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Beginner

[PACKET TRACER]OSPF VLINK Inter-Area Routing - 1 Network Not Properly Advertising

Hello All,

PacketTracer_OSPF_VLink_Topology.PNG

Been working at this for hours and I'm not sure why the 20.0.0.0 network is not always being advertised to Router12 and Router13. 

On occasion when I run "show ip route" on Router12 and Router13 I see the 20.0.0.0 network, but most times it isn't there. All of the settings in "show run"/"show run | begin ospf" seem to be appropriately configured on each of the routers, but for some reason exclusively the 20.0.0.0 network isn't advertised to the last 2 routers; none of the other networks give the same problem. I was wondering if anyone could help me deduce why?

 

Thank you guys so much. I'm genuinely confused. Is this something that involves distance from the ABR and the LSAs displayed. i don't totally understand LSAs yet, but I'm working to figure them out. I just want to understand why that one network isn't being advertised for now.

 

Thanks again guys. I appreciate it.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: [PACKET TRACER]OSPF VLINK Inter-Area Routing - 1 Network Not Properly Advertising

Might be a bug on packet tracer. Although daisy-chaining virtual-links is not the best of options, it creates a contiguous area 0 tunnel.

 

I've gone through your packet tracer file, and in fact, when the ospf process is reloaded, sometimes, 30.0.0.1 will be considered an ABR for the OSPF process (under the show ip ospf border-routers output), that is where the route will be installed on the RIB, however, on both broken and working state scenarios, the required LSA type 1 for area 0 with the ABR information will be there, it will just not install it... (in addition to that, the clear ip ospf process command is also bogus on Packet Tracer, it doesn't cleanup the border-routers information every time).

 

To avoid going way to far on explanation, lets keep it simple:


* The 20.0.0.0/24 route, being a summary route (LSA3), will resolve its next hop by it using its closest ABR based on the OSPF Database information.


Who is the ABR originating this route?, it can be either 30.0.0.1 or 20.0.0.1. Based on the OSPF table information, 30.0.0.1 is the closest, however, the OSPF process, sometimes, it doesn't consider it an ABR.

 

Router#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 1 subnets
O       10.0.0.0/24 [110/4] via 40.0.0.1, 00:19:00, GigabitEthernet0/0
     20.0.0.0/24 is subnetted, 1 subnets
O IA    20.0.0.0/24 [110/3] via 40.0.0.1, 00:00:10, GigabitEthernet0/0
     30.0.0.0/24 is subnetted, 1 subnets
O IA    30.0.0.0/24 [110/2] via 40.0.0.1, 00:19:00, GigabitEthernet0/0
     40.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       40.0.0.0/24 is directly connected, GigabitEthernet0/0
L       40.0.0.2/32 is directly connected, GigabitEthernet0/0
     50.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       50.0.0.0/24 is directly connected, GigabitEthernet0/1
L       50.0.0.1/32 is directly connected, GigabitEthernet0/1

Router#show ip ospf border-routers 
OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 40.0.0.1 [2] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 0, SPF 2
i 60.0.0.1 [2] via 50.0.0.2, GigabitEthernet0/1, ABR, Area 0, SPF 2
i 30.0.0.1 [3] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 0, SPF 3  ----- It is considered a border router.
i 20.0.0.1 [4] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 0, SPF 4
i 40.0.0.1 [1] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 3, SPF 1
i 60.0.0.1 [1] via 50.0.0.2, GigabitEthernet0/1, ABR, Area 4, SPF 1

When the route is not displayed, it is because the 30.0.0.1 is not considered ABR.

But for what reason it could not be considered an ABR?  All it needs it is an LSA type 1 from Area 0 (where the tunnel is daisy chained) with the ABR flag on it:

(from #show ip ospf data router)  

LS age: 1555
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 30.0.0.1 ------------- 30.0.0.1 LSA Type 1
  Advertising Router: 30.0.0.1
  LS Seq Number: 80000003
  Checksum: 0x7a12
  Length: 48
  Area Border Router ----------------- It is an ABR
  Number of Links: 2

    Link connected to: a Virtual Link
     (Link ID) Neighboring Router ID: 20.0.0.1
     (Link Data) Router Interface address: 20.0.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Virtual Link
     (Link ID) Neighboring Router ID: 40.0.0.1
     (Link Data) Router Interface address: 30.0.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

The difficult part is that, in both broken and workin state, the database entry is always there, with the ABR flag, so I'm suspecting an issue with packet tracer processing that ABR status from time to time.

 

I would suggest to try reproducing this scenario on GNS3, Eve-ng or VIRL instead of PT because of its bugs and limitations.

 

4 REPLIES 4
Cisco Employee

Re: [PACKET TRACER]OSPF VLINK Inter-Area Routing - 1 Network Not Properly Advertising

Might be a bug on packet tracer. Although daisy-chaining virtual-links is not the best of options, it creates a contiguous area 0 tunnel.

