ICND2 Chapter 9

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    Chapter 9 OSPF OSPF Protocols and Operation

    OSPF Configuration

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    Do I know this?Go through the Quiz-

    5 minutes

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    1. Which of the following affects the calculation ofOSPF routes when all possible default valuesare used?

    a. Bandwidth

    b. Delay

    c. Load

    d. Reliabilitye. MTU

    f. Hop count

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    1. Which of the following affects the calculation ofOSPF routes when all possible default valuesare used?

    a. Bandwidth

    b. Delay

    c. Load

    d. Reliabilitye. MTU

    f. Hop count

    Answer: A

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    2. OSPF runs an algorithm to calculate the

    currently best route. Which of the following terms

    refer to that algorithm?a. SPF

    b. DUAL

    c. Feasible successord. Dijkstra

    e. Good old common sense

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    2. OSPF runs an algorithm to calculate the

    currently best route. Which of the following terms

    refer to that algorithm?a. SPF

    b. DUAL

    c. Feasible successord. Dijkstra

    e. Good old common sense

    Answer: A & D

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    3. Two OSPF routers connect to the same VLAN usingtheir Fa0/0 interfaces. Which of the following settings onthe interfaces of these two potentially neighboringrouters would prevent the two routers from becomingOSPF neighbors?

    a. IP addresses of 10.1.1.1/24 and 10.1.1.254/25,respectively

    b. The addition of a secondary IP address on one router'sinterface, but not the other

    c. Both router interfaces assigned to area 3

    d. One router is configured to use MD5 authentication, andthe other is not configured to use authentication

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    3. Two OSPF routers connect to the same VLAN usingtheir Fa0/0 interfaces. Which of the following settings onthe interfaces of these two potentially neighboringrouters would prevent the two routers from becomingOSPF neighbors?

    a. IP addresses of 10.1.1.1/24 and 10.1.1.254/25,respectively

    b. The addition of a secondary IP address on one router'sinterface, but not the other

    c. Both router interfaces assigned to area 3

    d. One router is configured to use MD5 authentication, andthe other is not configured to use authentication

    Answer: A &D

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    4. Which of the following OSPF neighborstates is expected when the exchange of

    topology information is complete so thatneighboring routers have the same LSDB?

    a. Two-way

    b. Fullc. Exchange

    d. Loading

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    4. Which of the following OSPF neighborstates is expected when the exchange of

    topology information is complete so thatneighboring routers have the same LSDB?

    a. Two-way

    b. Fullc. Exchange

    d. Loading

    Answer: B

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    5. Which of the following is true about an existing OSPFdesignated router?

    a.A newly connected router in the same subnet, with ahigher OSPF priority, preempts the existing DR tobecome the new DR.

    b.A newly connected router in the same subnet, with alower OSPF priority, preempts the existing DR tobecome the new DR.

    c. The DR may be elected based on the lowest OSPFRouter ID.

    d. The DR may be elected based on the highest OSPFRouter ID.

    e. The DR attempts to become fully adjacent with everyother neighbor on the subnet.

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    5. Which of the following is true about an existing OSPFdesignated router?

    a.A newly connected router in the same subnet, with ahigher OSPF priority, preempts the existing DR tobecome the new DR.

    b.A newly connected router in the same subnet, with alower OSPF priority, preempts the existing DR tobecome the new DR.

    c. The DR may be elected based on the lowest OSPFRouter ID.

    d. The DR may be elected based on the highest OSPFRouter ID.

    e. The DR attempts to become fully adjacent with everyother neighbor on the subnet.

