Router - Ospf

Front 1

What is a neighbor table?

Back 1

Is a list of connected OSPF speakers (devices running OSPF) which is built up by exchanging hello messages. OSPF sends out hellos every 10 secs on broadcast links and every 30 secs on non-broadcast links

Front 2

What are the formal states a neighbor relationship transitions through in the process of forming a full adjacency?

Back 2

Down=no info/hellos received, Init=Has received a hello but no RID, 2 way=bi-directional communication has been established, Exstart=DR & BDR are elected, Exchange=Exchange Database Descriptor packets (DDP), Loading=Exchange of Link-state info, Full=Routers are fully adjacent, all router and network LSAs are exchanged and the router databases are fully synchronized

Front 3

What is a Designated Router and a Backup Designated Router?

Back 3

On a broadcast segment a BR is the 1 router assigned the duty of maintaining adjacencies with all other segment routers. It reduces the number of adjacencies which reduces b/w use. Routers advertise changes to the "all DRs" multicast address 224.0.0.6, and DRs advertise the LSA using the "ALL SPF" multicast address of 224.0.0.5. For redundancy a BDR is also elected

Front 4

What is an OSPF area?

Back 4

It is a logical grouping of routers that are running OSPF with identical topological databases. Each area runs SPF separately and summaries are passed between each area

Front 5

What is an internal OSPF router?

Back 5

Is a router in which all interfaces lie within a single area. Its sole function is to route within the area. It needs to know process, area and Router Ids, As well as identifying the participating interfaces.

Front 6

What is the role of an Internal Router?

Back 6

Its responsible for maintaining a current and accurate database of every LSA within the area. Its also responsible for forwarding data to other areas by the shortest path. Flooding of LSAs is confined to the area and all interfaces on this router are within the area.

Front 7

What is a Backbone Router?

Back 7

The design of OSPF require all areas to be connected through a single area, known as the backbone area or Area 0. This is a router within that area.

Front 8

What is the role of an Area Border Router (ABR)?

Back 8

Is responsible for connecting 2 or more areas. It holds a full topology database for each area to which its connected and sends LSA updates between the areas. These LSA updates are summary updates of the subnets within an area, but are sent as Type 3 LSAs only if summarization is configured on the ABR.

Front 9

What is a Autonomous System Boundary Router (ASBR)?

Back 9

They are typically located in the backbone area and connect to other routing domains. e.g. to internet or EIGRP

Front 10

What is the router ID?

Back 10

Is the mechanism by which a router is identified within the OSPF domain. Its used to identify LSAs in the OSPF network and is needed by a router to be able to participate in the OSPF domain. It can be defined by "router id" cmd, if cmd not present the highest IP of loopback interface, or lastly by highest IP of first active interface.

Front 11

What is the process ID?

Back 11

Is simply a mechanism to allow more than 1 process to be configured on a router

Front 12

What is an Area ID?

Back 12

Is a mechanism of grouping routers that share full knowledge of OSPF-derived routes within that OSPF area.

Front 13

What is the Transmit timer and its default setting?

Back 13

Determines the estimated number of seconds that it takes to send an LSA to a neighbor. Default is 1 sec.

Front 14

What is the Retransmit timer and its default setting?

Back 14

States the number of seconds to wait for an acknowledgment before retransmitting an LSA. Default 5 secs. Should always be bigger than transmit timer to avoid possibility of flapping.

Front 15

How do hello timers work and what are the defaults?

Back 15

OSPF sends out hellos at regular intervals to discover neighbors and to verify the neighbor list remains current. Neighbors time out after the dead time expires (by default 4 x hello time). Defaults are broadcast & point-to-point - 10 secs , non-broadcast & point-to-multipoint - 30 secs

Front 16

Whats the 6 LSA types?

Back 16

Type1 Router link=Generated by router & lists neighbors and costs, Type2 Network Link=Sent out by DR and lists all routers on the segment its adjacent too, Type3 Network Summary Link=ABRs generate to send between areas listing prefixes available in a given area, Type4 AS external ASBR Summary Link=ABRs produce to advertise ASBRs presence cost, Type5 External Link LSA=ASBR produces and floods throughout As advertising external routes, Type7 NSSA External LSA=Produced by ASBR, and is type 5 tunneled through the NSSA.

Front 17

Why are LSAs labeled with sequence numbers?