 

I've gone through your packet tracer file, and in fact, when the ospf process is reloaded, sometimes, 30.0.0.1 will be considered an ABR for the OSPF process (under the show ip ospf border-routers output), that is where the route will be installed on the RIB, however, on both broken and working state scenarios, the required LSA type 1 for area 0 with the ABR information will be there, it will just not install it... (in addition to that, the clear ip ospf process command is also bogus on Packet Tracer, it doesn't cleanup the border-routers information every time).

 

To avoid going way to far on explanation, lets keep it simple:


* The 20.0.0.0/24 route, being a summary route (LSA3), will resolve its next hop by it using its closest ABR based on the OSPF Database information.


Who is the ABR originating this route?, it can be either 30.0.0.1 or 20.0.0.1. Based on the OSPF table information, 30.0.0.1 is the closest, however, the OSPF process, sometimes, it doesn't consider it an ABR.

 

Router#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 1 subnets
O       10.0.0.0/24 [110/4] via 40.0.0.1, 00:19:00, GigabitEthernet0/0
     20.0.0.0/24 is subnetted, 1 subnets
O IA    20.0.0.0/24 [110/3] via 40.0.0.1, 00:00:10, GigabitEthernet0/0
     30.0.0.0/24 is subnetted, 1 subnets
O IA    30.0.0.0/24 [110/2] via 40.0.0.1, 00:19:00, GigabitEthernet0/0
     40.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       40.0.0.0/24 is directly connected, GigabitEthernet0/0
L       40.0.0.2/32 is directly connected, GigabitEthernet0/0
     50.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       50.0.0.0/24 is directly connected, GigabitEthernet0/1
L       50.0.0.1/32 is directly connected, GigabitEthernet0/1

Router#show ip ospf border-routers 
OSPF Process 1 internal Routing Table

Codes: i - Intra-area route, I - Inter-area route

i 40.0.0.1 [2] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 0, SPF 2
i 60.0.0.1 [2] via 50.0.0.2, GigabitEthernet0/1, ABR, Area 0, SPF 2
i 30.0.0.1 [3] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 0, SPF 3  ----- It is considered a border router.
i 20.0.0.1 [4] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 0, SPF 4
i 40.0.0.1 [1] via 40.0.0.1, GigabitEthernet0/0, ABR, Area 3, SPF 1
i 60.0.0.1 [1] via 50.0.0.2, GigabitEthernet0/1, ABR, Area 4, SPF 1

When the route is not displayed, it is because the 30.0.0.1 is not considered ABR.

But for what reason it could not be considered an ABR?  All it needs it is an LSA type 1 from Area 0 (where the tunnel is daisy chained) with the ABR flag on it:

(from #show ip ospf data router)  

LS age: 1555
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 30.0.0.1 ------------- 30.0.0.1 LSA Type 1
  Advertising Router: 30.0.0.1
  LS Seq Number: 80000003
  Checksum: 0x7a12
  Length: 48
  Area Border Router ----------------- It is an ABR
  Number of Links: 2

    Link connected to: a Virtual Link
     (Link ID) Neighboring Router ID: 20.0.0.1
     (Link Data) Router Interface address: 20.0.0.2
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Virtual Link
     (Link ID) Neighboring Router ID: 40.0.0.1
     (Link Data) Router Interface address: 30.0.0.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

The difficult part is that, in both broken and workin state, the database entry is always there, with the ABR flag, so I'm suspecting an issue with packet tracer processing that ABR status from time to time.

 

I would suggest to try reproducing this scenario on GNS3, Eve-ng or VIRL instead of PT because of its bugs and limitations.

 

Beginner

Re: [PACKET TRACER]OSPF VLINK Inter-Area Routing - 1 Network Not Properly Advertising

This was one of the most informative posts I have ever read. Thank you a ton for it. 

I didn't quite understand what VirtualLinks really did (and am still no expert) but seeing, "daisy-chaining virtual-links is not the best of options, it creates a contiguous area 0 tunnel," I get it. I didn't realize that it actually extended the area per se. I thought it enabled only the neighboring router to connect to another Area it wouldn't ordinarily be able to communicate with. 

 

Hmmm... I don't know if he did, I'm watching Chris Bryant's Tutorials which are gold to me, but this is the first I've heard of the "show ip ospf border-routers output" so i'll be sure to explore it. I don't fully understand LSA types yet, but will be sure to read up on them more as well. Additionally, wow, I didn't even realize that such changes could take place regarding the ABRs. Guess that's part of PT's bugginess.

 

I'll definitely try to reproduce it in GNS3 and check my results.

 

Thanks a lot for the answer.

Enthusiast

Re: [PACKET TRACER]OSPF VLINK Inter-Area Routing - 1 Network Not Properly Advertising


Not sure if PT supports Virtual links ; not too long ago we could not do 2nd OSPF Area
I would use GNS3 for advanced labbing
and check if OSPF virtual links are on cCCNA topics
Beginner

Re: [PACKET TRACER]OSPF VLINK Inter-Area Routing - 1 Network Not Properly Advertising

Yea. It looks like the most modern version today supports it, albeit in a bit of a buggy fashion, but I'll definitely try to reproduce this in GNS3.

 

Thanks for the guidance indeed.

 

 

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