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NO LSA Type 2 For Stubs Links

sujirou
Beginner
Beginner

hey guys, i hope you all are doing well. just have a quick one. total noob here btw so my apologies if this seems dumb.

on my lab, as i follow one of Keith Barker's video (screenshot), upon verification on R1, i noticed that R4's network 10.0.4.0/24 (supposedly a LAN of its own) isn't included in R4's LSA Type2. Only networks directly connected to its interfaces fa5/0 (to R3) & gi2/0 (to R2) are created. i would expect stubs are included in "Network LSAs" considering they're technically "networks", are they not?  

nonetheless, they're learned as "OSPF" routes most probably through LSA Type 1. i'm definitely missing something here.

enlighten me, oh wise ones! :(

- jiro

Screen Shot 2020-07-14 at 6.16.24 AM.png


 

 

1 Accepted Solution

Accepted Solutions

Hello @sujirou ,

@Martin L is right there is no real advantage for OSPF to create an LSA type 2 for stub networks because there are no OSPF neighbours out there and no election of DR/BDR takes place.

The network LSA is needed if two or more OSPF routers share a common segment the OSPF network type is broadcast (the default for LAN interfaces) and the network LSA describes from the point of view of the DR and using as link-id the DR LAN ip address all the nodes connected to the common segment.

 

Just to be clear in modern networks in point to point links to avoid DR/BDR election and the creation of an unnecessary LSA type 2 is best practice to use ip ospf  network point-to-point (only two neighbors on the link).

 

So the expectation to see a stub network to generate its own network LSA is not correct. OSPF attempts to minimize and optimize the number of LSAs in the database and the network LSA for a stub network is useless as it provides no additional info as the list of nodes on segment is empty.

Stub networks are listed and advertised in router LSA, this is correct and sufficient to describe the topology within the area.

 

Hope to help

Giuseppe

 

View solution in original post

11 Replies 11

Martin L
VIP Advisor VIP Advisor
VIP Advisor

 

I think show ip ospf database router x.x.x.x   will show you what you looking for. try , let me know

 

Regards, ML
**Please Rate All Helpful Responses **

my example:

R1#sh ip ospf database router 3.3.3.3

OSPF Router with ID (192.168.0.41) (Process ID 1)

Router Link States (Area 0)

LS age: 547
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 3.3.3.3
Advertising Router: 3.3.3.3
LS Seq Number: 80000002
Checksum: 0xB5C3
Length: 60
Number of Links: 3

Link connected to: a Stub Network
(Link ID) Network/subnet number: 3.3.3.3
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1

Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.0.34.0
(Link Data) Network Mask: 255.255.255.0
Number of TOS metrics: 0
TOS 0 Metrics: 10

Link connected to: a Transit Network
(Link ID) Designated Router address: 10.0.13.3
(Link Data) Router Interface address: 10.0.13.3
Number of TOS metrics: 0
TOS 0 Metrics: 10


but if you do network adv-router R3 does not show whole story

R1#sh ip os da ne adv-router 3.3.3.3

OSPF Router with ID (192.168.0.41) (Process ID 1)

Net Link States (Area 0)

Routing Bit Set on this LSA
LS age: 536
Options: (No TOS-capability, DC)
LS Type: Network Links
Link State ID: 10.0.13.3 (address of Designated Router)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0xEB89
Length: 32
Network Mask: /24
Attached Router: 3.3.3.3
Attached Router: 192.168.0.41