Ask the Expert: Open Shortest Path First (OSPF)

[ciscomoderator]

With Vignesh R. P.

Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to learn and ask questions to Cisco expert Vignesh R. P. about how to configure and troubleshoot Open Shortest Path First (OSPF)

Open Shortest Path First (OSPF) is classified as an Interior Gateway Protocol (IGP).  This means that it distributes routing information between routers belonging to a single Autonomous System. The OSPF protocol is based on link-state or SPF technology. This is a departure from the Bellman-Ford base used by traditional TCP/IP internet routing protocols.

Vignesh R. P. is a customer support engineer in the Cisco High Touch Technical Support center in Bangalore, India, supporting Cisco's major service provider customers in routing and MPLS technologies. His areas of expertise include routing, switching, and MPLS. Previously at Cisco he worked as a network consulting engineer for enterprise customers. He has been in the networking industry for 8 years and holds CCIE certification in the Routing & Switching and Service Provider tracks.

Remember to use the rating system to let Vignesh know if you have received an adequate response. 

Vignesh might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation on the  Network Infrastructure sub-community discussion forum shortly after the event. This event lasts through through June 28, 2013. Visit this forum often to view responses to your questions and the questions of other community members.

[Karthi Sivalingam]

Hi Vignesh,

I have the doubt for the below commands.Could you please carify what is the use of below commands.If we enables below command what will be the advantage

timers throttle spf 5 20 5000

timers throttle lsa all 10 100 5000

timers lsa arrival 80

[Vignesh Rajendran Praveen]

Hello Karthi,

timers throttle spf 5 20 5000

SPF Throttling feature makes it possible to configure SPF scheduling in millisecond intervals and to potentially delay shortest path first (SPF) calculations during network instability. SPF is scheduled to calculate the Shortest Path Tree (SPT) when there is a change in topology. One SPF run may include multiple topology change events.

The interval at which the SPF calculations occur is chosen dynamically and is based on the frequency of topology changes in the network. The chosen interval is within the boundary of the user-specified value ranges. If network topology is unstable, SPF throttling calculates SPF scheduling intervals to be longer until topology becomes stable.

SPF calculations occur at the interval set by the timers throttle spf command. Here the start interval is set at 5 milliseconds (ms), the wait interval at 20 milliseconds, and the maximum wait time is set at 5000 milliseconds.

The wait interval between SPF calculations doubles when at least one topology change event is received during the previous wait interval. Once the maximum wait time is reached, the wait interval remains the same until the topology stablizes and no event is received in that interval.

If the first topology change event is received after the current wait interval, the SPF calculation is delayed by the amount of time specified as the start interval. The subsequent wait intervals continue to follow the dynamic pattern.

If the first topology change event occurs after the maximum wait interval begins, the SPF calculation is again scheduled at the start interval and subsequent wait intervals are reset according the parameters specified in the timers throttle spf command.

timers throttle lsa all 10 100 5000

This command controls the generation (sending) of LSAs. The first LSA is always generated immediately upon an OSPF topology change, and the next LSA generated is controlled by the minimum start interval (10). The subsequent LSAs generated for the same LSA are rate-limited until the maximum interval (5000) is reached. The "same LSA" is defined as an LSA instance that contains the same LSA ID number, LSA type, and advertising router ID.

Prior to the OSPF LSA Throttling feature, LSA generation was rate-limited for 5 seconds. That meant that changes in an LSA could not be propagated in milliseconds, so the OSPF network could not achieve millisecond convergence.

timers lsa arrival 80

This command controls the minimum interval for accepting the same LSA. If an instance of the same LSA arrives sooner than the interval that is set, the LSA is dropped. It is recommended that the arrival interval be less than or equal to the hold-time interval of the timers throttle lsa all command.

Hope this clarifies.

Thanks & Regards,

Vignesh R P

[Karthi Sivalingam]

Hi,

In OSPF one area to other area shoud be connect via backbone area0 or we can connect directly between the areas ,if we connect between area whether ospf route will be shared between the area or not.

[Vignesh Rajendran Praveen]

Hello Karthi,

For two non-zero areas to exchange routes between them, both need to be connected via area 0.

The reason behind this is that OSPF uses type 1 and type 2 LSAs to advertise routes within an area and uses type 3 LSA to advertise routes between areas. And the router will not generate any type 3 LSA unless it is connected to area 0.

Hope this clarifies.

Thanks & Regards,

Vignesh R P

[sukesh tandon]

hi vignesh

A lot of confusion going through my mind about the network types in OSPF.As I have read many articles about the NBMA network i am unable to make a clear picture in my mind.

For example  :   If i put network type non-broadcast in a LAN network then would it be considered as a NBMA network ...????

Also many other questions such as where exactly u put a difference between a NBMA and BMA...???

and is multiaccess segment called a broadcast network or whether by default it is a broadcast..???

Totally confused..please Help

Thanks in Advance

[Vignesh Rajendran Praveen]

Hi Sukesh,

If you use the network type Non-Broadcast on a LAN interface, it is treated as a NBMA interface. See the below example.

R2#sh run | b router ospf

router ospf 100

router-id 2.2.2.2

log-adjacency-changes

network 2.2.2.2 0.0.0.0 area 0

network 172.31.23.0 0.0.0.255 area 0

neighbor 30.30.30.30

!

R2#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

30.30.30.30       1   FULL/DR         00:01:59    172.31.23.3     FastEthernet2/1/1

R2#

R2#sh ip ospf interface fastEthernet 2/1/1

FastEthernet2/1/1 is up, line protocol is up

  Internet Address 172.31.23.2/24, Area 0

  Process ID 100, Router ID 2.2.2.2, Network Type NON_BROADCAST, Cost: 1

  Transmit Delay is 1 sec, State BDR, Priority 1

  Designated Router (ID) 30.30.30.30, Interface address 172.31.23.3

  Backup Designated router (ID) 2.2.2.2, Interface address 172.31.23.2

  Flush timer for old DR LSA due in 00:02:40

  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5

    oob-resync timeout 120

    Hello due in 00:00:07

  Supports Link-local Signaling (LLS)

  Cisco NSF helper support enabled

  IETF NSF helper support enabled

  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 0 msec, maximum is 0 msec

  Neighbor Count is 1, Adjacent neighbor count is 1

    Adjacent with neighbor 30.30.30.30  (Designated Router)

  Suppress hello for 0 neighbor(s)

R2#

If you understand Frame Relay & Ethernet, you would be able to understand the differences between  BMA & NBMA.

Thanks & Regards,

Vignesh R P

[mohankrishna2012]

Hi Vignesh,

Can you kidly explain the operation and purpose of the below command

ip ospf demand-circuit

Regards

Mohan