Back 17

So that older and newer versions of an advertisement can be distinguished and decide whether to update the current entry in the topology table.

Front 18

What are the routing table LSA codes and there meanings?

Back 18

They show where the entry was generated from. O= 1.Router or 2.Network Link generated by the router or DR and propagated within an area, OIA=3 or 4 summary links generated by ABRs and propagated into backbone area and between ABRs, OE1 or OE2=5.Summary/external Link either E1 is internal + external cost or E2 only external cost.

Front 19

What is the metric used by OSPF and how its calculated?

Back 19

It is the cost and is calculated by 1000,000,000bps/link speed. It is applied to the outgoing interface, and the routing process will select the lowest cumulative cost to a remote network.

Front 20

What are the 5 OSPF packet types?

Back 20

Hello=establishes communication with directly attached neighbors, Database Descriptor(DBD)=sends list of RIDs from whom the router has an LSA, Link-state Requests(LSR)=follow DBDs to ask for any missing LSAs, Link-state Update(LSU)=Replies to LSR with requested data, Link-state acknowledgement(LSAck)=Confirm receipt of link-state info

Front 21

What IP protocol is OSPF traffic marked as?

Back 21

All OSPF traffic is transmitted inside IP packets marked as IP protocol 89

Front 22

Whats the benefit of creating multiple areas in OSPF?

Back 22

It is a lot easier to maintain and manage. It reduces the number of SPF calculations because the size of the topology table is reduced. This lessens the likelihood of a change in the network, and thus, SPF calculations. This also reduces the CPU, memory and network b/w consumption.

Front 23

Why must the Backbone Area be at the centre of all other areas and physically connect to all other areas?

Back 23

Because OSPF expects all areas to inject routing info into the backbone, and in turn, the backbone will disseminate that info into other areas

Front 24

How many paths will OSPF load-balance with by default?

Back 24

Cisco routers will perform equal-cost load balancing for upto 4 paths, but this parameter can be increased by configuration up to as many as 16 paths.

Front 25

What should you do if a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network?

Back 25

1.Make sure both routers are configured with same IP mask, MTU, Interface Hello timer, OSPF hello timer and OSPF dead interval, 2.Make sure both neighbors are in same area, 3.Use Debug and show cmds to trace the problem.

Front 26

What happens if there are routes with different routing information srcs to the same dst?

Back 26

The router will choose the path with the lowest AD. If both are OSPF, OSPF will select the lower type advertisements first, and choose lower costs to break ties.

Front 27

What are the benefits of summarization of IP subnets between areas?

Back 27

Reduces the size of the routing table, which in turn reduces CPU and memory requirements. It also speeds up convergence. The greater the summarization, the smaller and fewer the LSA packets that need to be propagated

Front 28

What are the 2 types of summarization that exist in OSPF?

Back 28

Interarea summarization is performed at the ABR, and creates Type 3 and Type 4 LSAs. These routes are sent between areas. External summarization is performed at the ASBR, and creates Type 5 LSAs. These routes are sent into OSPF from another routing protocol.

Front 29

How does a ABR forward summary LSAs?

Back 29

It forwards Type 3 LSAs to other ABRs and Type 4 LSAs to the ASBRs. It also forwards Type 3 LSAs from other areas into its own area. If the ABR has multiple links in the same area, it also forwards Type 1 and 2 LSAs in its capacity as an internal router.

Front 30

What are external routes and how are there costs calculated?

Back 30

They are routes that didn't originate in the OSPF domain and have been redistributed from another routing protocol (routing domain) or static routing. An external route is introduced into the OSPF domain by an ASBR. External routes discovered by OSPF can have there paths calculated in 1 of 2 ways. E1=cost of path to ABR is added to external cost to reach next hop router outside the AS, E2=External cost of path from ASBR. E2 is default, but E1 is preferred over E2 if 2 equal-cost paths exist.

Front 31

What are the rules when creating a Virtual Link?

Back 31

Both routes must share a common area, the transit area cant be a stub area, and one of the routers must be connected to area 0.

Front 32

What happens if OSPF identifies a problem or a link goes down?

Back 32

As soon as OSPF identifies a problem it modifies its LSAs accordingly and sends updated OSPFs to the remaining neighbors with full adjacency. Being event driven, this improves convergence times.

Front 33

When should a DR be manually selected using priorities?

