OSPF

OSPF Neighbor Relationships

1. Determine your Router ID

• Hard-coded using the router-id command

• Highest active loopback IP address

• Highest active interface IP address

1. Add interfaces to the link state databases (network command)

2. Send hello message on chosen interfaces

• sends HELLO packet to multicast address 224.0.0.5 (All OSPF Routers)

• Once every 10 seconds on broadcast/p-2-p networks

• Once every 30 seconds on NBMA networks

1. Receive Hello

• Check hello / dead interval

• check netmasks

• check area id

• check authentication password

1. Send reply hello

• If listed as a neighbour in hello packet, reset dead timer

• if not, add as new neighbor

1. Master - Slave relationship determined

• Determined by priority, router-id breaks tie

• Master sends database description (DBD) first

• Slave sends DBD second

1. DBD are acknowledged and reviewed

• Slave requests details (link state requests LSR)

• Master sends updates (link state updates LSU)

• Master requests details (LSR)

• Slave sends updates (LSU)

1. Neighbors are synchronized

• Full State

• Now run Dijkstra SPF Algorithm

Router ID

1. Statically configured with router-id x.x.x.x command

2. Highest IP address on any active loopback interface

3. Highest IP address on any active physical interface

Router Types

• Internal Router - All interfaces in the same area

• Backbone Router - All interfaces in area zero

• Area Border Router (ABR) - At least one interface in area zero and one interface in another area
• Only place that can summarize internal routes
• Generates a default route for certain area types

• Autonomous system boundary router (ASBR) - Any router redistributing into OSPF
• Only place that can summarize external routes

OSPF Packets

• Type 1: Hello packet - This is used to establish and maintain adjacency with other OSPF routers.

• Type 2: Database Description (DBD) packet - This contains an abbreviated list of the LSDB of the sending router and is used by receiving routers to check against the local LSDB. The LSDB must be identical on all link-state routers within an area to construct an accurate SPF tree.

• Type 3: Link-State Request (LSR) packet - Receiving routers can then request more information about any entry in the DBD by sending an LSR.

• Type 4: Link-State Update (LSU) packet - This is used to reply to LSRs and to announce new information. LSUs contain several different types of LSAs.

• Type 5: Link-State Acknowledgment (LSAck) packet - When an LSU is received, the router sends an LSAck to confirm receipt of the LSU. The LSAck data field is empty.

Hello Packet

1. Router ID

2. Area ID *

3. Address of originating interface

4. Authentication Information *

5. Hello Interval *

6. Dead Interval *

7. Priority

8. DR/BDR/ IDs

9. Neigbour Router IDs

OSPF Metric

Single-area OSPF configuration

router ospf 1
	router-id 1.1.1.1
	network 192.168.1.0 0.0.0.255 area 0
	
	# Best practice is an exact match
	network 192.168.1.1 0.0.0.0 area 0
	
	#add everything, not recommended
	network 0.0.0.0 255.255.255.255 area 0

router ospf 1
	router-id 1.1.1.1
	
int fa1/0
	ip ospf 1 area 0

router ospf 1
	area 0
		interface gi 0/1/0/1
commit

show ip ospf neighbor

show ip route

show ip ospf

show ip ospf interface

Authentication

int Fa1/0
	ip ospf authentication message-digest
	ip ospf message-digest-key 1 md5 cisco

router ospf 1
	area 0
		interface gi 0/1/0/1
			authentication message-digest
	    message-digest-key 1 md5 cisco
commit