OSPF Interview Questions & Answer Part-7 | OSPF Optional Bits

Interview Questions & Answer:

• What is optional bit & totally how many optional bits?
• Optional bits are only in hello packet?
• Is it effect OSPF Neighbor relation of two routers, if optional bit mismatch in LSA?
• What will be the status of OSPF Neighbor, if optional bit mismatch in Hello?
• What is difference b/w N & P bit?
• What is demand circuit bit?
• Is MC bit is in LSA-6?
• Which bit is used to identify opaque LSA,s?
• How ABR will convert LSA-7 to LSA-5 in NSSA/
• How loop will avoid in OSPF MPLS VPN Scenario?
• What is Link-local signaling bit?
• Which bit was using for Quality of service (QoS) purpose in OSPF?

Optional field are in Hello, Database description (DBD) & all LSA with different reaction in each packet types.

Optional Bits in Hello Packet:

In hello packet with other parameters there is one Optional field 8 bits, if two nodes have different or mismatched optional bits then OSPF neighbor relation will not up & no routes sharing between two neighbors.

Optional Bits in DBD Packet:

When optional bit is mismatch in Database description packet then device will not forward some LSA, s to the neighbor node.

Optional Bits in LSA:

In LSA due the optional bit difference the SPF algorithm will not apply to create routing table on some specific routes.

Now let me explain all eight optional bits in details.

• DN Bit
• O Bit
• DC Bit
• L Bit
• NP Bit
• MC Bit
• E Bit
• MT Bit

DN Bit:

In MPLS MP-BGP Layer 3 VPN DN bit is used. When on PE router OSPF routes learn & advertised in BGP then the same routes will advertise to the OSPF domain again this time it will create loop, to avoid loop in this scenario DN bit must be ON. I will explain this scenario MPLS section in details.

O Bit:

O bit is using in Opaque LSA 9/10/11, when this bit is ON means router support Opaque LSA,s.

DC Bit:

In early days of Networking & transmission field the dial up demand circuit was using due to expensive link & no need to up all time.

The circuit which up only when required, in other word Demand circuit.

As we all know that OSPF are sending periodic hello packet & LSA, s to sync Database & routing table of Devices. Due to this the link was up almost all the time.

To achieve the Demand circuit feature, DC Bit was introduced & use in these type of link.

Due to DC bit the periodic hello packet & LSA, s refresh are suppressed.

L Bit:

As any communication in Network through OSPF is defined in some fixed packet format.

We cannot change the format of OSPF packet to send or receive our new introduced date as per requirement.

If in some situation the different or arbitrary data format is required then it will create issue through OSPF, to solve this issue tried to introduce new packet type etc. but finally one new optional bit is introduced called Link-local signaling ( L bit).It will send Hello & DBD packet.

NP Bit:

Here two bits are combined N and P using same space, as both are using in one special area type NSSA LSA-7.

N bit is used in hello packet; N bit is ON in all routers in NSSA area, when N bit is set then devices support LSA-7 & establish OSPF neighbor relationship.

P bit is in NSSA LSA-7 header. The main purpose of P bit is to give signal to ABR that convert LSA-7 to LSA-5 to advertise NSSA external routes to other area that I already explain in OSPF special area section with example.

In simple words N bit is represent that this area is NSSA & P bit is used to convert NSSA External routes LSA-7 to LSA-5 on ABR.

MC Bit:

As I already explained in LSA Types article that LSA-6 was assigned to carry multicast traffic in Network. Multicast Capable (MC Bit) is used that this device is support multicast extension of OSPF (MOSPF).

E Bit:

External bit is for external routing capability. If this bit is ON means that this device will accept LSA-5 external routes.

NOTE:

MT Bit:

This bit was assigned to represent multi-topologies & that the devices will support Type of service (TOS) feature on node.

Multi-topologies for different type of traffic like unicast, multicast, voice & date etc.