CCIE Pursuit Blog

April 21, 2008

Question Of The Day: 21 April, 2008

Topic: OSPF

Give two methods of introducing a default route into an OSPF NSSA.

Click Here For The Answer


Yesterday’s Question

Question Of The Day: 18 April, 2008 

Topic: OSPF

Configure an OSPF dead-interval of 1 second on interface fa0/0 of r1.

Answer:  There are two ways to accomplish this.

Okay, I boned this question up due to being in a hurry as well as my own ignorance (a lethal combination).  I wrote the question thinking that the only answer would be to use OSPF Fast Hellos.  I assumed that the minimum OSPF dead timer that you could set without using Fast Hellos was 4 seconds.  I was wrong.

ip ospf dead-interval

r1(config)#int fa0/0
r1(config-if)#ip ospf dead-interval ?
  <1-65535>  Seconds
  minimal    Set to 1 second

So technically you can set the dead-interval to 1 second without using sub-second OSPF hellos.

Current OSPF Adjacencies:
r1(config-router)#do sh ip os nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/DR         00:00:36    10.1.12.2       FastEthernet0/0
r1(config-router)#do sh ip route os
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.1.12.2, 00:09:36, FastEthernet0/0

So let’s set our dead-interval to 1 second on both sides of the link:

r1(config)#int fa0/0
r1(config-if)#ip ospf dead-interval 1

r2(config)#int fa0/0
r2(config-if)#ip ospf dead-interval 1

Technically you will be able to form an adjacency, but it’s going to be pretty short-lived:

r1(config-if)#do sh ip os nei

r1(config-if)#do sh ip os nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   INIT/DROTHER    00:00:00    10.1.12.2       FastEthernet0/0
r1(config-if)#
*Mar  1 00:23:55.043: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on FastEthernet0/0 from INIT to DOWN, Neighbor Down: Dead timer expired
r1(config-if)#do sh ip os nei

r1(config-if)#

Your log is going to fill up with these error messages:

*Mar  1 00:24:55.027: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on FastEthernet0/0 from INIT to DOWN, Neighbor Down: Dead timer expired
*Mar  1 00:25:05.071: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on FastEthernet0/0 from INIT to DOWN, Neighbor Down: Dead timer expired
*Mar  1 00:25:15.075: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on FastEthernet0/0 from INIT to DOWN, Neighbor Down: Dead timer expired
*Mar  1 00:25:25.047: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on FastEthernet0/0 from INIT to DOWN, Neighbor Down: Dead timer expired
*Mar  1 00:25:35.071: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on FastEthernet0/0 from INIT to DOWN, Neighbor Down: Dead timer expired

Notice that the timestamp on each error shows that they’re 10 seconds apart?  Here’s why:

r1(config-if)#do sh ip os int fa0/0 | i Type|interval
  Process ID 100, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Timer intervals configured, Hello 10, Dead 1, Wait 1, Retransmit 5

We send a hello packet every 10 sends.  They neighbors try to establish the adjacency (that’s why we briefly saw the neighbor in an INIT state), but after 1 second OSPF is declaring the neighbor down due to the dead timer expiring.

So….technically you would be correct if you used ‘ip ospf dead-interval 1’ in that the dead-interval would be set to 1 second, but you’d never get the neighbor adjacency to establish unless you make the hello timer less than the dead timer.  To set the hello timer to less than one second you need to use OSPF Fast Hellos.

Configure this on both sides of the link:
r1(config)#int fa0/0
r1(config-if)#ip ospf dead-interval minimal hello-multiplier 4

r1(config-if)#do sh ip os int fa0/0 | i Type|interval
  Process ID 100, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Timer intervals configured, Hello 250 msec, Dead 1, Wait 1, Retransmit 5

This sets the dead timer to 1 second but also sends out 4 hello packets per second.  As you can see our adjacency is up and we’re learning OSPF routes:

r1(config-if)#do sh ip os nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/DR         964 msec    10.1.12.2       FastEthernet0/0
r1(config-if)#do sh ip route os
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/2] via 10.1.12.2, 00:05:28, FastEthernet0/0

 

 

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3 Comments »

  1. […] by Question Of The Day: 21 April, 2008 « CCIE Pursuit — April 21, 2008 @ 2:01 […]

    Pingback by Question Of The Day: 18 April, 2008 « CCIE Pursuit — April 21, 2008 @ 2:01 pm | Reply

  2. hey,
    nice suff around there ! “the question of the day” is a very nice feature !

    so about today, supposed that the nssa area is 1 and command run on a ABR, (for an ASBR, we need default route for the methode 1 work ):

    rtr03(config-router)#area 1 nssa ?
    default-information-originate Originate Type 7 default into NSSA area
    no-redistribution No redistribution into this NSSA area
    no-summary Do not send summary LSA into NSSA
    translate Translate LSA

    1 : rtr03(config-router)#area 1 nssa default-information-originate
    so the first method will propagate the a type 7 LSA into the nssa area.

    2 : rtr03(config-router)#area 1 nssa no-summary
    this command will advertise a type 3 LSA default route and remove the type 3/4 summary advertisement

    🙂

    Comment by lessaid — April 21, 2008 @ 6:38 pm | Reply

  3. […] Question Of The Day: 21 April, 2008  […]

    Pingback by Question Of The Day: 22 April, 2008 « CCIE Pursuit — April 22, 2008 @ 7:59 am | Reply


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