Knowledge Base OSPF Packet Types

Interface MTU- Contains the MTU value of the outgoing interface. For virtual-links, this field is set to 0x0000. (2-bytes)

Options- Same as Options field in a Hello packet (1-byte)

I- Initial Bit. Indicates this is the first in the series of DBD packets (1-bit)

M- More bit. Indicates whether the DBD packet is the last in the series of packets. Last packet has a value of 0, while all previous packets have a value of 1. (1-bit)

MS- Master/ Slave bit. Master=1, Slave=0 (1-bit)

DD Sequence Number- Used to sequence the collection of DBD packets. The initial value should be unique. The sequence number then increments by 1 until the complete database description has been sent.(4-bytes)

LSA Header- This field contains the LSA headers describing the local router’s database. (variable length)

During the DBD packet exchange, a Master/ Slave relationship is established between the neighbors. The router with the highest Router ID becomes the Master and initiates DBD packet exchange. The Interface MTU should match on neighbors otherwise FULL adjacency is not reached.

Interface MTU Mismatch

19:24:22.879: OSPF: Rcv DBD from on Serial1/0 seq 0xA9A opt 0x52 flag 0x7 len 32 mtu 1400 state EXCHANGE

19:24:22.883: OSPF: Nbr has smaller interface MTU

19:26:01.591: %OSPF-5-ADJCHG: Process 1, Nbr on Serial1/0 from EXCHANGE to DOWN, Neighbor Down: Too many retransmissions

19:27:01.591: %OSPF-5-ADJCHG: Process 1, Nbr on Serial1/0 from DOWN to DOWN, Neighbor Down: Ignore timer expired

The following packet capture shows a sample OSPF database. The DBD packets contain LSA header which contain the summary of this database i.e the Type of LSA, Link State ID, Advertising router, etc. The neighbors then request further information about these LSAs using Link State Request packets.

OSPF Database

R1# show ip ospf database OSPF Router with ID ( (Process ID 1) Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

10.1.1. .1.1. x E 0x00C .1.1. .1.1. x x0031E5 3