OSPF Virtual-Link / Redistribution

Hello there, It's really funny how OSPF can give you a very hard time just playing around for a while. Let me shoot you guys an interesting topology that is getting me a little headache.

* R3 and R4 are in AREA 1

* R5, R6 are in AREA 1,0

* R8 is in AREA 0

R5#show ip ospf inter bri

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Fa0/1        1     0               58.0.0.5/24        10    BDR   1/1

Se0/0        1     0               56.0.0.5/24        64    P2P   1/1

Lo0          1     1               5.5.5.5/32         1     LOOP  0/0

Se0/1        1     1               35.0.0.5/24        64    P2P   1/1

R6#show ip ospf inter bri

Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C

Se0/0        1     0               56.0.0.6/24        64    P2P   1/1

Fa0/0        1     0               68.0.0.6/24        10    BDR   1/1

Lo0          1     0               6.6.6.6/32         1     LOOP  0/0

Fa0/1        1     1               46.0.0.6/24        10    DR    1/1

 

* R5 redistribute RIP (Lo99-99.99.99.99) into OSPF

 

R5#show run | s router

router ospf 1

 log-adjacency-changes

 redistribute rip subnets

router rip

 version 2

 network 99.0.0.0

 no auto-summary

 

                          R3-----------R4

                           |                 |

                           |                 |

(RIP-Redistribute) R5-----------R6

                                    |

                                    |

                                   R8

=============NO PROBLEM========================

At this point everything is working as expected and you can ping 99.99.99.99 from R3 through R3-R5 serial connection

R3#show ip route 99.99.99.99

Routing entry for 99.99.99.99/32

  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 64

  Last update from 35.0.0.5 on Serial0/1, 00:00:15 ago

  Routing Descriptor Blocks:

  * 35.0.0.5, from 5.5.5.5, 00:00:15 ago, via Serial0/1

      Route metric is 20, traffic share count is 1

 

R3#show ip cef 99.99.99.99

99.99.99.99/32, version 89, epoch 0, cached adjacency to Serial0/1

0 packets, 0 bytes

  via 35.0.0.5, Serial0/1, 0 dependencies

    next hop 35.0.0.5, Serial0/1

    valid cached adjacency

============PROBLEM=========================

IF YOU CREATE A VIRTUAL-LINK BETWEEN R3-R5 THE PING TO THE REDISTRIBUTED 99.99.99.99 FAILED. Why ?????

When the vitual-link is created the route to reach 99.99.99.99 changed to go all the way around R3-R4-R6-R5 and failed

R3#show ip route 99.99.99.99

Routing entry for 99.99.99.99/32

  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 40

  Last update from 34.0.0.4 on FastEthernet0/0, 00:07:26 ago

  Routing Descriptor Blocks:

  * 34.0.0.4, from 5.5.5.5, 00:07:26 ago, via FastEthernet0/0

      Route metric is 20, traffic share count is 1

R3#show ip cef 99.99.99.99

99.99.99.99/32, version 83, epoch 0, cached adjacency 34.0.0.4

0 packets, 0 bytes

  via 34.0.0.4, FastEthernet0/0, 0 dependencies

    next hop 34.0.0.4, FastEthernet0/0

    valid cached adjacency

==============================================

The database is not modified since the external route is the same with or without the virtual-link:

R3#show ip ospf database external

            OSPF Router with ID (3.3.3.3) (Process ID 1)

Type-5 AS External Link States

  Routing Bit Set on this LSA

  LS age: 495

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 99.99.99.99 (External Network Number )

  Advertising Router: 5.5.5.5

  LS Seq Number: 80000003

  Checksum: 0x718B

  Length: 36

  Network Mask: /32

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

 

External Route Tag: 0

 

Any answer that let me relax this weekend and don't crush my head ???

Comments

  • Here's my take - where does the ping fail - what happens to a traceroute?

    It not clear from your diagram which links are Ethernet and which are serial. Have you set any OSPF costs or is this left to the native bandwidths?

    When you add the virtual link on R3 it is now in area 0.  It looks like virtual link is using the path via R3-R4-R6-R5 as it's cheaper than between R3 and R5.  See what happens if you change the cost of the link between R3 and R5.

    So with the virtual link in place the forwarding metric is better via the multihop path.

    Try changing the OPSF cost on R3 for the interface towards R5 - make this less than 40 and see what happens

  • Hello my Welshydragon friend :), I knew that you would help me to enjoy this weekend jajajajaja.

    You are right, changing the metric in the serial connection works fine, but to be more accuared and know the root of the problem Daniel gave me a good advice as well. Disable capability-transit.

    Check it out

    https://learningnetwork.cisco.com/message/377349#377349

    Thanks anyway.... a really good thing in these forums is that there a lot of alternatives that you can learn in the way.

    See you around.

  • I would try to feedback INE because It's very painfull to insert just a picture in the post.

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