OSPF: 21 TRICKY QUESTIONS


QUESTIONS

QUESTIONS & SOLUTIONS


 

QUESTIONS

R1 ——- R2

On R1, we have process id of 100.  On R2: we have process id of 200. Will R1 & R2 form neighbor ship or not ?

 

In OSPF, Give one example of neighbor ship  &  give one example of adjacency ?

 

In ” sh ip ospf database “,  do we get topology of same area to which logged in device belongs  or, we get topology of complete OSPF network ?

 

LSA-3 is generated by which type of router ? ABR or ASBR ?

 

LSA-4 is generated by which type of router ? ABR or ASBR ?

 

In case of area type, NSSA, default route is already present or it has to be configured manually ?

 

In ” sh ip ospf int br ” shows OSPF enabled interfaces, which are physical, virtual or both ?

 

To configure any area as a STUB area, do we need to put stub area configuration on ABR, ASBR or all the routers of area ?

 

In OSPF states, 2-way state is normal or, a problem ? 

 

In Broadcast network type, DR & BDR maintains which type of relationship ?

 

In DR / BDR election, firstly which is elected DR or BDR ?

 

In ” sh ip ospf database ” output, what is the difference between fields, LINK id & ADV router ?

 

On a single router, if we have connected two different networks with different process id’s configured, will they communicate or not. If yes, what needs to be done, to communicate with each other ?

 

Redistribution is done in case of different protocols only or it can be done in case of same protocol also ?

 

How to check if Hello packets are transferring or not, between OSPF neighbors without using debug command ?

 

Can we check errors in OSPF, without using debug command ?

 

What do we mean by Link state in OSPF ?

 

How to check, if there is any NSSA area present in the OSPF network ?

 

In NSSA, external routes enters as LSA-7. But, if LSA-7 is not required, what is the command to stop LSA-7 ?

 

In NSSA, we have one ASBR & two ABR’s. which ABR will do LSA-7 to LSA-5 conversion ?

 

In NSSA, while receiving routes through LSA-7, can we receive selected subnets or stop few subnets from receiving ?


 

QUESTIONS  &  SOLUTIONS

R1 ——- R2

On R1, we have process id of 100.  On R2: we have process id of 200. Will R1 & R2 form neighbour ship or not ?

Ans.

OSPF process id is locally significant to router. It doesn’t matter if in a single area, there are ” n ” number of processes running. Process ID is just to create two instance on a single router. OSPF has nothing to do with area or anything.

Practical Scenario

when we acquire new company, that is going to be connected with current network. So, OSPF neighbour ship will be formed, but I don’t want all the current routes to be advertised to the new company, so process id is the solution with selective redistribution.

Reference Link

https://networkproxy.wordpress.com/2014/07/11/ospf-tshoot-understanding-process-id-topology/

https://networkproxy.wordpress.com/2014/07/14/ospf-designing-ospf-failover-avoiding-asymmetric-loop/

 

 

In OSPF, Give one example of neighbour ship  &  give one example of adjacency ?

Ans. 

Neighbors are routers who have exchanged  Hello packets, but not LSA information and don’t have same LSDB. Example : Two DROther routers.

Adjacent routers are routers who have exchanged Hello packet, LSA information and have same LSDB. Example: DR & BDR

Reference Link

https://networkproxy.wordpress.com/2014/02/04/difference-between-neighborship-and-adjacency/

 

In ” sh ip ospf database “,  do we get topology of same area to which logged in device belongs  or, we get topology of complete OSPF network ?

Ans.

Complete topology can be seen

 

LSA-3 is generated by which type of router ? ABR or ASBR ?

Ans.

ABR

 

LSA-4 is generated by which type of router ? ABR or ASBR ?

Ans.

ABR

 

In case of area type, NSSA, default route is already present or it has to be configured manually ?

Ans.

Default route has to manually configured

Command: Area 1 nssa default-information-originate

Reference Link

https://networkproxy.wordpress.com/2014/07/11/ospf-concepts-stub-area-insight/

 

In ” sh ip ospf int br ” shows OSPF enabled interfaces, which are physical, virtual or both ?

Ans.

Both, Physical & Virtual interfaces

 

To configure any area as a STUB area, do we need to put stub area configuration on ABR, ASBR or all the routers of area ?

Ans.

Every router has to be configured, as per STUB area configuration.

 

In OSPF states, 2-way state is normal or, a problem ? 

Ans.

2-way state is normal. This state exists in case of BROADCAST network type.

All the DROTHER routers stays in 2-way state with each other. DROTHER routers maintains full state with the DR.

 

In Broadcast network type, DR & BDR maintains which type of relationship ?