    Answer: D&E

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    6. Which of the following network commands,following the command router ospf 1, tells thisrouter to start using OSPF on interfaces whoseIP addresses are 10.1.1.1, 10.1.100.1, and10.1.120.1?

    a. network 10.0.0.0 255.0.0.0 area 0

    b. network 10.0.0.0 0.255.255.255 area 0c. network 10.0.0.1 255.0.0.255 area 0

    d. network 10.0.0.1 0.255.255.0 area 0

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    6. Which of the following network commands,following the command router ospf 1, tells thisrouter to start using OSPF on interfaces whoseIP addresses are 10.1.1.1, 10.1.100.1, and10.1.120.1?

    a. network 10.0.0.0 255.0.0.0 area 0

    b. network 10.0.0.0 0.255.255.255 area 0c. network 10.0.0.1 255.0.0.255 area 0

    d. network 10.0.0.1 0.255.255.0 area 0

    Answer: B

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    7. Which of the following network commands,following the command router ospf 1, tells thisrouter to start using OSPF on interfaces whose

    IP addresses are 10.1.1.1, 10.1.100.1, and10.1.120.1?

    a. network 0.0.0.0 255.255.255.255 area 0

    b. network 10.0.0.0 0.255.255.0 area 0

    c. network 10.1.1.0 0.x.1x.0 area 0d. network 10.1.1.0 255.0.0.0 area 0

    e. network 10.0.0.0 255.0.0.0 area 0

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    7. Which of the following network commands,following the command router ospf 1, tells thisrouter to start using OSPF on interfaces whose

    IP addresses are 10.1.1.1, 10.1.100.1, and10.1.120.1?

    a. network 0.0.0.0 255.255.255.255 area 0

    b. network 10.0.0.0 0.255.255.0 area 0

    c. network 10.1.1.0 0.x.1x.0 area 0d. network 10.1.1.0 255.0.0.0 area 0

    e. network 10.0.0.0 255.0.0.0 area 0

    Answer: A

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    8. Which of the following commands list the OSPF

    neighbors off interface serial 0/0?

    a. show ip ospf neighborb. show ip ospf interface

    c. show ip neighbor

    d. show ip interfacee. show ip ospf neighbor interface serial 0/0

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    8. Which of the following commands list the OSPF

    neighbors off interface serial 0/0?

    a. show ip ospf neighborb. show ip ospf interface

    c. show ip neighbor

    d. show ip interfacee. show ip ospf neighbor interface serial 0/0

    Answer: A, B & E

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    9. OSPF routers R1, R2, and R3 attach to the same VLAN. R2 hasbeen configured with the ip ospf authentication message-digestinterface subcommand on the LAN interface connected to thecommon VLAN. The show ip ospf neighbor command lists R1 andR3 as neighbors, in an Init and Full state, respectively. Which of thefollowing are true?

    a. R3 must have an ip ospf authentication message-digest interfacesubcommand configured.

    b. R3 must have an ip ospf authentication message-digest-keyinterface subcommand configured.

    c. R1's failure must be because of having configured an incorrectOSPF authentication type.

    d. R1's failure may or may not be related to authentication.

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    9. OSPF routers R1, R2, and R3 attach to the same VLAN. R2 hasbeen configured with the ip ospf authentication message-digestinterface subcommand on the LAN interface connected to thecommon VLAN. The show ip ospf neighbor command lists R1 andR3 as neighbors, in an Init and Full state, respectively. Which of thefollowing are true?

    a. R3 must have an ip ospf authentication message-digest interfacesubcommand configured.

    b. R3 must have an ip ospf authentication message-digest-keyinterface subcommand configured.

    c. R1's failure must be because of having configured an incorrectOSPF authentication type.

    d. R1's failure may or may not be related to authentication.

    Answer: B & D

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    10.An OSPF router learns about six possible routes toreach subnet 10.1.1.0/24. All six routes have a cost of55, and all six are interarea routes. By default, how many

    of these routes are placed in the routing table?a. 1

    b. 2

    c. 3

    d. 4e. 5

    f. 6

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    10.An OSPF router learns about six possible routes toreach subnet 10.1.1.0/24. All six routes have a cost of55, and all six are interarea routes. By default, how many

    of these routes are placed in the routing table?a. 1

    b. 2

    c. 3

    d. 4e. 5

    f. 6

    Answer: D

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    Any Questions?

