Hi M,
That’s right, I don’t think you can still see it in the LSDB somewhere.
Rene
Thank you! Very easy to understand.
-Raul
Hi Rene,
As per RFC 2328 virtual links over unnumbered interfaces will fail.
I did 2 experiments on Virtual link creation.
1: experiment having numbered interfaces between nancy and donna which works fine.
2: Unnumbered interface between nancy and donna where Virtual link doesnt work.
Is there any method/way to create virtual links over unnumbered interfaces. Assume Nancy,donna and susan all are connected through unnumbered interfaces.
Thanks,
Hi Harmeet,
I haven’t tested this but from what I’ve read, it’s only possible if you have a route between the two ABRs that are configured for the virtual link.
Rene
1 Like
Hello Rene,
Thank you again for a great explanation on OSPF.
A quick questions, I hope – what if I had Area 3 hanging off of area 2 how would i create a virtual link to Area 0, I assume that would not be possible since we need at least one area a hop away from area 0?
Abdool,
Theoretically, this is possible, but only if you “cheat.” So, assume you have a network like this
R1–AREA 0–R2–AREA 1–R3–AREA 2–R4–AREA 3
Where R2, R3, and R4 border the areas shown.
Virtual links are performed between Routers using their Router-ID, and they must have an Area in common. The problem is that R4 and R2 have no area in common, so the virtual link will never come up. On the other hand, if you create a virtual link for R3 (which works fine using R2 and R3 as the endpoints), then try create a virtual link for R4 like this:
(config-router)#area 2 virtual-link 3.3.3.3
You will get this error:
%OSPF-4-ERRRCV: Received invalid packet: mismatched area ID, from backbone area must be virtual-link but not found
Basically, it is complaining that one end of the virtual-link is not terminating with an ABR connected to Area 0 (which is the definition of an ABR).
The only way around this is to trick R4 and R2 into believing they have a common link in Area 1. You can do this by creating a GRE tunnel between R4 and R2. This, however, has its own pitfalls in that you can easily get into a recursive loop, where you will get this error:
Midchain parent maintenance for IP midchain out of Tunnel0 - looped chain attempting to stack
Which means that the source or destination of the tunnel is being learned through the tunnel itself–which is impossible.
Regardless, if you are going through the trouble of creating a GRE tunnel into Area 1, why not just do it into Area 0 and forget about a virtual-link?
So the answer is: Technically it is possible, but why bother? 
1 Like
I agree. Why bother. I was just wondering if it was achievable. Thanks for the clear response.
Hi Rene,
2 questions:
- Will there be a neighbor FULL relationship between the end routers between whom we have established the virtual link ?
- Will there be a neighbor FULL relationship between the routers present in both the sides of the split area 0 ?
Hi Mansi,
When you establish a virtual link, both routers will show a full adjacency. Here’s an example:
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
192.168.23.2 0 FULL/ - 00:00:32 192.168.12.2 OSPF_VL0
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 0 FULL/ - 00:00:27 192.168.12.1 OSPF_VL0
Rene
1 Like
Renee
Hope you are well
I have had a good practice with virtual links I really like the concept and it seems to work, however in my lab I have a dilemma, well a sort of dilemma I have set up the example similar to your top example … (area 0 on far left, area 1 in middle, then area 2 on far right), but basically I am running a virtual link between area 0 and area 2 but I am also running my backbone router in area 0 via another area area 5 so I am running two virtual links to get to my other areas … where the dilemma comes into play is I am also doing summarization on my ABR’s again not a problem there in terms of actually doing the summarisation using the area range command with conventional subnet mask. But the problem is more of not knowing what SHOULD be done … because it actually works or seems to work in the lab. So in relation also to your example on summarisation which uses donna and nancy with areas 1 and 2 as well as area 0 see : https://networklessons.com/cisco/ccnp-route/how-to-configure-ospf-summarization/ … imagine that I am using virtual links to extend additional routers beyond areas 1 and 2 (extra hop) via virtual link to get to additional areas … which have been created 3 and 5. (area 0 via area 1 to area 3 & area 0 via area 2 to area 5) So, the summarisation I have applied (which incidentally is for discontigous subnets) is based on following logic:
On Area 0 ABR (directly adjoining Area 1 & 2): summarise area 1’s & 2’s subnets on this ABR (area 0 cannot be summarised presumably not allowed in the rules ??)
