OSPF Flashcards by Scott Brandt

What kind of routing protocol is OSFP?

Distance Vector
Link State

Link State

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Does OSPF only know about it’s neighbor’s routes or have a picture of the entire network topology

OSPF is a Link State routing protocol so it knows about the while network topology.

A Distance Vector protocol like RIP or EIGRP only learns about routes from its directly connected neighbors.

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What kind of packets are used in OSPF to exchange routing infrormation?

LSA - Link State Advertisements are used to pass routing updates between routers.

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List steps taken in OSPF route learning.

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List OSPF Packet Types

Hello Packet - Router sends and listens for these on enabled interfaces
DBD - DataBase Descriptor - Info about the networks a router knows about
LSR - Link State Request - asking for more information about a link
LSA - Link State Advertisement - A routing update
LSU - Link State Update - contains a list of LSA’s that should be updated, typicially flooded out
LSAck - Recieving routers acknowledge LSAs

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What is the number called that is listed when enabling OSPF on a router?

i.e. R1(config)# router ospf 100

the Process ID

This is a locally, not globaly significant number.
This does not have to be the same on other neighbor routers.
Usually only one Process ID is used on a router no matter how many interfaces it has.

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What is the number denote that is entered after the network address and wildcard mask in OSPF?

i.e:

R1(config)# router ospf 1
R1(config)# network 192.168.0.0 0.0.255.255 area 0
R1(config)# network 10.0.0.0 0.255.255.255 area 0

The area number will be the same on all routers that are sharing routing info in their databases with each other.

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In OSPF, if you enter the following commands and hit enter, what will happen reguarding the wildcard mask?

R1 (config) # router ospf 100
R1(config-router) network 10.0.0.0

You’ll get an % Incomplete command error because OSFP does not default to classful network/wildcard masks.

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In OSPF when entering the command:
R1(config-router) network 192.168.0.0 0.0.255.255 area 0

Will the network 192.168.0.0/16 be advertised?

Probably No.
Only if an interface on that router is configured on that exact network / mask combination. If it only has interfaces configured with 192.168.10.0/24 and 192.168.20.0/24, those spific networks will be advertised, not the entire 192.168.0.0/16 network.

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Command to show OSPF running config

R1# show run | section ospf

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command to show detailed information about OSPF:

R1# show ip protocols

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Command to show interfaces setup for OSPF?

R1# show ip ospf interface brief

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command to show nearby OSPF devices:

R1# show ip ospf neighbor

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Command to view OSPF database

R1# show ip ospf database

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How is the default Router ID chosen in OSPF?

Highest loopback interface IP.

If no loopback interfaces, then highest interface IP address.

Can also manually be set.

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Command to mamually config Router ID

R1(config) router ospf 100
R1(config-router)# router-id 2.2.2.2

(Doesnt have to be an IP address on that router but must be in the form of an IPv4 address)

Commands to configure OSPF passive interface

Commands to default to all passive OSPF interfaces and the allow some interfaces to not be passive:

Command to configure OSPF default route:

R1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.1
R1(config)# router ospf 100
R1(config-router)# default-information originate

What does O*E2 indicate in the show ip routes table?

The route was a static route on a router that was injected (redistributed) into OSPF.

What does O IA indicate in the show ip routes table?

The route was learned from an ABR - Area Boundry Router and is an ‘Inter Area’ Route that’s located in another OSPF area.

In OSPF, does manually setting the router ID imediatly take effect?

No. OSPF must be manually restarted for the new update to take effect.

How is the OSPF metric calculated?

Based on path ‘cost’.
Cost is derieved from the interface bandwidth.

How is the ‘Cost’ in OSPF referenced and be modified?

Based on 100 / interface bandwidth.

If this is less than 1, the protocol rounds up to 1

should be adjusted by:

R1(config)# router ospf 1
R1(config-router)# auto-cost reference-bandwidth 100000 (in mbit)

Command to adjust the reference bandwidth in OSPF:

R1(config)# router ospf 1 R1(config-router)# **auto-cost reference-bandwidth 100000** (in mbit)

How can the OSPF metric be manipulated?

Manually changing the cost of the link. | (can also change the bandwidth setting but that is a bad idea and not recomended)

Command to view OSPF Cost setting of a router's interfaces:

R1# show ip ospf interface brief

What must be the same between two neighbor routers in order to from an adjacency in OSPF

The area number

Types of OSPF routers

* Backbone routers i.e. Area 0 * ABRs - Area Boundry Routers - part of multipe areas (normally 2) * Normal Area Router - All in the same area * ASBR - Autonomous System Boundry Router - connects to outer routing protocols or static routes

In OSPF, where is route summerazation done?

on the ABRs - Area Border Routers

In OSPF, will routes be automaticly or manually summarised?

Manually.

How many routes are shared if maunal summarization is not configured on ABRs - Area Border Routers that at the edge of differnet areas.

All routes will be shared between both areas if manual summarazation is not configured.

What are DR and BDR routers in OSPF?

Designated Routers Backup Designated Router 3+ routers can be DROTHER i.e. DR Other DR and BDR relationships arn't estlibilished for serial links. A DR and BRD will be estlabished for each multiaccess network segment. i.e. broadcast domain or non-braodcast multidevice link

How are DR and BRD elections decided in OSPF?

Highest roter priority wins and becomes DR (designated router). Next highest becomes backup DR. In case of tie, highest router ID is elected DR and then BDR. Priority 0 will force the router to be not be a DR or BDR. Not tested: If routers come online at different times, the first router online will be the DR, and then the next will be BDR, then others will be DRother.

What are the 7 OSPF relationship states?

0. **Down -** no recent information received 1. **Init** – the router received a Hello message from the other OSFP router 2. **2-way** – the neighbor has received the Hello message and replied with a Hello message of his own. A neighborship has been estliblished — election for DR/BDR may now occur — 3. **Exstart** – beginning of the LSDB exchange between both routers. Routers are starting to exchange link state information. 4. **Exchange** – DBD (Database Descriptor) packets are exchanged. DBDs contain LSAs headers. Routers will use this information to see what LSAs need to be exchanged. 5. **Loading** – one neighbor sends LSRs (Link State Requests) for every network it doesn’t know about. The other neighbor replies with the LSUs (Link State Updates) which contain information about requested networks. After all the requested information have been received, other neighbor goes through the same process 6. **Full** – both routers have the synchronized database and are fully adjacent with each other.

In EIGRPv6, where should the network command be entered to enable each patrisapating interface/port?

The interface section in EIGRPv6

In EIGRPv4, where should the network command be entered to enable each patrisapating interface/port?

The router EIGRP section in EIGRPv4 unlike EIGRPv6 which requires the network info to be entered into the interface config.