Subject: Re: [OSPF] ASBR election

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Wed, 17 May 2006 05:53:10 -0700 (PDT)

Pradeep,

>The rule above only applies to AS external LSA (type 5s).

A similar rule applies to external type 7 LSAs. From RFC 3101, Section 
2.4.

   ...If two NSSA routers, both
   reachable from one another over the NSSA, originate functionally
   equivalent Type-7 LSAs (i.e., same destination, cost and non-zero
   forwarding address), then the router having the least preferred LSA
   should flush its LSA.  (See [OSPF] Section 12.4.4.1.)  Preference
   between two Type-7 LSAs is determined by the following tie breaker
   rules:

      1. An LSA with the P-bit set is preferred over one with the P-bit
         clear.

      2. If the P-bit settings are the same, the LSA with the higher
         router ID is preferred.

Note that Type 5 LSAs that are not translations of Type 7 LSAs must 
never have forwarding addresses from a stub area or NSSA. Similarly Type 
7 LSAs must always have forwarding addresses from their respective NSSA. 
Hence there should be no issues with a functionally equivalent Type 5 
and Type 7 LSA pair. When this occurs the Type 5 LSA should be a 
translation of the Type 7 LSA.

Pat

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From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Wed, 17 May 2006 09:21:59 -0700 (PDT)

>If we follow the behavior specified in RFC 3101, Section 2.4

>" NSSA border routers must originate an LSA for the default destination
>   into all their directly attached NSSAs in order to support intra-AS
>   routing and inter-AS routing.  This default destination is advertised
>   in either a Type-3 LSA or a Type-7 LSA, as described in Section 2.7.
>   The default LSA's metric should be configurable. For Type-7 default
>   LSAs, the metric type (1 or 2) should also be configurable."

>there could be multiple type-7 defaults which are functionally same

>This would lead to ECMP for the default route.

Normally a Type 7 default LSA originated by an NSSA ABR will have a 0.0.0.0 
forwarding address and, as such, is not pruned as described in Section 2.4. 
When the forwarding address of the default LSA is non-zero the above text 
would seem to indicate that pruning should not occur. Admittedly the two 
texts do conflict. However it is clear that when the forwarding address of 
the default LSA is non-zero only one is installed, per the tie breaker rule 
in Section 2.5 Step 6(e):

          (e) If the current LSA is functionally the same as an
              installed LSA (i.e., same destination, cost and non-zero
              forwarding address) then apply the following priorities in
              deciding which LSA is preferred:

                 1. A Type-7 LSA with the P-bit set.

                 2. A Type-5 LSA.

                 3. The LSA with the higher router ID.

Hence I can't see an ECMP (equal cost multi-path?) in this case. This LSA 
pruning only limits the effect on the size of the LSDB. It has no effect on 
the external route calculation of type 5 and type 7 LSAs.

Pat

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From: "Hasmit Grover \(hasmit\)" <[email-masked]>
Date: Wed, 17 May 2006 11:18:33 -0700

> -----Original Message-----
> From: Pat Murphy - (650)329-4044 [mailto:[email-masked]] 
> Sent: Wednesday, May 17, 2006 9:22 AM
> To: Hasmit Grover (hasmit)
> Cc: [email-masked]; Pradeep BG (pradg)
> Subject: Re: [OSPF] ASBR election
> 
> Normally a Type 7 default LSA originated by an NSSA ABR will 
> have a 0.0.0.0 forwarding address and, as such, is not pruned 
> as described in Section 2.4. 
> When the forwarding address of the default LSA is non-zero 
> the above text would seem to indicate that pruning should not 
> occur. Admittedly the two texts do conflict. However it is 
> clear that when the forwarding address of the default LSA is 
> non-zero only one is installed, per the tie breaker rule in 
> Section 2.5 Step 6(e):
> 
> Hence I can't see an ECMP (equal cost multi-path?) in this 
> case. This LSA pruning only limits the effect on the size of 
> the LSDB. It has no effect on the external route calculation 
> of type 5 and type 7 LSAs.
> 

I agree there won't be ECMP in case of default LSA with forwarding
address. 

I was referring to ECMP because normally type 7 default LSA originated
by an NSSA ABR will have 0.0.0.0 forwarding address.

- Hasmit

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From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Wed, 17 May 2006 11:36:19 -0700 (PDT)

>by an NSSA ABR will have 0.0.0.0 forwarding address.

The default LSAs of stub areas and NSSAs with multiple border routers may 
load balance unless network implementers manually set the metrics of these 
advertisements to be different. Nowadays, with symmetric flow such a big 
deal for the successful stateful analysis of security appliances, it is 
almost imperative that this be done. From a routing perspective there is no 
requirement to change the spec. It is my hope that security will take a 
completely different more distributed architectural approach someday. I have 
seen indications that this is happening.

Pat

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From: "Pradeep BG \(pradg\)" <[email-masked]>
Date: Thu, 18 May 2006 20:45:13 +0530

Area 0 is connected to 2/3 on the SS-3

  

                             2/1
2/1

 

INTERNET

 

Hi

GUYS

WE Are INTERLINKING THIS PROBLEM with another scenario

My question is very simple

Have 3 routers , 2 routers ss-2 and ss-1 they are in area 1(Normal area)
and both of the routers 2/1 link is connected to internet so they are
now ASBR 

have one more router SS-3 which is ABR for area 0 and area 1 , if the 2
routers learn a prefix say (88.88.88.0) , in the routing table of ss-3
what should be displayed

1:whether it should load balance across 2 routers to the destination ?

2:or there is election b/t ss-1 and ss-2 where one router is selected as
the best to forward

3:if yes what is the criteria for selecting the best router

Does RFC say anything for such a problem :-) ?

1:If all the routers are connected through a hub like broadcast access
in the area 1 

or

2: if there is a pt to pt link b/t SS-2 and SS-3 and SS-1 and SS-3

What will be the routing table in ss-3 for both the above questions (how
to reach)

Thanks

-BG

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From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Fri, 19 May 2006 10:26:52 -0700 (PDT)

Pradeep,

I suspect our diversion into the subtleties of the OSPF external route 

>Does RFC say anything for such a problem :-) ?

If the advertisements are of equal cost and functionally different, OSPF may 
install both. If it does, whether or not or how the traffic is load balanced 
across the two paths is unspecified. Most major implementations, without 
explicit configuration or traffic engineering to the contrary, will load 
balance.

>What will be the routing table in ss-3 for both the above questions (how

Here it sounds like you are implying equal cost paths from ss-3 to 88.88.88.0. 
Again, what is stated above is correct. If you want preference defined between 
the advertisements of ss-1 and ss-2 then you must tweak, via configuration, the 
advertisements' Type 1 or Type 2 metrics when these external routes, ala 
88.88.88.0, are imported into the OSPF environments' of ss-1 and ss-2.

Pat