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OSPF LSA Type-1

Answered Question
Dec 17th, 2013
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I know that a LSA Type 1 is a Router LSA, and includes the information about itself, it's links, and the status of those links, to all other routers in the same area. My question is, the "links" on the LSA Type 1 for a Router, does that include only OSPF enabled interfaces or all interfaces with IP addresses?

Correct Answer by Peter Paluch about 3 years 8 months ago

Hi John,


I swear the day you sound stupid, is the day I jump off a high rise.


I do not recall anyone ever saying such a nice thing about me... Thank you from the bottom of my heart but please - I can sound stupid more easy than not. Do not rely on me that much


Now I believe the 'passive-interface gi0/0' command should not allow  hellos to be sent and received off of that interface, but it should  technically still be an OSPF-enabled interface.


In Cisco's OSPFv2, an OSPF-enabled interface is an interface covered by a network statement. It does not matter whether that interface is declared passive - in any case, it must fall under a configured network statement. The passive-interface disallows creating OSPF adjacencies over that interface but still allows that interface's network to be advertised. So if the interface belongs under some of the network statements, it will be included in the router's LSA1.


Your particular configuration snippet does not meet this requirement - there is no network statement covering the IP address 10.0.0.1 of the Gi0/0 interface. In this particular case, this interface would not be added to LSA1. If, however, you added, say, network 10.0.0.1 0.0.0.0 area 0 command then the 10.0.0.0/24 network would possibly be advertised in the LSA1.


I say possibly here because how the network would exactly be advertised depends on the topology. If the Gi0/0 is passive like in your example, it will always be advertised in LSA1 as a stub network. If the Gi0/0 was active but there was no other router on the segment, the network would be also advertised in LSA1 as a stub network. If, however, there was at least one more router on the segment, the network on the Gi0/0 interface would be advertised in the LSA2 representing that transit multiaccess network, and LSA1 would only indicate a link to the transit network by referencing the LSA2 of the DR.


Please feel welcome to ask further! The issues with LSAs often get cumbersome.


Best regards,

Peter

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Peter Paluch Tue, 12/17/2013 - 05:41
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Hello John,


I know that a LSA Type 1 is a Router LSA, and includes the information  about itself, it's links, and the status of those links, to all other  routers in the same area.


Quite correct. More precisely, exactly one LSA1 is generated for each router in an area (each router creates one LSA1 describing itself within an area), and a particular router's LSA1 describes its link to its directly connected objects in the network. They can be either


  • Point-to-point links to other routers
  • Links to transit networks
  • Stub networks
  • Virtual link (virtual adjacencies)


So an LSA1 does not describe links to all other routers in an area, rather, it describes links to all neighboring routers and other directly connected objects in an area as seen by the router originating this particular LSA1.


My question is, the "links" on the LSA Type 1 for a Router, does that  include only OSPF enabled interfaces or all interfaces with IP  addresses?


Only OSPF-enabled interfaces and links can be advertised in an LSA1.


Best regards,

Peter

JohnTylerPearce Tue, 12/17/2013 - 05:51
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Thanks for the excellent response as usually Peter. I swear the day you sound stupid, is the day I jump off a high rise.


Here is another question for you.


Let's say I have RouterA


RouterA has a link to another OSPF router and a directly connected gi0/0 link with 10.0.0.1/24


RouterA

-----------

router ospf 100

network 172.16.10.0 0.0.0.3 area 0 (Link to other OSPF RouteB)

pass int gi0/0


Now I believe the 'passive-interface gi0/0' command should not allow hellos to be sent and received off of that interface, but it should technically still be an OSPF-enabled interface.


So that "LINK" should still be include in LSA1's to other OSPF neighbors, even those it's configured to not send/receive hellos?


Just wanted to make sure I have that correct.

Correct Answer
Peter Paluch Tue, 12/17/2013 - 06:14
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  • Cisco Employee,

Hi John,


I swear the day you sound stupid, is the day I jump off a high rise.


