Ospf-transit-capability

image

 

Guys -

http://blog.internetworkexpert.com/2009/09/14/understanding-ospf-transit-capability/

I am trying to simulate ospf transit capability on  above design. I have create a loopback 5.5.5.0/25 on R5. I have setup a virtual link from R1-R2-R4 (Area 3 and Area 2) and setup a virtual link on R3-R4 as well. R1 has 3 link in AREA 0 , AREA 3 and AREA 14 (between direct link R1 and R4).

Now you can see that the best path to reach R5 loopback is gone thorugh R1 --> R4 (less hop count) however it is use AREA 3 (if active) or AREA 0 to reach R5 loopback. Please note I have advertised L1 : 1.1.1.0/24 in area 0 on R1 so if I shutdown the link S0/0 and S0/1 then R5 loopack is still show in routing table.

As per routing table on R1

R1#show ip ro
R1#show ip route 5.5.5.0
Routing entry for 5.5.5.0/24
  Known via "ospf 1", distance 110, metric 139, type inter area
  Last update from 192.168.12.2 on Serial0/1, 01:24:04 ago
  Routing Descriptor Blocks:
  * 192.168.12.2, from 4.4.4.4, 01:24:04 ago, via Serial0/1
      Route metric is 139, traffic share count is 1

Now I shutdown the link S0/0 and S0/1 , to make sure that I can still reach this prefix even two link fails.

R1(config)#int s0/0
R1(config-if)#sh
R1(config-if)#int s
*Mar  1 12:41:42.280: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on Serial0/0 from F
ULL to DOWN, Neighbor Down: Interface down or detached0/1
R1(config-if)#s
*Mar  1 12:41:44.264: %LINK-5-CHANGED: Interface Serial0/0, changed state to adm
inistratively down
*Mar  1 12:41:45.264: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0,
 changed state to downhut
R1(config-if)#
R1(config-if)#
*Mar  1 12:41:46.356: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on Serial0/1 from F
ULL to DOWN, Neighbor Down: Interface down or detached
R1(config-if)#
R1(config-if)#
*Mar  1 12:41:47.788: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on OSPF_VL1 from FU
LL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 12:41:48.344: %LINK-5-CHANGED: Interface Serial0/1, changed state to adm
inistratively down
*Mar  1 12:41:49.344: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/1,
 changed state to down
R1(config-if)#
R1(config-if)#
R1(config-if)#
R1(config-if)#do show ip route 5.5.5.0
Routing entry for 5.5.5.0/24
  Known via "ospf 1", distance 110, metric 129, type inter area
  Last update from 172.168.14.4 on Serial0/3, 00:00:53 ago
  Routing Descriptor Blocks:
  * 172.168.14.4, from 4.4.4.4, 00:00:53 ago, via Serial0/3
      Route metric is 129, traffic share count is 1

Now I up this interfaces and R5 loopback use AREA 3 to reach this prefix becuase of better cost (high BW). as per my understanding traffic always perfer AREA 0 to reach inter area route however after reading petr blog on tranist capability , it should go to R4 to reach R5 loopback (non-backbone area 14). Can anyone clear out my problem? By default capabilty transit is enabled so why it is still use AREA 0 or AREA 3 to reach R5 prefix?

The OSPF database for R5 loopback:

R1#show ip ospf database summary 5.5.5.0

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

                Summary Net Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 14 (DoNotAge)
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 5.5.5.0 (summary Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000001
  Checksum: 0xE8F3
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 65


                Summary Net Link States (Area 3)

  LS age: 228
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 5.5.5.0 (summary Network Number)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000011
  Checksum: 0x6961
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 75


                Summary Net Link States (Area 11)

  LS age: 162
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 5.5.5.0 (summary Network Number)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000002
  Checksum: 0x2875
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 139


                Summary Net Link States (Area 14)

  LS age: 162
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 5.5.5.0 (summary Network Number)
  Advertising Router: 1.1.1.1
  LS Seq Number: 80000002
  Checksum: 0x2875
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 139

  LS age: 1279
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(Network)
  Link State ID: 5.5.5.0 (summary Network Number)
  Advertising Router: 4.4.4.4
  LS Seq Number: 8000000E
  Checksum: 0xCE01
  Length: 28
  Network Mask: /24
        TOS: 0  Metric: 65                            (<<----  Better prefix via R4 in AREA 14)

Regards

Amit Chopra

 

 

 

 

Comments

  • Ok Guys , I change my topology a bit and seems ospf transit capability work :-

    New desgin

    image

    Now there is no area 3 between R1 and R2 but a extended AREA 2 among R1-->R2--->R4

    and I create a loopback 11 on R1 and put in area 11 :-

    router ospf 1
     router-id 1.1.1.1
     log-adjacency-changes
     network 1.1.1.1 0.0.0.0 area 0
     network 11.11.11.11 0.0.0.0 area 11
     network 172.168.14.1 0.0.0.0 area 14
     network 192.168.12.1 0.0.0.0 area 2
     network 192.168.13.1 0.0.0.0 area 0

    All game is play from R2 : - So the best path from 11.11.11.0/24 prefix from R2 is via R1 , there is a virtual link setup between R2-->R4 and ospf transit capability is on R2

    Here is routing table on R2 for 11.11.11.0/24

    R2#show ip route 11.11.11.0
    Routing entry for 11.11.11.0/24
      Known via "ospf 1", distance 110, metric 65, type inter area
      Last update from 192.168.12.1 on Serial0/1, 00:09:23 ago
      Routing Descriptor Blocks:
      * 192.168.12.1, from 1.1.1.1, 00:09:23 ago, via Serial0/1
          Route metric is 65, traffic share count is 1

    Now if I disabled (OFF) ospf transit capability on R2 , R2 will follow virtaul link (AREA 0) to reach this prefix means a long path :-

    R2(config)#router ospf 1
    R2(config-router)#
    R2(config-router)#no ca
    R2(config-router)#no capability t
    R2(config-router)#no capability transit
    R2(config-router)#
    R2(config-router)#do show ip route 11.11.11.0
    Routing entry for 11.11.11.0/24
      Known via "ospf 1", distance 110, metric 131, type inter area
      Last update from 172.16.24.4 on Ethernet2/0, 00:00:18 ago
      Routing Descriptor Blocks:
      * 172.16.24.4, from 1.1.1.1, 00:00:18 ago, via Ethernet2/0
          Route metric is 131, traffic share count is 1

    So it is work on R2 but on my first topology it was not work still seaching the reason why it is not work on my first scenario?

    Regards

    Amit Chopra

     

  • There must be an error in your configuration, imo.

    ASSUMING that the topology is configured like you say it is:

    1) If there is a vritual link from R1 to R4 across AREA 14

    2) If there is vritual link from R1 to R2 across Area 3 AND virtual link from R2 to R4 across Area 2

    3) The link between R1 and R4 is lowest cost

    Then R1 will use Area 14 to reach R5.

    Can't see it happening any other way, but someone feel free to correct me.

    Btw, you arean't testing real transit capability is this topology. This is just using virtual path directly to access inter-area path to R5.

  • Hi jkdrouter

     If there is a vritual link from R1 to R4 across AREA 14

    No There is no virtual link between R1 and R4 and if there is virtual link then sure it will use R4.

    Btw, you arean't testing real transit capability is this topology. This is just using virtual path directly to access inter-area path to R5

    The second topology has show the transit capability but not first not sure why?


  • I have setup a virtual link from R1-R2-R4 (Area 3 and Area 2) and setup a virtual link on R3-R4 as well. R1 has 3 link in AREA 0 , AREA 3 and AREA 14 (between direct link R1 and R4).

    Now you can see that the best path to reach R5 loopback is gone thorugh R1 --> R4 (less hop count) however it is use AREA 3 (if active) or AREA 0 to reach R5 loopback.

     

    I assumed that  you made a typo and ment R1-R4 in your first post, effectively making area 14 transit area by default (I don't see why you made virtual link between R3 and R4 as they are already in Area 0?).

    It's obvious now why router R1 won't use direct link to R4. R4 is advertising 5.5.5.0/24 lsa-3 intro area 14, but R1 has interfaces connected to Area 0 (both directly to R3 and via virtual link R1-R2-R4), so it MUST go trough area 0. Given that virtual link provides shorter intra path to R4, R1 will use that. When you shut the link to R2, R1 still has interface to Area 0 so it will go via R3. When you shut that link as well, now R1 is no longer abr just another router in area 14, it will use the only link it has to reach R4.

    So the only solution is to make virtual link between R1 and R4.

    As far as transit capability option goes... Lets assume you did make virtual link between R1 and R4 via Area 14. Let's also assume that in that Area 14 you have multiple paths/routers which connect R1 and R4 with the total cost LESS than the cost of direct serial link between R1 and R4.

    Without capability transit option enabled, the only way for R1 to reach nonbackbone area (5 in your case) via area 14 was to utilize virtual link and travel directly from R1 to R4 via direct serial link. WITH transit option enabled, however, R1 will use the lowest cost path in Area 14 to R4 wether or not the path is actually configured to belong to Area 0 (via virtual-link).

    HTH

     

  • Thanks mate , Your explanation is clear my doubts

    Thanks once again

    Cheers and take care

    Amit

Sign In or Register to comment.