Week 14: OSPF

Question 1

What is included in an LSA #3?

Answer 1

LSA 3 is an inter-area advertisement that contains the summary routes from area border router

Question 2

What is an Area Border Router (ABR) and why is it important?

Answer 2

ABRs have networks that are in multiple OSPF areas and they are a point where route summarization can be implemented.
Also they help to isolate network issues and keep routing databases smaller

Question 3

Why is route summarization really important?

Answer 3

It serves to control the size of the routing table and helps to reduce network traffic related to OSPF.

Question 4

Why choose OSPF over EIGRP?

Answer 4

If you are working on a mixed vendor environment
If you are in an environment where propriety solutions are forbidden
EIGRP natively supports IPv6

Question 5

Define OSPF

Answer 5

1) OSPF, or Open Shortest Path First, is a link-state, open-standard, dynamic routing protocol.
2) OSPF uses an algorithm known as SPF, or Dijkstra’s Shortest Path First, to compute internally the best path to any given route.
3) OSPF is classless and converges fairly quickly, using cost as it’s metric.
4) A router running OSPF creates its own database which contains information on the entire OSPF network, not simply neighbor’s routes like EIGRP.
5) This allows the router to make intelligent choices about path selection on its own instead of relying exclusively on neighbor information.

Question 6

Areas

Answer 6

1) OSPF is different from EIGRP in that it uses areas to segment routing domains. This helps partition routers into manageable groups if the layer 3 network begins to get large.
2) It all starts with area 0. Every OSPF network must contain an area 0, sometimes referred to as the backbone area and every additional area must be physically connected to area 0. From there, other areas are optional.

Question 7

Area Types

Answer 7

•Backbone area
Another name for area 0
•Regular area
Non-backbone area, with both internal and external routes
•Stub area
Contains only internal routes and a default route
•Totally Stubby Area
Cisco proprietary option for a stub area
•Not-So-Stubby area (NSSA)
Contains internal routes, redistributed routes, and optionally a default route

Question 8

Router Roles

Answer 8

• Internal: All interfaces in a single area
• Backbone: At least one interface assigned to area 0
• Area Border Router (ABR): A router that connects more than one area. ABRs contain a separate Link State Database, separating LSA flooding between areas, optionally summarizing routes, and optionally sourcing default routes.
• Autonomous System Boundary Router (ASBR): Has at least one interface in an OSPF area and at least one interface outside of an OSPF area.

Question 9

OSPF Metric

Answer 9

•Each interface is assigned a cost value based purely on bandwidth. The formula is:
Cost = (100Mbs/bandwidth)
•Higher bandwidth means a lower cost.

Question 10

What must match in order for routers running OSPF to for a neighbor adjacency?

Answer 10

• hello timer
• dead timer
• area ID
• authentication type
• password
• stub area flag

Question 11

Router ID

Answer 11

1) The SPF algorithm uses a Router ID to identify hops along a path. The problem, of course, is that routers don’t have a generic “router ID” built in.
2) use the highest IP address assigned to a loopback interface as the Router ID (RID) by default.
3) If no loopback is configured, it will use the highest IP address assigned to an active interface when the OSPF process begins.

Question 12

What are Designated Routers and Backup Designated Routers?

Answer 12

1) DR and BDR serve as the central point for exchanging OSPF routing information.
2) Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment.
3) DR will then distribute topology information to every other router inside the same area. This greatly reduces OSPF traffic.
4) The function of the DR is to distribute updates to the other routers connected on the multi-access network. The Backup Designated Router will take over the Designated roll if the DR fails.

Question 13

OSPF Election Process of a DR and BDR

Answer 13

1) The router with the highest OSPF priority will become a DR. By default, all routers have a priority of 1.
2) If there is a tie, a router with the highest Router ID (RID) wins the election.
3) The router with the second highest OSPF priority or Router ID will become a BDR.
NOTE – on point-to-point links a DR and BDR are not elected since only two routers are directly connected.

Question 14

Which routers using OSPF can summarize?

