CCIE Pursuit Blog

May 15, 2008

Question Of The Day: 15 May, 2008

Topic: OSPF

You are rolling out 10 Gigabit links across your core network.  You are running OSPF in your core.  You’ve noticed that the OSPF cost for the new 10 Gigabit links is 1.  This is the same as your FastEthernet and Gigabit links.  You are tasked with configuring the core routers so that the 10 Gigabit links will have a default OSPF cost of 10.

Click Here For The Answer


Yesterday’s Question

Question Of The Day: 14 May, 2008 

Topic: OSPF

r1 and r2 are running OSPF in area 0.  They have established a neighbor adjacency over a single serial interface.  Configure LSAs on this adjacency to set the DoNotAge bit.

Answer:

r1 (or r2)
interface s0/0
 ip ospf demand-circuit

By default, an LSA will be refreshed every 30 minutes regardless if there are any changes in the network topology.  This is sometimes referred to as a ‘periodic’ or ‘paranoid’ update.  You can disable this update with the ‘ip ospf flood-reduction’ command.  Another way that you can accomplish this is by configuring the link as an OSPF demand circuit.

ip ospf demand-circuit

Usage Guidelines
On point-to-point interfaces, only one end of the demand circuit must be configured with this command. Periodic hello messages are suppressed and periodic refreshes of link-state advertisements (LSAs) do not flood the demand circuit. This command allows the underlying data link layer to be closed when the topology is stable. In point-to-multipoint topology, only the multipoint end must configured with this command.

Before:
r2#show ip ospf database router 1.1.1.1 | i LS
  LS age: 3
  LS Type: Router Links
  LS Seq Number: 8000000C

After:
r1(config)#int s0/0
r1(config-if)#ip ospf demand-circuit

Real World Warning: This will reset your OSPF neighbor adjacency:

*Mar  1 00:14:11.271: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on Serial1/0 from FULL to DOWN, Neighbor Down: Interface down or detached
*Mar  1 00:14:15.051: %OSPF-5-ADJCHG: Process 100, Nbr 2.2.2.2 on Serial1/0 from LOADING to FULL, Loading Done

r2#show ip ospf database router 1.1.1.1 | i LS
  LS age: 5 (DoNotAge)
  LS Type: Router Links
  LS Seq Number: 8000000E
r2#show ip ospf interface s1/0

Serial1/0 is up, line protocol is up
  Internet Address 10.1.12.2/24, Area 0
  Process ID 100, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 64
  Run as demand circuit.
  DoNotAge LSA allowed. 
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:09
  Supports Link-local Signaling (LLS)
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 4 msec, maximum is 4 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1  (Hello suppressed)
  Suppress hello for 1 neighbor(s)

You only need to configure ‘ip ospf demand-circuit’ on one side of the link.  It will affect the DoNotAge bit on BOTH sides of the link (unlike the ‘ip ospf flood-reduction’ command):

r1#show ip ospf database router 2.2.2.2 | i LS
  LS age: 5 (DoNotAge)
  LS Type: Router Links
  LS Seq Number: 8000000C

This is good to keep in mind during a lab.  A task may ask you to set the DoNotAge bit on both sides on the link but restrict you to configuring only one side.

 

Advertisements

2 Comments »

  1. […] Click Here For The Answer […]

    Pingback by Question Of The Day: 14 May, 2008 « CCIE Pursuit — May 15, 2008 @ 6:09 am | Reply

  2. […] Question Of The Day: 15 May, 2008  […]

    Pingback by Question Of The Day: 19 May, 2008 « CCIE Pursuit — May 19, 2008 @ 9:00 am | Reply


RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Blog at WordPress.com.

%d bloggers like this: