OSPF Flashcards
1
Q
What characteristics do Link-state routing protocols have?
A
- 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.
2
Q
What is a LSA?
A
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.
3
Q
What is a LSDB?
A
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.
4
Q
What are the drawbacks of large OSPF area?
A
- 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.
5
Q
What type of area hierarchy does OSPF use?
A
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.
6
Q
What is an Internal router?
A
- 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.
7
Q
What is a Backbone Router?
A
a router that sits in the perimeter of the backbone area 0 and that has at least one interface connected to area 0.
8
Q
What is an Area Border Router (ABR)?
A
- 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.
9
Q
What is an Autonomous System Boundary Router (ASBR)?
A
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.
10
Q
How is OSPF cost calculate?
A
*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.
11
Q
What functions does OSPF perform?
A
- 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
12
Q
What are the five OSPF packets?
A
- 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.
13
Q
What is a Router ID?
A
*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.
14
Q
What is Router Priority?
A
*8-bit number that indicates a router’s priority. *Priority is used when selecting a DR and BDR.
15
Q
What are the OSPF states?
A
- 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
16
Q
What are the three types of OSPF networks?
A
- 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.
17
Q
What are the OSPF area types?
A
- Standard Area
- Backbone Area
- Stub Area
- Totally Stubby Area
- Not So Stubby Area
- Totally Not So Stubby Area
18
Q
How are DRs and BDRs elected?
A
- 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
19
Q
What is the default behavior of OSPF over NBMA interfaces?
A
by default, OSPF cannot automatically build adjacencies with neighbor routers over NBMA interfaces.
20
Q
What are the two official modes for OSPF in NBMA topologies?
A
- Nonbroadcast–the nonbroadcast (NBMA) mode simulates the operation of OSPF in broadcast networks. Neighbors must be configured manually, and DR and BDR election is required. This configuration is typcially used with full-mesh networks.
- Point-to-multipoint–this mode treats the nonbroadcast network as a collection of point-to-point links. In this environment, the routers automatically identify their neighboring routers but do not elect a DR and BDR. This configuration is typically used with partial-mesh networks.
The choice between nonbroadcast and point-to-multipoint modes determines how the Hello protocol and flooding work over the nonbroadcast network.
21
Q
What are the default OSPF over Frame-Relay modes?
A
- the default OSPF mode on a point-to-point Frame Relay subinterface is the point-to-point mode.
- the default OSPF mode on a Frame Relay multipoint subinterface is the nonbroadcast mode.
- the default OSPF mode on a main Frame Relay interface is also the nonbroadcast mode.
22
Q
What are the LSA types?
A
1 -- Router LSA 2 -- Network LSA 3 & 4 -- Summary LSAs 5 -- Autonomous system external LSA 6 -- multicast OSPF LSA 7 -- defined for NSSAs 8 -- External attributes LSA for Border Gateway Protocol (BGP) 9, 10, or 11 -- Opaque LSAs
23
Q
What do Type 1 LSAs do?
A
- generated by every router in the OSPF domain
- not flooded outside the area they originate in
- describe router’s directly connected links (what are its link costs, and who are its neighbors)
- used to build graph for intra-area SPF (shortest path tree)
24
Q
What do Type 2 LSAs do?
A
- network LSA
- generated by DR on broadcast and non-broadcast network types
- not flooded outside the area they originate in
- describe who is adjacent w/DR
- used to reduce redundant info in the database
25
What do Type 3 LSAs do?
* network summary LSA
* generate by ABR
* flooded from area 0 into non-transit area and vice versa
* describe ABR's reachability to links in other areas (includes cost by thides ABR's actual path to destination)
* SPF not run to reach ABR advertised routes, instead logic is:
- ABR can reach link A via SPT in cost X
- I can reach ABR via...
26
What do Type 4 LSAs do?
* ASBR summary LSA
* generated by the ABR to describe the ASBR
* flooded from area 0 into non-transit area and vice versa
* describe ABR's reachability to ASBRs in other areas (includes cost, but hides ABR's actual path to destination.
* SPF not run to reach inter-area ASBR, instead logic is:
- ABR can reach ASBR via SPT in cost X
- I can reach ABR via SPT in cost Y
- I can reach ASB via SPT in cost X + Y
27
What do Type 5 LSAs do?
* External LSA
* generated by ASBR
* flooded to all non-stub areas
* describes routes ASBR is redistributing:
- -metric
- -metric type (type 1=E1, type 2=E2 [default])
28
What do Type 6 LSAs do?
*this type of LSA is used for multicast applications
29
What do Type 7 LSA do?