Back 33

In a partial mesh network (eg hub-and-spoke), and the chosen DR should be a router with permanent virtual circuits (PVCs) connecting to all other routers. The DR idea doesn’t work well in a partial mesh because the elected router may not be able to reach all other routers.

Front 34

What type of networks don’t involve a DR/BDR election?

Back 34

Point-to-multipoint and point-to-point network types, only the non-Broadcast and Broadcast network types have the DR/BDR election.

Front 35

Which network types offer neighbor recognition?

Back 35

Non broadcast OSPF networks like NBMA cant dynamically discover there neighbors, so the neighbors must be manually identified. However Broadcast networks like point-to-point can.

Front 36

What OSPF network types are available on Cisco router interfaces?

Back 36

Broadcast Multiaccess, Point-to-Point, Point-to-Multipoint (default is Point-to-Multipoint broadcast, but nonbroadcast is available), and Nonbroadcast Multiaccess (NBMA)

Front 37

Broadcasts and multicasts aren't supported on which OSPF network type?

Back 37

NBMA is used to accurately model X.25 and frame relay environments in multiple-access networks where there are no intrinsic broadcast and multicast capabilities. In an NBMA configuration, OSPF sends HELLO packets to each router one at a time rather than multicasting them. The HELLO timer is extended from 10 to 30 secs and dead router timer from 40 to 120 secs.

Front 38

What steps need to be performed to configure a router to support OSPF in a non-broadcast environment with the NBMA OSPF network type?

Back 38

Because NBMA is the default OSPF network type, its only really necessary to identify neighbors.

Front 39

To identify neighbors. If "ip ospf priority" cmd is already set, the 2 of the higher values is used. Poll interval is rate that are hellos still sent to inactive routers (default 120secs).

Back 39

router(config-if)# neighbor [ip add] [priority [num]] [poll interval [secs]] [cost [num]]

Front 40

What steps need to be performed to configure a router to support OSPF point-to-multipoint?

Back 40

Point-to-multipoint forms an adjacency automatically along any PVC, which causes more overheads but is more resilient than the NBMA option. No DR is elected and neighbors are automatically discovered, so only need to change from the default OSPF NBMA network type using "ip ospf network point-to-multipoint" cmd

Front 41

What is needed to configure an OSPF broadcast network type?

Back 41

Configuring a broadcast interface just involves changing the OSPF network type, no neighbor statements are required. The broadcast OSPF network type works best with a fully meshed network.

Front 42

Which network types are best applied to full-mesh topologies?

Back 42

Multiaccess topologies, such as those found in NBMA and broadcast OSPF network types, rely on DRs and so are best applied to full mesh-topologies. The only real difference is that Broadcast network types are capable of discovering there neighbors automatically. Ethernet links are automatically recognized as broadcast links.

Front 43

Which OSPF network types are Cisco proprietary, and which are RFC compliant?

Back 43

Non-broadcast and point-to-multipoint (broadcast) are RFC Complaint, whilst point-to-multipoint (non-broadcast), broadcast and point-to-point are Cisco proprietary

Front 44

What is the default OSPF network type for a physical serial interface?

Back 44

Non-broadcast Multiaccess. Multipoint subinterfaces are also treated like other interfaces, so their default OSPF network type is also NBMA. Multipoint interfaces on Cisco routers automatically start in non-broadcast mode. Point-to-point subinterfaces are treated by the router exactly as if they were point-to-point circuits. These interfaces automatically use the point-to-point OSPF network type.

Front 45

Whats a stub area?

Back 45

Is an area that wont except external routes (Type 5). They are replaced by a default route, and internal routers send external traffic to the closest ABR. Useful to protect slowest or less powerful routers from being overwhelmed with routes from outside. An area that has a single ABR or an ABR where the choice of exit ABR is not important is a good candidate for a stub area.

Front 46

Whats a totally stubby area?

Back 46

An area that doesn’t accept summary LSAs from other areas (Type 3 & 4) or external summary LSAs (Type 5). Protects internal routers by minimizing routing tables and summarizing everything outside the area with a default route. Is Cisco proprietary.

Front 47

Whats a Not-So-Stubby-Area (NSSA)?

Back 47

Are stubby areas that can have ASBRs. Since stubby areas don’t support Type 5 LSAs, they use Type 7 LSAs to disguise external information and the ABR converts the Type 7 to Type 5 when its sent to Area 0. Can also have Not-so-totally-stubby areas. NSSA external routes show up as N1 or N2 instead of E1 or E2 in the routing table. Note the ABR so a NSSA doesn't automatically generate a default route.

