OSPF
OSPF is a vendor neutral open standard.
It establishes adjacencies and passes link state advertisements with other routers in an area.
Creates a link state database from the LSA's it receives.
Will choose what it believes to be the best path and injects it into the routing table.
LSA's
LSA is the information that a packet called an LSU (Link State Update) sends and receives.
Hello is used to discover OSPF neighbors and confirm reachability. It is used in election processes as well.
Link State Request is used to get specific information from a neighbor.
Link State Acknowledgement is used to confirm it received an LSU
Neighbors/Adjacencies
Neighbors exchange information and hello messages.
Adjacencies are neighbors but they exchange link state updates and database description (DD) packets.
To form a neighborship the following requirements must be met:
Matching area, authentication, subnet, timers, and stub flags.
LSA Types
Type 1: Router LSA contains information on directly attached networks.
Type 2: Network LSA is made for each transit network inside an area which a DR is elected.
Type 3: Summary LSA is sent from one area to another and is used to advertise a network in the source area.
Type 4: Summary ASBR LSA is created by an ABR to tell members of an area how to reach an ASBR
Type 5: AS External LSA is created by an ASBR to advertise networks imported from a different AS.
Type 6: OSPF Group LSA(Multicast) use PIM instead.
Type 7: NSSA SLA is sent from an ASBR into an NSSA to advertise networks from a different AS.
Type 8: OSPF external attributes and link local LSA (OSPFv3) used to carry BGP through OSPFv2. Sends IPv6 through OSPFv3.
Type 9: OSPF link scope opaque(v2) and intra-area prefix(v3), used to carry additional info with v2 and address info/network adverts in v3.
Type 10: Area scope opaque used to send additional info such as MPLS TE info.
Type 11: Same as Type 10 but not flooded into stubs.
Network Types and Elections
Areas
Metrics
Configuration
Network advertisements are done with wild card masks, and it does not explicitly advertise networks like BGP. It advertises that network if it has an interface that is configured for that network.
OSPF is configured in router configuration mode.
It requires a process ID. This ID is not required to be the same on each routing instance and is only locally important. Multiple routing instances (multiple OSPF processes with separate IDs) can run on the same router.
(conf)#router ospf "process id(positive integer)"(config-router)#network "network address" "wildcard mask" area "area id"
Auto-cost reference-bandwidth is configured in Mb/s. For reference 1Gb/s is 100000. This must be set on all routers within area to function properly.
Reference bandwidth can be changed with the following.
(config-router)#auto-cost reference-bandwidth "100000"NX-OS (Nexus) Configuration
On nexus devices you configure OSPF under the interfaces themselves instead of a network command under the OSPF process.
We will need to enable the OSPF feature.
(config)#feature ospf
We can then create the OSPF process and then add the interfaces to it.
(config)# router ospf "id"
(config-router)# default-information originate
Adding the interfaces. If it is not configured with "no switchport" you will want to do that as well as any other configurations such as IP.
(config)#int "int"
(config-if)# no switchport
(config-if)# ip address "address/cidr"
(config-if)# ip route ospf "id" area "area id"
(config-if)# no shut
This adds the interface to the ospf process.