Subject: [OSPF] ASBR election

From: "Pradeep BG \(pradg\)" <[email-masked]>
Date: Wed, 17 May 2006 12:45:17 +0530

Hi
 
Guys
 
I am stuck with a problem
 
1:If we are adversting the same route from 2 ASBR  What does RFC say for
this ? think it should be highest ip address  if yes can u tell me which
RFC says that and the section pleases :-)?
 
in SS-3 in the routing table which will be selceted as the best path
(the link cost to reach both the ASBR is same for SS-3)
 
if area 0 is connected let us assume it is conected to SS-3 area 0 is
getting updates from SS-3 , 

SS-1 ASBR 11.11.11.1
SS-3 ASBR 12.12.12.1

                           area1               area1
        (ASBR)SS-1----------------SS-3----------SS-2(ASBR)
                                             |
                                             |(Area 0)
                                             |
                                             |
                                          SS-4  
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From: Acee Lindem <[email-masked]>
Date: Wed, 17 May 2006 05:58:55 -0400

Hi Pradeep,

>RFC says that and the section pleases :-)?
 From RFC 2328, section 12.4.4.1:

                    In figure 16, suppose instead that both RTA and RTB
                    exchange BGP information with RTX.  In this case,           
                    RTA and RTB would originate the same set of AS-
                    external-LSAs.  These LSAs, if they specify the same
                    metric, would be functionally equivalent since they
                    would specify the same destination and forwarding
                    address (RTX).  This leads to a clear duplication of
                    effort.  If only one of RTA or RTB originated the
                    set of AS-external-LSAs, the routing would remain
                    the same, and the size of the link state database
                    would decrease.  However, it must be unambiguously
                    defined as to which router originates the LSAs
                    (otherwise neither may, or the identity of the
                    originator may oscillate).  The following rule is
                    thereby established: 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.  Flushing an LSA
                    is discussed in Section 14.1.

>getting updates from SS-3 , 
The rule above only applies to AS external LSA (type 5s). I'm not sure I
understand your scenario.

Hope this helps,
Acee

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