Explanation: The five types of OSPF packets are: Hello, Database description, Link-state request, Link-state update, and Link-state ACK. There is no Link-state response packet; the neighbor router sends a Link-state update packet as a response to the Link-state request packet if there is an update in the routing table.
The problem occurs most frequently when attempting to run OSPF between a Cisco router and another vendor's router. The problem occurs when the maximum transmission unit (MTU) settings for neighboring router interfaces don't match.
There are five types of OSPF areas: Backbone area (area 0), Standard area, Stub area, Totally stubby area, and No so stubby area (NSSA).
OSPF Neighborship RequirementsThe devices must be in the same area. The devices must have the same authentication configuration. The devices must be on the same subnet. The devices hello and dead intervals must match.
OSPF Router Types
- Internal Router. —A router with that has OSPF neighbor relationships only with devices in the same area.
- Area Border Router (ABR) —A router that has OSPF neighbor relationships with devices in multiple OSPF areas.
- Backbone Router.
- Autonomous System Boundary Router (ASBR)
OSPF packets are unicast on an NBMA network. P2MP—No link is P2MP type by default. P2MP must be a conversion from other network types such as NBMA. On a P2MP network, OSPF packets are multicast to 224.0.
The Summary (Type 3) LSA is used for advertising prefixes learned from the Type 1 and Type 2 LSAs into a different area. The Area Border Router (ABR) is the OSPF device that separates areas and it is this device that advertises the Type 3 LSA.
An OSPF area contains one set of link-state information, although each router within the area will process that information individually to form its own SPF tree. OSPF process IDs are locally significant and are created by the administrator. Router IDs uniquely identify each router.
In an OSPF broadcast network, OSPF elects one router to serve as the designated router (DR) and another router on the segment to act as the backup designated router (BDR). This minimizes the amount of repetitive information that is forwarded on the network. OSPF forwards all messages to the designated router.
Routers R1 and R2 have been elected as DR and BDR because they have the highest and the second highest router ID (100.0. 0.0 and 90.0.
The LSDB (Link State Database) is the database that OSPF builds and is based on the information that it has found in LSAs (Link State Advertisements). The LSDB is synchronized between routers within the same area.
Answers Explanation & Hints:Regardless of which OSPF area a router resides in, the adjacency database, routing table, and forwarding database are unique for each router. The link-state database lists information about all other routers within an area and is identical across all OSPF routers participating in that area.
OSPF uses two IP multicast addresses on broadcast and point-to-point networks: 225.0.0.5 for all OSPF routers and 224.0. 0.6 for all DR/BDR (designated router/backup designated router) routers. Using IP multicast addresses is more efficient than using broadcast addresses.
OSPF Type 4 ASBR Summary LSA
A type 4 LSA identifies the ASBR and provides a route to the ASBR. The link-state ID is set to the ASBR router ID. All traffic that is destined to an external autonomous system requires routing table knowledge of the ASBR that originated the external routes.OSPF is the most widely used but it is not the only choice. With that said, it is the most standardized IGP and that allows for optimal vendor interoperability. OSPF is primarily used for internal routing because it's a link-state routing protocol.
OSPF and BGP belong to the Application Layer.
Each OSPF router passes along information about the routes and costs they've heard about to all of their adjacent OSPF routers, called neighbors. OSPF routers rely on cost to compute the shortest path through the network between themselves and a remote router or network destination.
OSPF uses five message types:
- Hello.
- Database Description (DD)
- Link State Request.
- Link State Acknowledgement.
- Link State Update.
EIGRP vs OSPF: Scalability.The scalability of the OSPF is higher than EIGRP because EIGRP is complicated and vendor-specific and incapable of migrating from one vendor to the other. Conversely, OSPF is an open standard and simple protocol through which the network can be scaled easily.
Internet Engineering Task Force (IETF)
To enable OSPF on a Cisco router and advertise interfaces, the following tasks are required:
- Use the command router ospf process ID to start OSPF.
- Use the network command to enable the interfaces.
- Identify area assignments.
- (Optional) Assign the router ID.
If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
OSPF supports/provides/advantages –
- Both IPv4 and IPv6 routed protocols.
- Load balancing with equal cost routes for same destination.
- VLSM and route summarization.
- Unlimited hop counts.
- Trigger updates for fast convergence.
- A loop free topology using SPF algorithm.
- Run on most routers.
- Classless protocol.
LSA 4 is generated by the ABR. Link ID in that is the router ID of ASBR and Adv. Router is the ASBR itself. LSA 4 is required to tell the routers in other areas how to reach the ASBR to get to external network eventually.
LSA Types - Quick Overview
- LSA Type 1: OSPF Router LSA.
- LSA Type 2: OSPF Network LSA.
- LSA Type 3: OSPF Summary LSA.
- LSA Type 4: OSPF ASBR Summary LSA.
- LSA Type 5: OSPF ASBR External LSA.
- LSA Type 6: OSPF Group Membership LSA.
- LSA Type 7: OSPF Not So Stubby Area (NSSA) External LSA.
Full state is the normal operating state of OSPF that indicates everything is functioning normally. In this state, routers are fully adjacent with each other and all the router and network Link State Advertisements (LSAs) are exchanged and the routers' databases are fully synchronized.
OSPF supports complex networks with multiple routers, including backup routers, to balance traffic load on multiple links to other subnets. Routers form adjacencies when they have detected each other. This detection is initiated when a router identifies itself in a Hello protocol packet.
The OSPF protocol is a link-state routing protocol, which means that the routers exchange topology information with their nearest neighbors. The topology information is flooded throughout the AS, so that every router within the AS has a complete picture of the topology of the AS.
OSPF DR BDR ELECTION
- Router with the highest OSPF priority will become a DR. Router with the second highest OSPF priority or router ID will become a BDR (By default, all routers have a priority of 1)
- In case of a tie, Router with the highest router ID wins the election.
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