Subject: [Isis-wg] Questions on Attached Bit

From: "Suresh Boddapati" <[email-masked]>
Date: Wed, 19 Dec 2001 20:28:00

Hi,
I have a few questions on the usage of the Attached bit. As I understand it, 
the attached bit enables an L1 router to figure out who its closest L2 
router is.

1)Is it right to assume that the attached bit is meaningfuly only when set 
in L1 LSPs and not in L2 LSPs? If not, when would it be set in L2 LSPs?

2)Is the attached bit meaningful only in LSP Zero and not in any other LSPs? 
Section 7.2.9.2 of ISO 10589 says "regenerate its Level 1 LSP with LSP 
number zero" upon detecting a change in the attached bit status. But Section 
7.3.4.3 does not list the attached bit as one of the fields that are 
meaningful ONLY if present in LSP zero.

3) Technical Corrigendum 1 modifies subclause 7.2.9.2 and states
"...The IS considers itself attached if either
a) it can reach at least one other area using the corresponding routing 
metric or
b) it has at least one enabled reachable address prefix with the 
corresponding metric defined."

Can someone elaborate on b). Is it referring to at least one external prefix 
being advertised by the IS itself, due to say redistribution from other 
protocols?

Also, the requirement of being able to reach at least one other area is not 
clear. Consider the following example.

IS A(L1) ------IS B(L1L2)------IS C(L2)--------IS D(L1L2)-----IS E(L1)
Area X          Area X         Area X          Area X,Y       Area Y

IS A is an L1 router with manually configured area X and so on....

Even though IS B is connected to the L2 backbone, it will not advertise 
itself as attached (since it can reach only Area X) and therefore IS A 
cannot reach any system in Area Y as it will not install a default route 
pointing towards IS B. Is this a bad configuration? Am I missing something?

Thanks a bunch in advance.

Regards,

Suresh

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