I do not recall anyone ever saying such a nice thing about me... Thank you from the bottom of my heart but please - I can sound stupid more easy than not. Do not rely on me that much


Now I believe the 'passive-interface gi0/0' command should not allow  hellos to be sent and received off of that interface, but it should  technically still be an OSPF-enabled interface.


In Cisco's OSPFv2, an OSPF-enabled interface is an interface covered by a network statement. It does not matter whether that interface is declared passive - in any case, it must fall under a configured network statement. The passive-interface disallows creating OSPF adjacencies over that interface but still allows that interface's network to be advertised. So if the interface belongs under some of the network statements, it will be included in the router's LSA1.


Your particular configuration snippet does not meet this requirement - there is no network statement covering the IP address 10.0.0.1 of the Gi0/0 interface. In this particular case, this interface would not be added to LSA1. If, however, you added, say, network 10.0.0.1 0.0.0.0 area 0 command then the 10.0.0.0/24 network would possibly be advertised in the LSA1.


I say possibly here because how the network would exactly be advertised depends on the topology. If the Gi0/0 is passive like in your example, it will always be advertised in LSA1 as a stub network. If the Gi0/0 was active but there was no other router on the segment, the network would be also advertised in LSA1 as a stub network. If, however, there was at least one more router on the segment, the network on the Gi0/0 interface would be advertised in the LSA2 representing that transit multiaccess network, and LSA1 would only indicate a link to the transit network by referencing the LSA2 of the DR.


Please feel welcome to ask further! The issues with LSAs often get cumbersome.


Best regards,

Peter

JohnTylerPearce Tue, 12/17/2013 - 07:48
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Thanks for the help Ray, I appreciate it.


I have another quick question if you dont mind (sorry for the after question after).


I know exactly what 'soft reconfiguration inbound' is and what it does. But do you know where in

memory it's stored, and or if you can view it as a file to speak? I've tried looking for this answer for months lol.

Granted I've been doing ALL kinds of other studying for CCIEv5 new subjects.

Peter Paluch Tue, 12/17/2013 - 11:05
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Hi John,


Ehm... Ray?


I know exactly what 'soft reconfiguration inbound' is and what it does. But do you know where in memory it's stored, and or if you can view it as a file to speak?


Well, some books say that the per-neighbor database for soft reconfig is kept totally separate from the RIBs used by BGP (and obviously, it is in the router's RAM). However, I do not believe that is the case because that would be hugely inefficient (if we can speak about efficiency at all, considering the hack soft reconfig is). My personal feeling is that IOS keeps the received prefixes just once in memory but it maintains an indication whether the prefix was accepted or denied by an inbound policy, and it maintains a copy of the set of attributes as-received and another copy of attributes as-modified by the inbound policy. This way, you do not need to store the entire set of received routes twice - you remeber them only once, but you also keep a track on whether the route would be accepted or filtered, and if it was accepted, which attributes have changed and how, so that at all times, you know how the routes looked like before and after the inbound policy was applied.


The unfiltered set of routes from a particular neighbor can be show by show ip bgp neighbor X.X.X.X received-routes command. If no soft reconfig is configured for that neighbor, the command will merely tell you about that and quit.


Best regards,

Peter

JohnTylerPearce Tue, 12/17/2013 - 12:01
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I need to stop studying so much and sleep for a change lol....


Thanks for all the help again. It's weird that Cisco doesn't have this information in any of there documentation anywhere. YOu would think they would go "It's stored in two separate file in RAM, or its stored in one location blah" etc etc...

Peter Paluch Tue, 12/17/2013 - 12:29
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Hi John,


It's weird that Cisco doesn't have this information in any of there  documentation anywhere. YOu would think they would go "It's stored in  two separate file in RAM, or its stored in one location blah" etc etc...


Well, for most purposes, this is an implementation detail that does not really impact how the feature works. There is always a degree of uncertainty about how deep you should describe things you have implemented.


Best regards,

Peter

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