Subject: Links included in routing calculation

From: Santhosh Ram <[email-masked]>
Date: Thu, 5 Sep 2002 00:13:03 +0530

Hi,
In the book "OSPF Anatomy of an internet routing protocol" in section 4.2.4
under the heading "LS Age Field" author says,

"After an LSA's originating router has failed, it can therefore take as long
as an hour for the LSA to be removed from other router's link-state database
Such an LSA is certainly advertising out-of-date information; however, OSPF
guarantees that the LSA will not interfere with the routing table calculation.
How? By requiring that a link be advertised by the routers at both the ends
of the link before using the link in the routing calculation"

Can someone please help me understand what the last line in the above quote
means ?

One more question I had is, when a router LSA is generated should the router
always advertise its router ID as a stub link ? If not, what are the cases
in which it should be advertised ? In the RT3's router-LSA for Area 1 given
in section 12.4.1.4 of RFC 2328 the router ID is not advertised in the router
LSA where as a router LSA in the above mentioned book(In section 4.3) does
include the router ID in the type 1 LSA.

thanks,
Santhosh.

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From: Acee Lindem <[email-masked]>
Date: Thu, 5 Sep 2002 10:27:24 -0400

> of the link before using the link in the routing calculation"

Hi Santhosh,

Yes. The LSAs have to have a bi-directional link (i.e., there must be a
full neighbor adjacency) in order to be used in the SPF calculation.

> include the router ID in the type 1 LSA.

Implicitly advertising the router ID as a stub link is an implementation choice.
It is not part of the specification. Since it increases the size of every
OSPF router's route table in the area, I'd opt against it. If an implementation
does choose implicit advertisement, the implementation should assure that
the router ID always corresponds to an interface address (so that the route
is valid).

--
Acee