OSPF Interview Questions & Answer Part-6 | OSPF Packet | Hello | LSA | Header
Summary Insights:
- OSPF Headers | OSPF Packet Header | Hello Packet Header | LSA Header.
- Our focus for today is to learn about OSPF packet, Hello & LSA header that will help us during OSPF troubleshooting & Interview Questions.
OSPF Packet Header:
OSPF packet is not carrying by TCP/UDP like other protocols, But ospf are directly encapsulated in IP Packet with protocol number is 89.The size of OSPF Packet header is 24 bytes.
OSPF header consists of below parameters.
- Version
- Type
- Packet Length
- Router ID
- Area ID
- Checksum
- AuType
- Authentication
Wireshark Packet capture of OSPF Header:
There are total five types of packet that mentioned in OSPF Header.
- Type-1 : Hello Packet
- Type-2 : DBD
- Type-3 : LSR
- Type-4 : LSU
- Type-5 : LSAck
If you want to learn about Packet type in details that I already explained before. Click Here OSPF Packet Types & States
Hello Packet Header:
The packet headers for all 5 types are same that’s why I will explain only Hello packet header.
Below attributes are in Hello packet.
- Network Mask
- Hello interval
- Optional bits
- Router priority
- Router Dead interval
- Designated Router
- Backup designated router
- Neighbor
Packet capturing screenshot
Here the important field in hello packet is optional bits for capability of OSPF protocol in different scenario. The eight Optional bits I will explain in my next post with complete details & usage purpose.
OSPF LSA Header:
As we have total 11 types of LSAs. Any router in OSPF domain should advertise one or more type of LSAs that’s depend the position/role of the device. But one router cannot advertise all 11 types of LSAs.
LSA Header contains below.
- Age
- Option
- LS Type
- Link state ID
- Advertising router
- Sequence Number
- Checksum
- Length
- The age field started the time from which the LSA is generated by the router.
- Option bit will discuss in next post.
- Type field represent type of LSA i-e router LSA-1,LSA2 etc.
- Sequence number is increasing when LSA is forwarded router by router its showing the latest of LSA, if the sequence number is not match then send LSR to neighbor & ask for the updated routes then LSU will be responded by peer.
Link State ID is depend on link type & LSA Type, for example if Router LSA is advertised in one area & link type is point to point then in this case Link state ID is Route ID of neighbor which advertised the LSA.
Below table is combination of Link & LSA Type with Link stat ID.
