© 2006 Cisco Systems, Inc. All rights reserved. BSCI v3.031 Configuring OSPF OSPF Packet Types

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF Packet Types

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF Packet Header Format

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Neighborship: The Hello Packet

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Establishing Bidirectional Communication

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Discovering the Network Routes

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Adding the Link-State Entries

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Router A notifies all OSPF DRs on DR notifies others on Maintaining Routing Information

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Sequence Numbering Each LSA in the LSDB maintains a sequence number. The sequence numbering scheme is a 4-byte number that begins with 0x and ends with 0x7FFFFFFF. OSPF floods each LSA every 30 minutes to maintain proper database synchronization. Each time the LSA is flooded, the sequence number is incremented by one. Ultimately, an LSA sequence number will wrap around to 0x When this occurs, the existing LSA is prematurely aged to the maximum age (one hour) and flushed. When a router encounters two instances of an LSA, it must determine which is more recent. The LSA having the newer (higher) LS a sequence number is more recent.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v LSA Sequence Numbers and Maximum Age Every OSPF router announces a router LSA for those interfaces that it owns in that area. Router with link ID has been updated eight times; the last update was 48 seconds ago. RTC# show ip ospf database OSPF Router with ID ( ) (Process ID 10) Router Link States (Area 1) Link ID ADV RouterAge Seq#Checksum Link count x xB x x3F44 2

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v debug ip ospf packet R1#debug ip ospf packet OSPF packet debugging is on R1# *Feb 16 11:03:51.206: OSPF: rcv. v:2 t:1 l:48 rid: aid: chk:D882 aut:0 auk: from Serial0/0/0.2 Debug of a single packet Shows fields in OSPF header

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary There are five OSPF packet types: hello, DBD, LSU, LSR, and LSAck. The Hello protocol forms logical neighbor adjacency relationships. A DR may be required to coordinate adjacency formations. The exchange protocol passes through several states (down, init, two-way, exstart, and exchange) before finally reaching the goal of full state. Full state means that databases are synchronized with adjacent routers.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary (Cont.) LSAs are sent on change but are also sent every 30 minutes to ensure database integrity. The maximum time that an LSA will stay in the database, without an update, is 1 hour. The LSA sequence number is incremented every time it is advertised. Each LSA in the LSDB has a sequence number, which is incremented by one each time the LSA is flooded. When a router encounters two instances of an LSA, it must determine which is more recent. The LSA having the newer (higher) LSA sequence number is more recent. Use the debug ip ospf packet command to verify that OSPF packets are flowing properly between two routers.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v