Ans.

Full state

 

In DR / BDR election, firstly which is elected DR or BDR ?

Ans.

BDR is selected first, then BDR becomes DR.

 

In ” sh ip ospf database ” output, what is the difference between fields, LINK id & ADV router ?

Ans.

ADV router  represents the RID of the router.

LINK id presents the prefix advertised.

Example

new#sh ip ospf database
OSPF Router with ID (204.79.34.216) (Process ID 1)

Type-5 AS External Link States
Link ID   ADV Router      Age Seq# Checksum Tag
0.0.0.0     204.79.34.216 1693 0x8000008B 0x00749C 1
1.1.1.0       204.79.34.216 1693 0x8000008B 0x0002DB 64513
2.3.4.4      204.79.34.216 1702 0x8000008B 0x009340 64513

 

On a single router, if we have connected two different networks with different process id’s configured, will they communicate or not. If yes, what needs to be done, to communicate with each other ?

Ans.

Two different processes on a single router do not communicate at all, until & unless we are doing some redistribution.

Reference Link

https://networkproxy.wordpress.com/2014/07/11/ospf-tshoot-understanding-process-id-topology/

https://networkproxy.wordpress.com/2014/07/14/ospf-designing-ospf-failover-avoiding-asymmetric-loop/

 

Redistribution is done in case of different protocols only or it can be done in case of same protocol also ?

Ans.

Redistribution is simply modification of parameters.

So, Redistribution can be done in different protocols or, within same protocol also, as per requirement.

In OSPF, generally, we perform redistribution between different processes running on a single router.

 

How to check if Hello packets are transferring or not, between OSPF neighbours without using debug command ?

Ans.

Use command: show ip ospf traffic

Example

4507#sh ip ospf traffic

Interface statistics:Interface Loopback0  

OSPF packets received/sent

Invalid Hellos DB-des LS-req LS-upd LS-ack Total

Rx: 0 0 0 0 0 0 0

Tx: 0 0 0 0 0 0 0

OSPF header errors

Length 0, Auth Type 0, Checksum 0, Version 0,

Bad Source 0, No Virtual Link 0, Area Mismatch 0,

No Sham Link 0, Self Originated 0, Duplicate ID 0,

Hello 0, MTU Mismatch 0, Nbr Ignored 0,

LLS 0, Unknown Neighbor 0, Authentication 0,

TTL Check Fail 0,

OSPF LSA errors

Type 0, Length 0, Data 0, Checksum 0

 

Can we check errors in OSPF, without using debug command ?

Ans.

Use command: show ip ospf traffic

Example

4507#sh ip ospf traffic

Interface statistics:Interface Loopback0  

OSPF packets received/sent

Invalid Hellos DB-des LS-req LS-upd LS-ack Total

Rx: 0 0 0 0 0 0 0

Tx: 0 0 0 0 0 0 0

OSPF header errors

Length 0, Auth Type 0, Checksum 0, Version 0,

Bad Source 0, No Virtual Link 0, Area Mismatch 0,

No Sham Link 0, Self Originated 0, Duplicate ID 0,

Hello 0, MTU Mismatch 0, Nbr Ignored 0,

LLS 0, Unknown Neighbor 0, Authentication 0,

TTL Check Fail 0,

OSPF LSA errors

Type 0, Length 0, Data 0, Checksum 0

 

In NSSA, external routes enters as LSA-7. But, if LSA-7 is not required, what is the command to stop LSA-7 ?

Ans.

Use command: router ospf 1 Area 1 nssa no-redistribution

Reference Link

https://networkproxy.wordpress.com/2014/07/11/ospf-concepts-stub-area-insight/

 

In NSSA, we have one ASBR & two ABR’s. which ABR will do LSA-7 to LSA-5 conversion ?

Ans.

ABR, which is having lower cost to the ASBR, will convert LSA-7 into LSA-5

Secondly, Router ID is compared, if condition leads to tie.

Reference Link

https://networkproxy.wordpress.com/2014/07/11/ospf-concepts-stub-area-insight/

 

In NSSA, while receiving routes through LSA-7, can we receive selected subnets or stop few subnets from receiving ?

Ans.

For example, you have a RIP learned route 141.108.10.0/24 that is injected into the OSPF NSSA Area 1.  You do not want this route to be leaked into the rest of the OSPF areas

Command

router ospf 1 summary-address 141.108.10.0 255.255.255.0 not-advertise

Reference Link

https://networkproxy.wordpress.com/2014/07/11/ospf-concepts-stub-area-insight/

 

What do we mean by Link state in OSPF ?

 

How to check, if there is any NSSA area present in the OSPF network ?

 

 

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