Subject: Re: Network Mask 0 for Summary Net LSAs

From: John Pecola <[email-masked]>
Date: Thu, 1 Sep 2005 15:42:29 -0700

Acee,

If a type 3 LSA with LSId A.B.C.0 is received but with
the network mask of 0.0.0.0, shouldn't it be
considered a bad prefix-mask combination? Otherwise, a
0.0.0.0/0 route will be added corresponding to this
LSA.

Or, is appendix E applicable in this case also?

John

>Hi
>
>What should ospf do if it receives a type 3 summary
>LSA with a non-zero link-state id but a zero network
>mask? Should it be considered an invalid LSA or
should
>it be stored in the database but not used in route
calculation.
>  
>
>Hi John,
>
>OSPF allows the setting of the host bits in type 3
>LSIDs. What makes
you think the above is invalid?
>Thanks,
>Acee

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From: Acee Lindem <[email-masked]>
Date: Thu, 1 Sep 2005 19:28:09 -0400

>Or, is appendix E applicable in this case also?
Yes. In fact, there was a gated derivative that always advertised
an LSID of 255.255.255.255 for default.

Acee

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From: Acee Lindem <[email-masked]>
Date: Thu, 1 Sep 2005 19:50:12 -0400

> an LSID of 255.255.255.255 for default.

Actually, I looked at RFC 2328 and there are a couple places where
it explicitly states  that the default destination will have an LSID of 
DefaultDestination.
Also, I can see some advantage to NOT allowing a non-zero LSID for
default (irrespective of the version of gated that advertised 
255.255.255.255).

 From section 12.1.4:

            When an AS-external-LSA (LS Type = 5) is describing a
            default route, its Link State ID is set to
            DefaultDestination (0.0.0.0).

 From section 12.4.3.1:

                In a stub area, instead of importing external routes
                each area border router originates a "default summary-
                LSA" into the area. The Link State ID for the default
                summary-LSA is set to DefaultDestination, and the metric
                set to the (per-area) configurable parameter
                StubDefaultCost.  Note that StubDefaultCost need not be
                configured identically in all of the stub area's area
                border routers.

From section  12.4.4 
 
            AS-external-LSAs describe routes to destinations external to
            the Autonomous System.  Most AS-external-LSAs describe
            routes to specific external destinations; in these cases the
            LSA's Link State ID is set to the destination network's IP
            address (if necessary, the Link State ID can also have one
            or more of the network's "host" bits set; see Appendix E for
            details).  However, a default route for the Autonomous
            System can be described in an AS-external-LSA by setting the
            LSA's Link State ID to DefaultDestination (0.0.0.0).    

The main impetus for not allowing it is that one would not have to walk the
whole summary database for an summary or AS calculation for default (since
any LSID could be a valid candidate with one or more host bits set). Hence,
it seems to me we would be better off not allowing this case. 

Acee

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From: Acee Lindem <[email-masked]>
Date: Thu, 1 Sep 2005 19:54:27 -0400

Acee Lindem wrote:

> it explicitly states  that the default destination will have an LSID 
> of DefaultDestination.

If it isn't apparent, the above argument applies to incremental SPF 
calculation as
described in sections 16.5 and 16.6 of RFC 2328.