Answer 14

In OSPF, you can only summarize routes at an ABR/ASBR on the link facing the inbound towards the network.

Question 15

What multicast addresses are used in an OSPF environment?

Answer 15

OSPF uses the destination addresses 225.0.0.5 for all OSPF routers and 224.0.0.6 for communication between the DR/BDR (Designated Router and Backup Designated Router).

Question 16

Which routers form a neighbor relationship with the DR/BDR?

Answer 16

All routers form a neighbor relationship with the DR/BDR but not between DROTHER’s (Non DR or BDR Routers)

Question 17

What is the purpose of the DR?

Answer 17

The DR’s purpose is to provide a central source for routing updates and to reduce traffic. All other routers within an area send their Link State Advertisements to the DR.

Every OSPF area has a DR and BDR

Question 18

Are DR/BDR routers elected on Point-to-Point links?

Answer 18

DR/BDR routers are not elected on point-to-point networks as there are only two routers on the link.

Question 19

OSPF Networks: Broadcast

Answer 19

1) The Broadcast network type is the default for an OSPF enabled ethernet interface.
2) The Broadcast network type requires that a link support Layer 2 Broadcast capabilities.
3) The Broadcast network type has a 10 second hello and 40 second dead timer.
4) An OSPF Broadcast network type requires the use of a DR/BDR.

Question 20

OSPF Networks: Point-to-Point

Answer 20

1) A Point-to-Point OSPF network type does not maintain a DR/BDR relationship.
2) The Point-to-Point network type has a 10 second hello and 40 second dead timer.
3) Point-to-Point network types are intended to be used between 2 directly connected routers.

Question 21

OSPF Networks: Point-to-Multipoint

Answer 21

1) OSPF treats Point-to-Multipoint networks as a collective of point-to-point links.
2) Point-to-Multipoint networks do not maintain a DR/BDR relationship.
3) Point-to-Multipoint networks advertise a hot route for all the frame-relay endpoints.
4) The Point-to-Multipoint network type has a 30 second hello and 120 second dead timer.

Question 22

Point-to-Multipoint Non-Broadcast

Answer 22

1) Same as Point-to-Multipoint but requires static neighbors. Used on Non-broadcast layer 2 topologies.
2) Gives you the ability to define link cost on a per neighbor basis.

Question 23

OSPF Networks: Loopback

Answer 23

1) The default OSPF network type; only available to loopback interfaces.
2) Advertises the interface as a host route; changeable by configuring the interface as point-to-point.

Question 24

Totally Stubby Area

Answer 24

A totally stubby area is only connected to the backbone area. A totally stubby / totally stub area does not advertise the routes it knows. It does not send any Link State Advertisements. The only route a totally stub area receives is the default route from an external area, which must be the backbone area. This default route allows the totally stub area to communicate with the rest of the network.

Question 25

Backbone (Area 0)

Answer 25

The backbone is the first area you should always build in any network using OSPF and the backbone is always Area 0 (zero).

Question 26

Stub Area

Answer 26

Stub areas are connected only to the backbone area. Stub areas do not receive routes from outside the autonomous system, but do receive the routes from within the autonomous system, even if the route comes from another area.

Question 27

Not-So-Stubby (NSSA)

Answer 27

Type 7 LSA’s are created by the Autonomous System Boundary Router and forwarded via the stub area’s border router (ABR) to the backbone. This allows the other areas to learn routes that are external to the OSPF routing domain.

Question 28

OSPF Router-ID

Answer 28

The OSPF Router ID identifies a specific router in the OSPF topology. The Router ID is either a) the IP address assigned to the loopback interface, or b) the IP address of the interface with the highest IP address number. Using the loopback interface makes a more stable OSPF environment as the loopback interface is always up, unlike physical interfaces, which can fail.

Question 29

OSPF Timers

Answer 29

• **HELLO timer - 10 seconds by default
• **DEAD timer - Usually 4x the HELLO timer (40 seconds by default)
• **Link State Database Updates - Whenever the OSPF route table changes, or 30 minutes from the last change, whichever occurs first.