Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
New Member

Doubt on LSA updates

Hi,

Is there any difference between DROthers LSA updates and DR LSA updates( not in multicast address) ? In the context , whether DR also advertises as soon as it receives any updates from DROthers ?

14 REPLIES
Cisco Employee

Re: Doubt on LSA updates

The DR receives the LSAs from all of its adjacencies (on address 224.0.0.6) and floods them to all other adjacencies (on address 224.0.0.5).

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
New Member

Re: Doubt on LSA updates

Hi,

I am not aksing the mulicast address, but in the perspective of whether it sends immediately as soon as it receives.Also, does it send both Router-LSA and Network- LSA, whenver it sees any change ?Could you please explain it more.

Thanks,

Vijay

New Member

Re: Doubt on LSA updates

Hi ,

Kindly, someone explain for the above thread on OSPF.

Thanks,

Vijay

Purple

Re: Doubt on LSA updates

Hi Vijay,

A perusal of Section 13.3 of RFC2328 would probably answer your questions better but here's a summary:

* if a DR receives an LSA from the Backup DR, then it will wait to see LSAcks from all other routers on the segment (since all the DROthers should have already received that LSA from the Backup DR). If it does not receive an LSAck from any of these routers, it will unicast that LSA to those router after the re-transmission interval expires.

* If the LSA is received from a DROther, it is immediately flooded to AllSPFRouters.

The above procedure applies to *all* LSA types.

HTH,

Paresh.

New Member

Re: Doubt on LSA updates

Hi,

For your first point, LSAcks in this case are send as multicasts, what happens if BDR receives LSAck from a neighbour in that segment but not DR?(consider someway its missed in the link between the neighbour and DR).

Also could you please explain the five circumstances described in Table 19 'Sending link state acknowledgements' of RFC2328.

Thanks,

Vijay

Purple

Re: Doubt on LSA updates

Hi Vijay,

I've attached a file with my interpretation of table 19 from RFC2328. Hope that clears up your issues...

Regards,

Paresh.

New Member

Re: Doubt on LSA updates

Hi,

Thanks for your explanation.

Still, I have some doubt:

1)Statement :The only time a BDR will flood an LSA out the received interface is when it sees that the DR has not flooded it over the interface.

Dobut: How can a BDR flood LSA back out receiving interface , it can flood LSA only after it is promoted to DR ?

2)Statement:In this case the LSA was originally flooded onto this link by the BDR and then subsequently reflected by the DR. The BDR needs to send an LSAck so that the DR will take the LSA off the re-tx list for the BDR.

Doubt: Why does the DR expects Ack from BDR for the implied Acknowledgement ? But not from DROther ?

Thanks,

Vijay

Purple

Re: Doubt on LSA updates

Respones to your queries:

1. The only time a BDR will send an LSA back out the receiving interface is when the DR is not seen to have flooded it. There could be a couple of reasons why this is the case:

a. The LSA packet sent by the DR somehow got lost OR

b. The DR has died. This typically takes the routerDeadInterval to detect. The retransmission timer on the BDR will expire long before that so it will send the packet out. Note that this is a transitory state.

2. Whoops.. that was a typo. It should have read:

In this case the LSA was originally flooded onto this link by the BDR and then subsequently reflected by the DR. The BDR needs to send an LSAck so that the **DROthers** will take the LSA off the re-tx list for the BDR. This is because when a DROther receives an LSA, it will add it to the re-tx list for all adjacent neighbors, which for a DROther are the DR and the BDR. Since the LSA was sent by the DR, the only neighbor to whose re-tx list the LSA is added is the BDR. If the BDR does not send a delayed ACK, DROthers on the segment may start re-tx'ing the LSA to the BDR.

Hope that helps,

Paresh.

New Member

Re: Doubt on LSA updates

Hi,

Thanks for your indepth explanation.

I have one more doubt:

Consider, in a transit network, there are 5 routers(DR,BDR,DROther1,DROther2,DROther3).In that, DROTher2 goes down which is identified by DROther1 before DR or BDR.

1)In that case, DROther1 sends its new Router LSA to DR/BDR.But,Once the DR receives LSA,which LSA it sends first(Router or Network)?

2)Consider after DR received LSA from DROther1(modified for DROther2), DR also identifies DROther2 is dead.Whether it also sends Router LSA ? or How its prevented ?

Thanks

Vijay

Thanks,

Vijay

Purple

Re: Doubt on LSA updates

Here you go:

1. DROthers can only be neighbors of each other, not adjacencies. Therefore, when DROther2 goes down, there is nothing in DROther1's router-LSA that changes. Adjacencies to multiacess segments are identified by a transit link to the DR in router-LSAs, which has not changed. As a result, DROther1 does not have to send out a new router-LSA.

2. Like the previous case, nothing in the DR's router-LSA will change. However, it will originate a new network-LSA to signify that it has one less adjacency now.

Paresh.

New Member

Re: Doubt on LSA updates

Hi,

Once again thanks.

From your explanation,I got that DR won't send Router LSA to DROthers, it sends Router LSAs only to rest of the area.Correct me if I am wrong.

Thanks,

Vijay

Purple

Re: Doubt on LSA updates

Vijay,

Not sure if that's what I meant. If the DR's router-LSA changes for any reason, it will flood it to all neighbors. It's just that in the case you described, there was not going to be a change in the DR's router-LSA.

Paresh.

New Member

Re: Doubt on LSA updates

Hi,

consider that the DR also has a point-to-point connection to other router, in that case, if that router goes down,it should send new Router LSAs.So, it should also be sent to DROthers also.Correct me if I am wrong.

Thanks

Vijay

Purple

Re: Doubt on LSA updates

Yes, the DR will always flood to AllSPFRouters on LAN segments. Therefore, all routers on the LAN segment will get the flooded LSA.

Paresh.

211
Views
12
Helpful
14
Replies
CreatePlease to create content