cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
775
Views
30
Helpful
11
Replies

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



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


this is precisely what i was looking for --> a stub link's entry in the LSA Type 2 data when you type that command. i would expect a stub to turn up along with transits since it's still a network :(

hi Martin thank you for responding. would this not yield a result showing TYPE 1 (router) LSA only? which it does on my lab, btw. as previously mentioned, this network was "..learned as "OSPF" routes most probably through LSA Type 1". i actually thought that because it shows up on this very command you suggested. but what im looking for is an LSA Type 2 (Network) entry.

paul driver
VIP Expert VIP Expert
VIP Expert

Hello

Could you try the following:
sh ip ospf database router | in Stub|(Link ID)


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

 

hi Paul, thank you for responding. i did that. it shows up (as expected) but only as Type1 LSA - coming from the "router" part of the command.

 

what i'm looking for is type2 when you issue a "sh ip ospf database network" command. 

Hello

 


@sujirou wrote:

 

hi Paul, thank you for responding. i did that. it shows up (as expected) but only as Type1 LSA - coming from the "router" part of the command.

 

what i'm looking for is type2 when you issue a "sh ip ospf database network" command. 


Why would you expect it to be a type2 LSA these LSA's are only shown for designated routers for the ospf area's
So a type 1 LSA would be for that network say in the bacbone area 0 (as shown in your OP) and if it was in a different area than the backbone the LSA for that networkwould become a type 3 summary or type 5 if redistibuted into ospf.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Martin L
VIP Advisor VIP Advisor
VIP Advisor

 

are you looking for specific format of output of database; looking for this Net Link States (highlight below)?

 

R1#sh ip ospf database

OSPF Router with ID (1.1.1.1) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1  1.1.1.1 5 0x80000002 0x0020B9 1
3.3.3.3  3.3.3.3 11 0x80000002 0x00B5C3 3

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum
10.0.13.3  3.3.3.3 11 0x80000001 0x00AD56

 

if so, you can only get it from sh ip ospf database - straight command on router, not from "remote" router. Getting info from perspective of R1 while R3 is DR, you need to do following:

step 1 show ip os databse to see Net type 2 and get Router ID that create it (DR), then , step 2, use sh ip ospf database router 3.3.3.3 to get more detailed info.  Link called Transit Network is LSA type 2 you looking for.  sh ip ospf database network output shows all attached routers to Type 2 LSA.

R3 is DR and connects to R1 ( BDR)  on 10.0.13.0/24; R1 shows 1 link count in Router Link States section.

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


R3 is DR and connects to R1 ( BDR) on 10.0.13.0/24; R1 shows 1 link count in Router Link States section. --let me clarify this just in case there is a confusion. OSPF has 11 LSA types; Router LSA type 1 has 4 link types: p2p, transit network, stub network, and virtual link. Type 2 Network LSA is listed by R1 as 1 link count in Router Link States section and then as Type 2 right after. This is called Transit Network - refers as pseudonode - as OSPF cannot show 2 and more routers connected to shared medium where election is held.

to see what links are under Link count (details); use sh ip ospf database router 1.1.1.1.  In my case of R1 has 1 link which is LSA type 2 listed as Transit network and connecting to R3

in case of R3, it has 2 stub networks (loopbacks) and Transit network connecting to R1.

Command sh ip os database network is used to show attached routers to type 2 , aka pseudonode

So, no, there is no LSA type 2 for stub links as stub is defined as dead-link which only has 1 router by osfp (no neighbors) and virtual link is special case.

 

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

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

 

 

Thank you, Sirs! all your responses are very well understood and very much appreciated! i know there was something i'm missing - that was to minimize data transfer across links as much as possible. perhaps additional overhead for router resources to have to sift through when in fact they offer little or no advantage to the overall performance as they would have when they were created. i just wanted to know the reason, and i think i got it. still a long way to go for me, but i'm getting there.  thanks to you, Good Sirs! hope y'all are safe and sound always.

warm regards and deepest respect!

jiro

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community:

Recognize Your Peers