Front 48

What certain characteristics unite all stub area types?

Back 48

Area 0 can`t be a stub, all routers in the area must agree that its a stub area, and no virtual links may traverse a stub area.

Front 49

What is a disadvantage of stub areas?

Back 49

Routers in a stub area will always route external traffic through the closest ABR. Sometimes this can cause inefficient routing if there is a better path.

Front 50

To check the CPU utilization

Back 50

router# show process cpu sorted

Front 51

To check the memory utilization

Back 51

router# show memory free

Front 52

To set the OSPF priority. DR & BDR are elected on basis of highest priority, whilst ties are broken in favor of highest IP. Doesn’t pre-empt, once elected serve until they are rebooted. The default is 1, whilst 0 prevents a router from being elected.

Back 52

router(config-if)# ip ospf priority [0-255]

Front 53

To clear the ospf process to force a DR/BDR election or a change of RID. Can alternatively reboot the router

Back 53

router# clear ip ospf process

Front 54

To configure OSPF as the routing protocol to use. The process number isn't globally significant, so doesn’t have to be same on every router in an area. It is possible to have more than 1 process number running on a router.

Back 54

router(config)# router ospf [process number]

Front 55

To define the interfaces that are to participate in OSPF and the area they reside in. All interfaces that match the network wildcard pattern will be active in the given ospf area. A router can have different interfaces in different areas.

Back 55

router(config-router)# network [network] [mask] area [area-num]

Front 56

To define the router ID. If it is changed, the router needs a restart or the OSPF routing process reinitialized before it takes effect.

Back 56

router(config-router)# router-id [ip add]

Front 57

To define the loopback interface. Is a virtual interface that is always active, since doesn't physically exist so cant go down. This is the preferred way to define a router ID.

Back 57

router(config)# interface loopback [num] router(config-if)# ip address [ip add] [mask]

Front 58

To manually override the default cost result from the cost equation. Lower values are preferred. The default calculation gives costs of 64 for serial link 10 for Ethernet, and 1 for both fast Ethernet and gig.

Back 58

router(config-router)# ip ospf cost [0 - 65,535]

Front 59

To change the numerator in the automatic cost calculation. Default is 100.

Back 59

router(config-router)# ospf auto-cost reference-bandwidth [1 to 4,294,967]

Front 60

To consolidate and summarize routes at an area boundary. Is configured on a ABR and dictates the networks that will be advertised out of the area. The "area id" is the area about which routes are to be summarized.

Back 60

router(config-router)# area [area-id] range [ip add] [mask]

Front 61

To summarize network addresses received from ISP and redistributed into OSPF. Is applied to ASBR. Advertises a single route for all the redistributed routes that are covered by a specified network address and mask. Is used to external routes before injecting them into OSPF.

Back 61

router(config-router)# summary-address [address] [mask]

Front 62

To create a stub area. All routers inside a stub or totally stubby area must be configured as stub routers since hello packets will have a stub flag which neighbors must agree on to be neighbors. Also all routers inside the area need too have default network set (0.0.0.0)

Back 62

router(config-router)# area [area-id] stub and network for routers in the stub area router(config-router)# network 0.0.0.0 255.255.255.255 area [area-id]

Front 63

To create a totally stubby area. Is configured on ABR and all other routes are configured as stub so that their stub-flag will agree. No interred or external LSAs are propagated into the area, and use a default route advertised into the area by the ABR to reach networks and hosts outside the area. Also all routers inside the area need to have default network set (0.0.0.0)

Back 63

router(config-router)# area [area-id] stub no-summary and network for routers in the stub area router(config-router)# network 0.0.0.0 255.255.255.255 area [area-id]

Front 64

To set the cost of the default route propagated into another area, if not specified is cost of ABR + 1. Is configure only on ABR, and the ABR will then automatically generate and advertise the route cost along with the default route. In stub and totally stubby areas the ABR replaces the external routes with a default route. Is useful when the stub area has more than 1 ABR, since allows the administrator to prefer a specific exit.

Back 64

router(config-router)# area [area-id] default-cost [cost]

Front 65

To create a virtual link between ABRs that share a common area, at least 1 must be connected to Area 0. The cmd states the transit area (area-id) and the Router ID (router-id) of the remote destination ABR. May transit many routers along the way, but appears to OSPF on remote ABRs as next hop.