Type 7 LSA allow injection of external routes through Not-so-Stubby-Areas (NSSA). Generally external routes are advertised by type 5 LSA but they are not allowed inside any stub area. That’s why Type 7 LSA is used, to trick OSPF. Type 7 LSA is generated by NSSA ASBR and is translated into type 5 LSA as it leaves the area by NSSA ABR, which is then propagated throughout the network as type 5 LSA.
30
What do Types 8, 9,10, and 11 LSAs do?
* used to work with BGP
* types 9,10,11: these LSA types are designated for future upgrades to OSPF for distributing application-specific information through an OSPF domain.
31
What is a OSPF virtual link?
* used to connect area 0 over a non-transit area
* a virtual link is a link that allows discontiguous area 0s to be connected, or a disconnected area to be connected to area 0, via a transit area.
The virtual link relies on the stability of the underlying intra-area routing. Virtual links cannot go through more than one area, nor through stub areas.
32
How do you change the OSPF Cost Metric?
you can use the commands "ip ospf cost", "bandwidth" or "auto-cost reference-bandwidth" to manipulate the cost metric.
33
What is a Standard Area?
it is an OSPF default area type that accepts link updates, route summaries, and external routes.
34
What is a Backbone Area?
it is an OSPF area in which all other areas connect to exchange route information.
35
What is a Stub Area?
it is an OSPF area type that does not accept information about routes external to the autonomous system, such as routes from non-OSPF sources. If routes need to route to networks outside the autonomous system, they use a default route, indicated as 0.0.0.0. Stub areas cannot contain ASBRs (except that the ABRs may also be ASBRs).
36
What is a Totally Stubby Area?
it is a Cisco proprietary area type that does not accept external autonomous system routes or summary routes from other areas internal to the autonomous system. If a route needs to send a packet to a network external to the area, it sends the packet using a default route.
37
What is a NSSA Area?
a not-so-stubby area is a area type which defines a special LSA type 7. NSSA offers benefits that are similar to those of a stub area. They do not accept information about routes external to the autonomous system, but instead use a default route for external networks. However, NSSAs allow ASBRs, which is against the rules in a stub area.
38
What is a Totally Stubby NSSA Area?
it is a Cisco proprietary area type that allows ASBRs, but does not accept external routes or summary routes from other areas. A default route is used to get to networks outside the area.
39
How do OSPF routes use the Hello protocol to build adjacencies?
after two routers establish neighbor adjacency using hello packets, they synchronize their LSDBs by exchanging LSAs and confirming the receipt of LSAs from the adjacent router. Routers on point-to-point links form a full adjacency with each other, whereas routers on LAN links only form a full adjacency with the DR and BDR.
40
What are the five fields in the hello packet that must match on neighboring routers?
Hello Interval, Dead Interval, Area ID, Authentication Password and Stub Area Flag.
41
What are the commands for verifying OSPF operation?
* show ip ospf
* show ip route
* show ip ospf interface
* show ip ospf neighbor
* show ip route ospf
* show ip protocols
* debug ip ospf events
* debug ip ospf adj
* debug ip ospf packet
42
How do you prevent a routing protocols' routing updates from being sent through the specified route interface?
* passive-interface default command to prevent a routing protocol's routing updates from being sent. *specified interface appears as a stub network,
* OSPF routing information is neither sent nor received through the specified interface.
43
What are the three kinds of OSPF routes?
intra-area (O)
interarea (O IA)
external (either 0 E1 or O E2)
44
What does link-state mean?
Link: the interface on the router
State: description of the interface and how it is connected to neighbor routers.
Link-state routing protocols operate by sending link-state advertisements (LSA) to all other link-state routers.
45
How often do LSAs age out?
Every 30 minutes each LSA will age out and will be flooded: the sequence number will increment by one. OSPF floods LSAs to make sure the LSDB stays up to date and when it does this, the sequence number will increase and OSPF will reset the max age when it receives a new LSA update.
Every LSA begins with 0X80000001 and ends with 0x7FFFFFFF. Once an LSA makes it to the last sequence number, it will wrap around and start again.
46
Does OSPF use TCP or UDP as a transport protocol?
OSPF does not use TCP or UDP. OSPF uses its own protocol for all of its packets--protocol ID 89.
47
What is the role of each OSPF packet?
* Hello: neighbor discovery, build neighbor adjacencies and maintain them.
* DBD: used to check if the LSDB between 2 routers is the same. The DBD is a summary of the LSDB.
* LSR: requests specific link-state records from an OSPF neighbor.
* LSU: sends specific link-state records that were requested. This packet is like an envelope with multiple LSAs in it
* LSAck: OSPF is a reliable protocols so it needs a packet to acknowledge the others.
