Re: Exit-Graceful Restart Condition

[Padma Pillay-Esnault <[e-mail]>]

ashok holla wrote:

>Hi,
>When myself and Sujay raised this issue, we thought about the
>possibility which Pama and Acee suggested.
>However, this will not work in all cases.
>
>Consider:
>
>TOPO------------------------ DUT---------------------------RTA
>          Area0                                Area1
>In this case, DUT will not get his pre-restart router of Area 1 as
>there no other router in the area to give it to him.
>Therefore, this solution fails. 
>In this case, suppose DUT had run the spf, he would have seen that the
>type-3 summary lsa he had advertised into area 0, is no longer
>consistent with the topology and then could have exited GR. However,
>this consistensy check can be done only after grace period terminates
>as RTA MAY come up within that time.
>So, for this case, there seems to be no solution.
>  
>
You have not taken into account one major premise in thr rfc -
For graceful restart to success you MUST have a helper to enable you to 
recover.
In the case above you have
1. A double failure
2. No helper in the area

With 2 - There can be no GR restart in the first place. Optimizing the 
detection is not possible if you do not even
 have the list of your previous neighbors.

>However, it does seem to be worthwhile to start deadTimers for all
>nbrs listed in the pre-restart router lsa when we get it for the first
>time.  (even though we have not received hello pkts from them). When
>any inactivity timer fires on the restarting router, he can quit GR.
>This can be done to impro convergence incase, topology is unstable.
>
>  
>
I would put that as an option rather than a "must".

Padma

>Is our understanding correct?
>
>regards,
>Ashok/Sujay
>
>
>On 5/25/05, Acee Lindem <[e-mail]> wrote:
>  
>
>>Padma Pillay-Esnault wrote:
>>
>>    
>>
>>>sujay wrote:
>>>
>>>      
>>>
>>>>Hi All,
>>>>
>>>>Considering RFC3623, Graceful Restart for OSPF V2 I have a problem;
>>>>
>>>>Ex:
>>>>
>>>> (network)---- DUT ---- RTA
>>>>
>>>>(All in the same area)
>>>>
>>>>Assume DUT undergoes a Graceful-Restart, it sends out grace LSA's to
>>>>RTA and other routers in the 'network'
>>>>
>>>>At the instant RTA recvs the Grace-LSA it also goes down.
>>>>
>>>>Is there a way in the RFC 3623 specified as to how DUT could Detect
>>>>this topology change and Exit GR *before* grace period expiry??
>>>>
>>>>
>>>>        
>>>>
>>>I do not think the RFC covers this case. You are in a double failure
>>>situation.
>>>
>>>I can think of a simple solution. When DUT reacquires its router-lsa.
>>>It can determine its previous
>>>neighbors. It can then decide that if within DeadRouterInterval that
>>>neighbor did not  respond
>>>to hellos to abort GR. This would be equivalent to the normal
>>>situation when a router goes down and its
>>>neigbors takes DeadInterval to bring down their adjacency
>>>      
>>>
>>I agree completely with Padma. One point, if RTA comes back up and
>>originates a new router LSA
>>then the DUT will detect the inconsistency (as specified in RFC 3623)
>>with it's pre-restart LSA and
>>terminate graceful restart.
>>
>>
>>    
>>
>>>Padma.
>>>
>>>      
>>>
>>>>For other routers in the network RTA is still reachable and traffic
>>>>Will be eventually dropped.
>>>>
>>>>Kindly correct me,
>>>>
>>>>Regds,
>>>>Ashok/Sujay
>>>>
>>>>
>>>>
>>>>        
>>>>
>
>
>  
>