Back 65

router(config-router)# area [area-id] virtual-link [router-id]

Front 66

To change the OSPF network type. Interfaces default to the NBMA OSPF network type in a non broadcast environment.

Back 66

router(config-if)# ip ospf network [broadcast | non-broadcast | point-to-multipoint [non-broadcast]]

Front 67

To show how OSPF is running on a particular router. Includes the number of times that the SPF routing algorithm has run (calculated the routing table), which is indicative of the stability of the network. Some of the other info it displays is the number of areas of which its a member, how many of its interfaces are in an area, authentication and the area range (summarization) that has been configured.

Back 67

router# show ip ospf [process-id]

Front 68

What cmd would show all the network link-state info?

Back 68

router# show ip ospf [process-id] [area-id] database network

Front 69

To display the contents of the routers topological database and the different LSAs that have populated the database. Shows RID, Advertised RID, linkstate age, sequence number and the checksum for the contents of the LSA. It can be tailored to show specific info from the database (eg Type2 LSAs) by specifying the appropriate parameter.

Back 69

router# show ip ospf database [router | network | summary | asbr-summary | nssa-external | external | database summary]

Front 70

To see how OSPF has been configured and is working on a particular interface. It will display info like the router, area & process ID, state DR, BDR and DROTHER (if neither DR or BDR) and priority number.

Back 70

router# show ip ospf interface [type] [mod/num]

Front 71

To view all the neighbors known to the router. Its also possible to focus on viewing info regarding neighbors that have been discovered on a particular interface. Useful to verify that all neighbors are present. This cmd shows info like r=the functional state (eg 2-way)

Back 71

router# show ip ospf neighbor [type] [mod/num] [neighbor-id] [detail]

Front 72

To show IP routing protocol configuration on a router. This includes the networks it advertises and indicates when next updates will occur. This cmd is excellent for troubleshooting configuration errors and understanding how the network is communicating about its routes.

Back 72

router# show ip protocols

Front 73

To display the routing table and networks of which the router is aware. It details how the network is known to the router and how the router discovered the route. Also shows preferred paths and gives next hop for each path. Good for understanding an IP network and allows you to analyze the flow of LSAs both within and between areas (LSA codes).

Back 73

router# show ip route

Front 74

Displays info about OSPF-related events, such as adjacencies, flooding info, DR selection and SPF calculation

Back 74

router# debug ip ospf events

Front 75

Displays info about packets received, generated and forwarded. Fast-switched packets do not generate messages.

Back 75

router# debug ip packet

Front 76

What cmds can be used to show which router on a LAN is the BDR?

Back 76

router# show ip ospf neighbor or router# show ip ospf interface

Front 77

Which cmd is used to show the state of adjacencies?

Back 77

router# show ip ospf neighbor

Front 78

To show the ABRs and ASBRs that the internal router has in its routing table in a table format with the areas, cost, RID of dst ABR/ASBR and next hop to ABR/ASBR. Is excellent for troubleshooting configuration errors and understanding how the network is communicating. In a multi area network this cmd can immediately indicate why users cant connect outside their area.

Back 78

router# show ip ospf border-routers

Front 79

To show the configured virtual links. Even though virtual-link partners aren't directly connected, they will show as neighbors because of the virtual connection. Shows the timers, cost, transit area and via interface.

Back 79

router# show ip ospf virtual-links

Front 80

To start log-adjacency-changes, is similar to debug but uses less resources. Whenever a change in the state of an adjacency occurs, a message is sent to syslog. So if you ran this cmd, then the "router(config-router)# network [network] [mask] area [area-num]" cmd afterwards, the ospf process would re-initialize all interfaces, requiring neighbors to be found and adjacencies formed which would all be logged.

Back 80

router(config-router)# log-adjacency-changes

Front 81

To set OSPF plaintext password on a per-interface basis. Other routers connected through this interface should have same password. Prior to IOS 12.0, authentication was set per-area

Back 81

router(config-if)# ip ospf authentication-key [password]

Front 82

To set a MD5 key and password. A key number is used in combination with a password to create a hash. More than 1 key may be active at one time.

Back 82

router(config-if)# ip ospf message-digest-key [key] md5 [password]

Front 83

To enable OSPF authentication using MD5

Back 83

router(config-if)# ip ospf authentication-key message-digest