its
preferred path to prefix 192.168.0.0/16 is through NSSA 1's border router
BR2 over NSSA 1 (cost 4 path).

However BR2 has two LSA external advertisements for the prefix
192.168.0.0/16, namely TR1's aggregated Type5 translation and ASBR2's Type7
LSA. Because the Type2 metric of TR1's Type5 LSA is 21, BR2 prefers ASBR2's
Type7 LSA whose metric is 20. If TR1's type 2 metric was 20 (i.e. +1 was not
added), then BR2 would prefer TR1's translated Type5 LSA because of the
lower ASBR path cost, namely 1 versus 4 (provided RFC1583Compatibility is
set to "enabled" - see RFC 3101 Section 2.5 Step 6.c). Hence a routing loop
would result for traffic forwarded by Area 0's Z2 router to addresses in
prefix 198.168.0.0/16 not subsumed by 192.168.1.0/24.

Note that the "+1" adjustment of the Type 2 metric first appeared in NSSA
RFC 1587. The NSSA RFC 3101 specification ensured compatibility with OSPF
RFC 2328 while providing backward compatibility with OSPF RFC 1583 and NSSA
RFC 1587.

Hope this is what you are looking for. It has been a while since RFC 3101
was published. Perhaps others can produce more concise examples.

Pat Murphy
[email-masked]

Date: Thu, 30 Dec 2010 09:09:41 +0530
From: p6 c6d6 <[email-masked]>
Subject: [OSPF] type-2 cost plus 1
Sender: [email-masked]
To: [email-masked]
Errors-to: [email-masked]

hi,

In nssa rfc.

==
3.2 Translating Type-7 LSAs into Type-5 LSAs

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.
==

1 is added to the type-2 cost. can anyone explain me with a topology ?

thanks