OSPF Flashcards
What characteristics do Link-state routing protocols have?
- They respond quickly to network changes.
- They send triggered updates when a network change occurs.
- They send periodic updates, known as link-state refresh, at long intervals, such as every 30 minutes.
What is a LSA?
LSA = (link-state advertisement)
- basic communication means of the OSPF routing protocol
- communicates the router’s local routing topology to all other local routers in the same OSPF area.
What is a LSDB?
LSDB = (link-state database)
- full picture of the network and it is built using all the link-state advertisements.
- Routers running link-state routing protocols collect routing information from all other routers in the network and then each router independently calculates its best paths to all destinations in the network, using Dijkstra’s SPF algorithm.
What are the drawbacks of large OSPF area?
- Frequent SPF algorithm calculations- routers spend many CPU cycles recalculating the SPF algorithm and updating the routing table if a change occurs.
- Large routing table–OSPF does not perform route summarization by default. If the routes are not summarized, the routing table can become very large, depending on the size of the network
- Large LSDB–Because the LSDB covers the topology of the entire network, each router must maintain an entry for every network in the area, even if not every route is selected for the routing table.
What type of area hierarchy does OSPF use?
OSPF uses a two-layer area hierarchy:
- Backbone area-an OSPF area whose primary function is the fast and efficient movement of IP packets.
- Regular (non backbone area)–an OSPF area whose primary function is to connect users and resources.
What is an Internal router?
- an internal router is a router that has all of their interfaces in the same area.
- All routers within the same area have identical LSDBs.
What is a Backbone Router?
a router that sits in the perimeter of the backbone area 0 and that has at least one interface connected to area 0.
What is an Area Border Router (ABR)?
- a router that has interfaces attached to multiple areas
- maintain separate LSDBs for each area to which they connect, and routes traffic destined for or arriving from other areas.
- ABRs connect area 0 to a nonbackbone area and are exit points for the area, which means that routing information destined for another area can get there only via the ABR of the local area. ABRs distribute this routing information into the backbone. The backbone routers then forward the information to the other ABRs. ABRs are the only point where address summarization can be configured. ABRs separate LSA flooding zones, and may function as the source of default routes. An area can have one or more ABRs.
What is an Autonomous System Boundary Router (ASBR)?
a router that has at least one interface attached to a different routing domain (such as another OSPF autonomous system or a domain using EIGRP). An OSPF autonomous system consists of all the OSPF areas and the routers within them. ASBRs can redistribute external routes into the OSPF domain and vice versa.
How is OSPF cost calculate?
*the default OSPF cost on Cisco routers is calculated using the formula
Cost=Reference Bandwidth / Interface Bandwidth.
*The reference bandwidth is a default value on Cisco routers which is a 100 Mbit interface. You divide the reference bandwidth by the bandwidth of the interface and you’ll get the cost.
The higher the bandwidth, the lower the cost.
What functions does OSPF perform?
- neighbor discovery, to form adjacencies
- flooding link-state information, to facilitate LSDBs being built in each router
- running SPF to calculate the shortest path to all known destinations
- populating the routing table with the best routes to all known destinations
What are the five OSPF packets?
- Hello: discover neighbors and builds adjacencies between them.
- Database Description (DBD): checks for database synchronization between routers.
- Link-state request (LSR): requests specific link-state records from another router.
- Link-state update (LSU): sends specifically requested link-state records.
- LSAck: acknowledges the other packet types
All five OSPF packets are encapsulated directly in an IP payload.
What is a Router ID?
*32-bit number that uniquely identifies the router. The highest IP address on an active interface is chosen by default unless a loopback interface address exists or the router ID is manually configured.
What is Router Priority?
*8-bit number that indicates a router’s priority. *Priority is used when selecting a DR and BDR.
What are the OSPF states?
- Down: no active neighbor detected
- Init: Hello packet received.
- Two-way: Router sees its own router ID in a received hello packet.
- ExStart: Master/slave roles determined.
- Exchange: DBDs (summary of LSDB) sent.
- Loading: Exchange of LSRs and LSUs, to populate LSDBs.
- Full: Neighbors fully adjacent
What are the three types of OSPF networks?
- Point-to-point: a network that joins a single pair of routers.
- Broadcast: a multiaccess broadcast network, such as Ethernet.
- Nonbroadcast multiaccess (NBMA): a network that interconnects more than two routers but that has no broadcast capability (ex. Frame Relay). There are five modes of OSPF operation available for NBMA networks.
What are the OSPF area types?
- Standard Area
- Backbone Area
- Stub Area
- Totally Stubby Area
- Not So Stubby Area
- Totally Not So Stubby Area
How are DRs and BDRs elected?
- The router with the highest priority value is the DR.
- The router with the second-highest priority value is the BDR.
- The default for the interface OSPF priority is 1. In case of a tie, the router ID is used. The router with the highest router ID become the Dr. The router with the second-highest router ID becomes the BDR.
- A router with a priority of 0 cannot become the DR or BDR. A router that is not the DR or the BDR is a DROTHER