On Area 1 ABR (via VL) (adjoining Area 3): summarise area 3’s subnets on this ABR
On Area 2 ABR (via VL) (adjoining Area 5): summarise area 5’s subnets on this ABR
So, ALL ABR’s subnets get summarised but the way I saw it is that with the VL’s (virtual link’s) I would have to summarise area 3 and 5’s subnets on area 1 and 2 ABR’s respectively and NOT or I COULD NOT do this on ABR 0 … is this correct ?
many Thanks as always Will
Will,
I am trying to visualize your area layout, but I need you to confirm. Would it be something like this?
5
|
3
|
0 - 1 - 2
If this is the case, you will not be able to do summarization in areas 1 and 3, because any area through which a Virtual-Link is created is considered a transit area. Transit areas are not allowed to do summarization. I previously wrote a forum response on this (see below). You might find it useful.
https://networklessons.com/topic/how-to-configure-ospf-summarization/page/3/#post-23736
1 Like
Hi Rene,
Thank you for these 2 scenarios. They are really explained well !
Ahmad
Dear Andrew P
Many thanks for your quick repsonse.
I am not sure you understood my question correctly so hopefully this will help as below to clarify :-
R5
Area 3
ONLY
NO SUMMARIZATION
|
|
|
|
R4
Area 1 (+ 3)
ABR
SUMMARIZATION
OF AREA 3 ONLY
(VL end point
for area 3)
|
|
|
|
R3
Area 1
ONLY
NO SUMMMARIZATION
(VL Transit Router)
|
|
|
R1 ----------------- R2 ----------- R6 --------------- R7 -------------------- R8
Area 0 Area 0 (+ 1 + 2) Area 2 Area 2 (+ 5) Area 5
ONLY ABR ONLY ABR ONLY
NO SUMMMARIZATION SUMMARIZATION > NO SUMMARIZATION SUMMARIZATION > NO SUMMARIZATION
OF BOTH AREA 1 + 2 OF AREA 5 ONLY
(VL Origin Points x 2 (VL End Point
to reach Area 3 + 5) for area 5)
R1# sh ip route ospf | include IA : showed summaries (/16) for Area 1, 2, 3 + 5
R5# sh ip route ospf | include IA : showed summaries (/16) for Area 1, 2 + 5
R8# sh ip route ospf | include IA : showed summaries (/16) for Area 1, 2 + 3
===================================================================================================================
Main Points:
> Area 0 DIRECTLY interconnects two areas, area's 1 + 2 via its ABR
> VL's configured on this ABR to interconnect area 3 (using transit area 1) and to interconnect area 5 (using transit area 2)
> summarisation on ABR's as follows:-
>> R2 (area 0 main ABR adjoining area's 1 + 2) - summarises area 1 + 2 subnets
>> R4 (area 1 + 3 ABR) - summarises area 3 subnets
>> R7 (area 2 + 5 ABR) - summarises area 5 subnets
My Question:
Am I summarising on the correct area’s on the relevant (correct) ABR’s ?
