Subject: [OSPF] Functionally equivalent as-external lsa

From: "Chitra Lakshmi Namadevan" <[email-masked]>
Date: Fri, 14 Jul 2006 10:40:19 +0530

Hi,

Functionally equivalent AS-external lsas are originated. Therefore the
lsa with higher advertising router id is installed and the other lsa is
flushed.

Topology:   

         dummy    dummy

         SW(*)    SW(*)

          |       |

+-----+   |       |   +-----+

|     |   |       |   |     |

|     +---+       +---+     |

|     |               |     |

|PSS9 +------ X ------|PSS11|

+-----+               +-----+

10.81.156.72          10.81.156.74

          3.3.3.0/24

 (*)dummy SW: only for up the Link of PSS9/PSS11.

Problem 1:

============

But the database of PSS11 has the AS-external lsa whose advertising
router is 10.81.156.72 along with the lsa advertised by 10.81.156.74.
This is the problem.

PSS9 sends a maxage lsa. When PSS11 receives this maxage lsa it should
flush the external lsa with advertising router 10.81.156.72. But it is
not flushed.

It is due to the following explanation found in RFC2328:

"If there is already a database copy, and if the database copy was
received via flooding and installed less than MinLSArrival seconds ago,
discard the new LSA (without acknowledging it) and examine the next LSA
(if any) listed in the Link State Update packet."

Therefore it discards the maxage lsa and so the lsa is not flushed and
it still remains.

Analysis:

PSS-11 discards the maxage lsa without any ls-ack. PSS-9 should not
flush the lsa and should send again a maxage lsa since it did not
receive a LS-ACK for the maxage lsa.

Kindly let me know if my analysis is correct.

Problem 2:

==========

Disconnect the cable and make the link up. Then the router with the
lower router id (10.81.156.72 - PSS9) has the as-external lsa advertised
by the higher router id (10.81.156.74 - PSS11). But it should have the
as-external lsa advertised by lower router id i.e. self lsa.

This is happening because the as-external lsa with higher router id is
not flushed. Therefore when PSS 9 originates the self as-external lsa,
it finds that functionally equivalent as-external lsa is still present
so it discards its self lsa.

Kindly let me know what mechanism can be followed to flush the
as-external lsa with higher router id when the neighbor is deleted.

Regards,

Chitra

**************** CAUTION - Disclaimer *****************
This e-mail contains PRIVILEGED AND CONFIDENTIAL INFORMATION intended solely for the use of the addressee(s). If you are not the intended recipient, please notify the sender by e-mail and delete the original message. Further, you are not to copy, disclose, or distribute this e-mail or its contents to any other person and any such actions are unlawful. This e-mail may contain viruses. Infosys has taken every reasonable precaution to minimize this risk, but is not liable for any damage you may sustain as a result of any virus in this e-mail. You should carry out your own virus checks before opening the e-mail or attachment. Infosys reserves the right to monitor and review the content of all messages sent to or from this e-mail address. Messages sent to or from this e-mail address may be stored on the Infosys e-mail system.
***INFOSYS******** End of Disclaimer ********INFOSYS***

_______________________________________________
OSPF mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/ospf

---

From: "Vishwas Manral" <[email-masked]>
Date: Fri, 14 Jul 2006 17:54:48 +0530

Hi Chitra,

> Problem 1:
Your analysis is correct, the LSA should not be removed if it is in
any of the neighbor lists.
If an Acknowledgement is not got the LSA needs to be retransmitted to
the neighbor.

> Problem 2:
If two routers, both reachable from one another, originate
functionally equivalent AS-external-LSAs (i.e., same destination, cost
and non-zero forwarding address), then the LSA originated by the
router having the highest OSPF Router ID is used. The router having
the lower OSPF Router ID can then flush its LSA.

Also note that any router with lower Router-ID CAN flush it s LSA, it
is an optimization to reduce the size of the OSPF DB.

Thanks,
Vishwas

On 7/14/06, Chitra Lakshmi Namadevan <[email-masked]> wrote:
>
>
>
>
>
> Hi,
>
>
>
> Functionally equivalent AS-external lsas are originated. Therefore the lsa with higher advertising router id is installed and the other lsa is flushed.
>
>
>
> Topology:
>
>          dummy    dummy
>
>          SW(*)    SW(*)
>
>           |       |
>
> +-----+   |       |   +-----+
>
> |     |   |       |   |     |
>
> |     +---+       +---+     |
>
> |     |               |     |
>
> But the database of PSS11 has the AS-external lsa whose advertising router is 10.81.156.72 along with the lsa advertised by 10.81.156.74. This is the problem.
>
> PSS9 sends a maxage lsa. When PSS11 receives this maxage lsa it should flush the external lsa with advertising router 10.81.156.72. But it is not flushed.
>
> "If there is already a database copy, and if the database copy was received via flooding and installed less than MinLSArrival seconds ago, discard the new LSA (without acknowledging it) and examine the next LSA (if any) listed in the Link State Update packet."
>
> Therefore it discards the maxage lsa and so the lsa is not flushed and it still remains.
>
> Analysis:
>
> PSS-11 discards the maxage lsa without any ls-ack. PSS-9 should not flush the lsa and should send again a maxage lsa since it did not receive a LS-ACK for the maxage lsa.
>
> Disconnect the cable and make the link up. Then the router with the lower router id (10.81.156.72 – PSS9) has the as-external lsa advertised by the higher router id (10.81.156.74 – PSS11). But it should have the as-external lsa advertised by lower router id i.e. self lsa.
>
> This is happening because the as-external lsa with higher router id is not flushed. Therefore when PSS 9 originates the self as-external lsa, it finds that functionally equivalent as-external lsa is still present so it discards its self lsa.
>
> Kindly let me know what mechanism can be followed to flush the as-external lsa with higher router id when the neighbor is deleted.
>
> Regards,
> Chitra
>