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OSPF ASBR Summary LSA Type 4

‎10-22-2016 08:06 AM

Hi Everyone,

Prepping for my JNCIS-ENT right now. Working through OSPF. Have a question about LSA Type 4. 

 

If you have a backbone Router connected to another AS(RIP,BGP,static routes etc..). Does it send out and an LSA Type 4 to it's Neighbors? If it does do the Neighbors add it to there OSPF Database?

 

I only ask because what I see when I do a "show ospf database" I see LSA 5's (External) but no LSA 4's (ABRsummary). Should I not see both. Here is a topology image if it helps any . R2 is the ASBR & R8 is exporting his loopback to R2 through eBGP. So should not R2 send Area 0 a LSA 4? Should R1,R3 & R4 have a ASBR Summary (LSA type 4) in their databases?
I am perplexed because I only see Externals and no ASBR Summary. Is this normal?  What is wierder is if I add R2 to another area (ie Area 2) it does have an LSA type 4 but only in Area 2.

 

Seems like there is no LSA type 4 in the Backbone... Is this normal in JUNOS?  Is this normal in most ospf implementations?

This is before adding Area 2 making R2 a backbone router and an ASBR

root@R2> show ospf database
OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 10.0.1.1 10.0.1.1 0x80000022 1126 0x22 0x8060 36
Router *10.0.1.2 10.0.1.2 0x8000001a 2552 0x22 0xe6c7 60
Router 10.0.1.3 10.0.1.3 0x80000017 1782 0x22 0xfab4 60
Router 10.0.1.4 10.0.1.4 0x8000001a 932 0x22 0xc9b 60
Network 10.0.0.1 10.0.1.1 0x80000008 757 0x22 0xa835 40
Summary 10.1.0.0 10.0.1.1 0x80000016 385 0x22 0x61b1 28
Summary 10.1.5.5 10.0.1.1 0x80000007 1862 0x22 0x28ec 28
Summary 10.4.0.0 10.0.1.4 0x8000000d 2220 0x22 0x4fc3 28
OSPF AS SCOPE link state database
Type ID Adv Rtr Seq Age Opt Cksum Len
Extern *10.200.0.0 10.0.1.2 0x8000000c 1014 0x22 0x1433 36
Extern 172.16.6.0 10.0.1.1 0x80000007 1495 0x22 0xc701 36

This is after adding it to Area 2 making it a backbone router, ABR and ASBR

root@R2> show ospf database
OSPF database, Area 0.0.0.0
Type ID Adv Rtr Seq Age Opt Cksum Len
Router 10.0.1.1 10.0.1.1 0x80000022 1136 0x22 0x8060 36
Router *10.0.1.2 10.0.1.2 0x8000001b 6 0x22 0xe7c4 60
Router 10.0.1.3 10.0.1.3 0x80000017 1792 0x22 0xfab4 60
Router 10.0.1.4 10.0.1.4 0x8000001a 942 0x22 0xc9b 60
Network 10.0.0.1 10.0.1.1 0x80000008 767 0x22 0xa835 40
Summary 10.1.0.0 10.0.1.1 0x80000016 395 0x22 0x61b1 28
Summary 10.1.5.5 10.0.1.1 0x80000007 1872 0x22 0x28ec 28
Summary *10.2.0.0 10.0.1.2 0x80000001 6 0x22 0x8b97 28
Summary 10.4.0.0 10.0.1.4 0x8000000d 2230 0x22 0x4fc3 28
OSPF database, Area 0.0.0.2
Type ID Adv Rtr Seq Age Opt Cksum Len
Router *10.0.1.2 10.0.1.2 0x80000001 6 0x22 0x21ea 36
Summary *10.0.0.0 10.0.1.2 0x80000001 6 0x22 0xa381 28
Summary *10.1.0.0 10.0.1.2 0x80000001 6 0x22 0x8f96 28
Summary *10.1.5.5 10.0.1.2 0x80000001 6 0x22 0x38e0 28
Summary *10.4.0.0 10.0.1.2 0x80000001 6 0x22 0x7da2 28
ASBRSum *10.0.1.1 10.0.1.2 0x80000001 6 0x22 0x80a1 28
OSPF AS SCOPE link state database
Type ID Adv Rtr Seq Age Opt Cksum Len
Extern *10.200.0.0 10.0.1.2 0x8000000c 1024 0x22 0x1433 36
Extern 172.16.6.0 10.0.1.1 0x80000007 1505 0x22 0xc701 36

Just trying to wrap my head around OSPF and that there is no lsa type 4 in the backbone is not what I expected. 

 

Post with 31 views. ASBR Summary
4 REPLIES 4
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Re: OSPF ASBR Summary LSA Type 4

‎10-22-2016 02:46 PM

So it appears to be normal. This article really explains it well.  

http://thenetworksherpa.com/ospf-what-really-triggers-type-4-lsa-origination/

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Re: OSPF ASBR Summary LSA Type 4

‎11-01-2016 10:20 PM

Before adding area 2, run this command>

root@R2> show ospf database extensive

Look at the forwarding address. Run said command after adding area 2 and check forwarding address in the Type 5. In your case R2 is the ABR/ASBR hence it generates the Tye5. However the Type is 4 is generated when the TYPE 5 is comining for from an ASBR in another Area. Since the standalone router was not in an Area, hence no Type 4 generated. However , even though is is not generated in every instance, it is only generated by the ABR as a result of a Type 5. It cannot be generated by itself without the presence of the Type 5. So your system is functioning as designed. 

[KUDOS PLEASE! If you think I earned it!
If this solution worked for you please flag my post as an "Accepted Solution" so others can benefit..]
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Re: OSPF ASBR Summary LSA Type 4

‎12-04-2018 08:17 AM

Hello,

I don't know if my response will still help ... I think OSPF LSA Type4 (ASBRSummary) role is to provide ASBR location in OSPF domain. So this  information  will only be present in area  other than which the ASBR is connected.  

   Let suppose  the below network graph

   OSPF LSA Type4 propagationOSPF LSA Type4 propagation

And let's considere 1,2, 3

1. Area 1 and area 2 are normal area

2. R1 propagate route to network A in OSPF domain

3. R4 propagate route to Nework A in OSPF domain

 ASBRSummary for R1 will appeared in area 0 & 2 while ASBRSummary for R4 will appeared in area 0 and 1. But both External routes (LSA Type 5)    network A  and  network B will appeared in all area.

besscou,
JNCIA-JunoS,JNCIS-SP
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Re: OSPF ASBR Summary LSA Type 4

‎12-05-2018 10:52 PM

Hi Ariel,

 

Find below detailed articles as to when does type 4 lsa get generated (generic) :

http://packetlife.net/blog/2008/apr/21/where-are-type-4-lsas-generated/

http://www.astorinonetworks.com/2011/06/06/understanding-ospf-type-4-lsas/

 

Type 4 LSA is basically to identify how to get to ASBR. where you are re-distributing external routes and where the ASBR is present will NOT have type 4 LSA. The ABR already has the entire network topology of both area’s that it borders. Therefore, when it sees the forwarding address of the type 5 external LSA is set to a RID of a router not in the area it is injecting the LSA, it knows it needs to inject a type 4 LSA as well.

 

-Manvita

 

 

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