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OSPF (Open Shortest Path First)
- A link-state routing protocol
- OSPF divides an AS into areas, defined as collections of networks, hosts, and routers.
- OSPF is a link state routing protocol that updates the routing table only when network changes occur, rather than at a predefined interval.
- Each router sends the state of its neighborhood to every other router in the area.
- OSPF supports variable length subnet masks (VLSM) and route summarization.
- OSPF routing tables are calculated by using Dijkstra’s algorithm.
- OSPF routers and links are grouped logically into areas that are identified by assigned numbers.
- All OSPF networks have at least one area with the default being area 0.
- If more than one area exists, area 0 is defined as the backbone area and is used to connect all other areas.
- Each area has its own link state databases.
- Stub areas are OSPF areas that have default routes, internal and inter-area routes, but do not flood external routes. A stub area has no routers or areas beyond it.
- Making Sense of OSPF Areas – some of the fundamentals of OSPF
- OSPF Design Guide
- Cisco IOS IP Command Reference, Volume 2 of 4: Routing Protocols, Release 12.3
- Cisco Routing Study Guide: Configuring OSPF in a Single Area
Different types of OSPF routers
|Internal Router||Responsible for maintaining a current and accurate database of subnets within the area. Forwards data to other networks using the shortest path.|
|Backbone Router||Has an interface connected to the backbone (Area 0)|
|Area Border Router (ABR)||Has interfaces in multiple areas with at least one interface in area 0. Connects other areas to the backbone and maintains routing information for each connected area.|
|Autonomous System Boundary Router (ASBR)||Router located between an OSPF autonomous system and a non-OSPF network. Used to redistribute routing information between networks. Must reside in a non-stub area.|
show ip ospf neighbor
A designated router (DR) is the router interface elected among all routers on a particular multiaccess network segment, generally assumed to be broadcast multiaccess. Special techniques, often vendor-dependent, may be needed to support the DR function on nonbroadcast multiaccess (NBMA) media. It is usually wise to configure the individual virtual circuits of a NBMA subnet as individual point-to-point lines; the techniques used are implementation-dependent.
Do not confuse the DR with an OSPF router type. A given physical router can have some interfaces that are designated (DR), others that are backup designated (BDR), and others that are non-designated. If no router is DR or BDR on a given subnet, the DR is first elected, and then a second election is held if there is more than one BDR
|BDR||Backup Designated Router|
A backup designated router (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with second highest priority at the time of the last election.
|process id||Internally used identification parameter for an OSPF routing process. It is locally assigned and can be any positive integer. A unique value is assigned for each OSPF routing process.|
|wildcard mask||A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly. A 255 indicates that you don’t care what the corresponding octet is in the network number. The wildcard mask is the inverse of the subnet mask The wildcard mask tells OSPF how many addresses are in the network|
|area #||Use area 0. (must have a backbone) OSPF routers and links are grouped logically into areas that are identified by assigned numbers. All OSPF networks must have at least one area with the default being area 0. If more than one area exists, area 0 is defined as the backbone area and is used to connect all other areas. Each area has its own link state databases. AREA terminology is created to provide a hierarchical structure and limit the multicast LSAs within routers of the same area — the default or otherwise core area is area 0 and all other areas connect directly to it|
- Configuring OSPF
- router ospf <process- id>
- Process-id: Internally used identification parameter for an OSPF routing process. It is locally assigned and can be any positive integer. A unique value is assigned for each OSPF routing process.
- Do not start more than one OSPF routing process on the same router.
- router id <id>
- network <network address> < wildcard mask> area <area #>
- wildcard mask : A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly. On the other hand a 255 indicates that you don’t care what the corresponding octet is in the network number.
- The “wildcard mask” is often referred to the “inverse mask”
- e.g. 126.96.36.199 0.255.255.255 area 0
- area #
- Example Commands
- Router(config)# router ospf 100
- Router(config-router)# network 10.0.25.0 0.0.0.255 area 0
- Router(config-router)# network 188.8.131.52 0.225.255.255 area 0
- router ospf <process- id>
To remove a network from OSPF
- no network <network address> <wildcard mask> area <area #>
- e.g. no network 184.108.40.206 0.255.255.255 area 0
OSPF loopback interface: Used to access the router if a physical interface is down.
Gateway of Last Resort (ip route 0.0.0.0 0.0.0.0 next-hop-ip/exit-interface)
- This command requires ip routing to be enabled.
- This command sets a default route for anything not in your routing table.
- After this command is entered it will show a “gateway of last resort” configured in your ip route table.
- Each OSPF router selects a router ID (RID) that must be unique on your network.
- It could be the highest IP address, or can be manually configured.
Best Practice (Manually Set the OSPF Router ID)
- It is considered best practice to always set the OSPF router-id manually, whether you happen to have a loopback or not.
- This is because there are certain OSPF functions that will break should the Router-ID ever change (an OSPF virtual link is a great example of this).
- If you rely on OSPF automatically choosing its Router-ID based on a loopback you have configured, there is no guarantee that the Router-ID wouldn’t change later (for example, you later needed another loopback for some other purpose, and this new loopback had a higher IP address than the first).
If you get into the habit of immediately setting the OSPF router-id as soon as you create the routing process, you will do yourself favors in the long run.
Expanding Lab-03 with Serial interfaces, Routers, and OSPF
- Due before 8pm Sat. 2021-10-30
- You must be in class this day, and submit a working simulation file, to receive full credit for completing this lab.
- Download the Lab-04 Assets from the Downloads page
- Important!!! Make sure to copy running-config startup-config BEFORE turning off a router to install the serial interface.
Read / Watch / Do
- Read Chapter :
- Do the Written Labs
- Answer the Review Questions
- Do not submit your answers for this chapter. The answers are in Appendix.