-----------------------------------------------------------
Many Thanks
Will V
Diagram did not tranlsate on copy and past Andrew please see this one instead :-
R5
Area 3
ONLY
NO SUMMARIZATION
|
|
|
|
R4
Area 1 (+ 3)
ABR
SUMMARIZATION
OF AREA 3 ONLY
(VL end point
for area 3)
|
|
|
|
R3
Area 1
ONLY
NO SUMMMARIZATION
(VL Transit Router)
|
|
|
R1 -------------- R2
Area 0 Area 0 (+ 1 + 2)
ONLY ABR
NO SUMMMARIZATION SUMMARIZATION >
OF BOTH AREA 1 + 2
(VL Origin Points x 2
to reach Area 3 + 5)
|
|
|
R6
Area 2
ONLY
NO SUMMARISATION
|
|
|
R7
Area 2 (+ 5)
ABR
SUMMARIZATION >
OF AREA 5 ONLY
(VL End Point
for area 5)
|
|
|
R8
Area 5
ONLY
NO SUMMARIZATION
Attempt 3 ignore last two diagrams not sorry about this :-
R5
Area 3
ONLY
NO SUMMARIZATION
|
|
|
|
R4
Area 1 (+ 3)
ABR
SUMMARIZATION
OF AREA 3 ONLY
(VL end point
for area 3)
|
|
|
|
R3
Area 1
ONLY
NO SUMMMARIZATION
(VL Transit Router)
|
|
|
R1
Area 0
ONLY
NO SUMMMARIZATION
|
|
|
|
R2
Area 0 (+ 1 + 2)
ABR
SUMMARIZATION >
OF BOTH AREA 1 + 2
(VL Origin Points x 2
to reach Area 3 + 5)
|
|
|
R6
Area 2
ONLY
NO SUMMARISATION
|
|
|
R7
Area 2 (+ 5)
ABR
SUMMARIZATION >
OF AREA 5 ONLY
(VL End Point
for area 5)
|
|
|
R8
Area 5
ONLY
NO SUMMARIZATION
Will,
I drew out a diagram based on your description, and I think I have an understanding of how your network is laid out.
R2 is an ABR with Areas 0,1,3
R4 is an ABR with Areas 1,3
R7 is an ABR with Areas 2,5
You have a virtual link between R7 and R2 that transits Area 2
You have a virtual link between R5 and R2 that transits Area 1
You are summarizing Area 3’s network into Area 1
You are summarizing Area 5’s network into Area 6
Have I got all that right?
Assuming I do, I notice you asked this
Am I summarising on the correct area’s on the relevant (correct) ABR’s ?
Is that your question? Are you having unexpected results?
Andrew
Thanks for reply almost there this is how it should be:-
R2 is an ABR with Areas 0,1,2
R4 is an ABR with Areas 1,3
R7 is an ABR with Areas 2,5
You have a virtual link between R7 and R2 that transits Area 2
You have a virtual link between R4 and R2 that transits Area 1
You are summarizing Area 3’s network into Area 1
You are summarizing Area 5’s network into Area 2
Following is more understandable I think :-
R8 Area 5 ------- R7 Area 2 (+5) ----- R6 Area 2 -------- R2 Area 0 (1 + 2) ------- R3 Area 1 ------ R4 Area 1 (+3) --------- R5 Area 3
|
|
R1 Area 0
I don’t actually see any problems in the lab with regards to summarisation at least on the surface level … 
but my question is around whether I am summarising on the correct area’s (boundaries) and on the correct ABR’s. So, I presume I would not have to summarise ADDITIONALLY Area 5 and Area 3’s subnets on the Area 0 ABR … because Area 0 ABR (or the backbone ABR) will learn those summarises (area 3 and 5 subnet summaries) from its OSPF upstream neighbours following full convergence of the network ??? so presumably summarisation is always done on the network boundary ? So, in terms of the lab test I seem to see all summaries as being okay example being on ‘R1 area 0’ where I see the R3 and R5 summaries in addition to the Area 1 and Area 2 summaries.
Andrew - just move the R1 Area 0 to the right so it is underneath the ‘R2 Area 0 (1 + 2)’
You are correct. Summarization is only done on the network boundaries, and only ABRs are allowed to summarize an OSPF area. These summaries are then propagated throughout the OSPF domain (with some exceptions for Stub zones), so it is not necessary to summarize elsewhere.