ICND2 – Chapter 11 – OSPF

Chapter 11 was chest deep in the OSPF protocol and there was a lot to highlight. Here are my notes:

Hello Messages

OSPF starts with OSPF sending Hello messages out the configured interfaced. Each Hello lists the router’s Router ID (RID).

OSPF RIDs are 32-bit dotted decimal numbers similar to IP address.

Hello messages are protocol type 89.

Hello messages multicast to

Hello messages include:

Area ID
Hello interval
Dead interval
Router priority
RID of the designated router
RID of the backup designated router
List of the router’s known neighbors

When the router sees it own RID in a received Hello message and all the parameters are agreed upon it goes into a 2-WAY state.

In order for OSPF to form a neighbor relationship, both sides must agree on:

Subnet mask on the subnet
Subnet number
Hello interval
Dead Interval
Area ID
Authentication Check (if configured)
Value of Stub Flag

If any parameter is different then they will not become neighbors.

On Ethernet interfaces the default Hello interval is 10 seconds and default dead interval is 4 * Hello interval (40 seconds)

If OSPF had a neighbor but the interface fails then it will be listed in a Down state.

The INIT state means that the neighbor relation is transitioning.

OSPF Routers on a common subnet will elect a Designated (DR) and a Backup Designated Router (BDR). All routers in that subnet then will send their updates only to the DR and the BDR not all the other routers in the subnet. Non-DR and Non-BDR routers in this configuration will show in a DROTHER state.

When only 2 routers share a subnet they will become fully adjacent and change to a FULL state.

The DR concept prevents a subnet from being overloaded with OSPF traffic.

The router sending the Hello with the HIGHEST OSPF priority becomes the DR.
If their is a tie then the HIGHEST RID wins.
Priority setting of 0 means the router will never be the DR or BDR. Otherwise, priority range is from 1-255.
New DR/BDR candidates do not preempt the current DR/BDR.

For ICND2, 2 OSPF interface types are discussed: point-to-point and broadcast.

The router that creates an LSA is obligated to re-flood it every 30 minutes. (by default)
Each LSA has a separate timer.

OSPF choose the least cost route between the router and the subnet by adding up the costs of each interface in the path.

OSPF Area are used for large networks. (Typically greater than 50 routers and a few hundred subnets)

Area Border Routers (ABR) connect Area 0 and another area.

All Areas must have a connection to Area 0.

You can run multiple instances of OSPF on a router by using different process IDs.

OSPF Configuration

Router(config)# router ospf 1 (where 1 is the process ID and not an AS)
Router(router)#network area 0

Network statements use wildcard masks like ACLs.

Optional statements:

Router(router)#router-id (manually set RID)

Configured RID is used first. If not configured, OSPF will use the HIGHEST virtual (loopback) address as the RID. Highest physical if no virtual exists. OSPF considers UP/DOWN interfaces to be fair game for RID.

Router(router)#auto-cost reference-bandwidth 1000 (changes the calculation for cost metric from default 100 to 1000)

Router(config-if)ip ospf hello-interval 5 (set on interface to change default Hello interval)
Router(config-if)ip ospf dead-interval 25 (set on interface to change default dead interval)
Router(config-if)ip ospf cost 1 (set on interface to change default Cost)
Router(config-if)bandwidth 1000 (set on interface to set Bandwidth of interface)

OSPF Authentication (most secure method)

Router(config-if)#ip ospf authentication message-digest
Router(config-if)#ip ospf message-digest-key 1 md5 mykey

OSPF will load balance across up to 16 equal cost links. However, only 4 are enabled by default. Use the maximum-path 16 router subcommand to allow 16 to be used.

Main OSPF SHOW Commands

Router#show ip ospf neighbors (lists all neighbors)
Router#show ip ospf interface FA0/0 (list area, neighbors, and timers on that interface)
Router#show ip ospf neighbor (show details about that neighbor)


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