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    OSPF Operation Neighbors

    How do I get to be a neighbor

    Database Exchange

    Only done with neighbors

    Route Calculation

    Once I have details, use SPF

    Pg 347

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    OSPF Neighbor Another router that connects to the same

    data link with which the first router can

    exchange OSPF information Match OSPF settings on both routers

    OSPF Hello lets us find neighbors

    Pg 347

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    OSPF Router ID Router ID is used to control OSPF

    Uniquely identify each router

    32 bit number IP Address

    Manual config if you want

    Pg 348

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    HELLO Can be neighbors if on the same subnet

    Router sends multicast OSPF Hello

    Listens for other Router Hellos

    Keep sending based on a timer

    Hello contains: RID

    Area ID

    Hello Interval Dead Interval

    Router priority

    Designated RID

    Backup Designated RID

    Pg 349

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    Any Questions?

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    Neighbors Getting a HELLO does not mean you become a

    neighbor

    Subnet mask used on the subnet Subnet number (as derived using the subnet mask

    and each router's interface IP address)

    Hello interval

    Dead interval OSPF area ID

    Must pass authentication checks (if used)

    Value of the stub area flag

    Pg 349

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    Neighbor States

    Once a router sees its own ID in HELLO, and meets the

    checks, it becomes neighbor States

    Down-interface down

    Init-sending Hello

    2-way-Router sees its ID in hello from next router

    Pg 350

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    OSPF Topology Sharing Step 1 Based on the OSPF interface type, the

    routers may or may not collectively elect a

    Designated Router (DR) and BackupDesignated Router (BDR).

    Step 2 For each pair of routers that need to

    become fully adjacent, mutually exchange the

    contents of their respective LSDBs. Step 3 When completed, the neighbors monitor

    for changes and periodically reflood LSAs while

    in the Full (fully adjacent) neighbor state.Pg 352

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    Choosing Designated ROUTER OSPF knows two kinds of networks

    Broadcast-Like Ethernet or point to multipoint

    Needs DR DR is hub of topology sharing. Everyone shares to

    DR who sumamrizess and sends out details

    Point to point

    No DR

    All routers share information

    Pg 352

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    Designated Router

    Pg 353

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    Electing Designated Routers The router sending the Hello with the highest OSPF

    priority setting becomes the DR.

    If two or more routers tie with the highest priority setting,the router sending the Hello with the highest RID wins.

    It's not always the case, but typically the router with thesecond-highest priority becomes the BDR.

    A priority setting of 0 means that the router does notparticipate in the election and can never become the DRor BDR.

    The range of priority values that allow a router to be acandidate are 1 through 255.

    If a new, better candidate comes along after the DR andBDR have been elected, the new candidate does notpreempt the existing DR and BDR.

    Pg 354

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    Any Questions?

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    Database Exchange Send a list of LSAs

    Not full details

    If a router needs more details, then the fullLSA is sent from neighbor

    Then they are fully completed

    converged

    Pg 354

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    Keeping up to date Hellos sent on interval

    LSAs updated every 30 minutes

    Individual routes, not all of them

    If there is a change, LSA is triggered

    Pg 355

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    Summary of OSPF states

    Pg 355

    Neighbor

    Stat

    Meaning

    Down A known neighbor is no longer reachable, often because of an

    underlying interface failure.

    Init An interim state in which a Hello has been heard from the neighbor, but

    that Hello does not list the router's RID as having been seen yet.

    Two-way The neighbor has sent a Hello that lists the local router's RID in the list

    of seen routers, also implying that neighbor verification checks all

    passed.Full Both routers know the exact same LSDB details and are fully adjacent.

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    Routing Tables from OSPF The OSPF database lists all routers and

    links

    Each router uses Dijkstra SPF algorithm tofind best path to destination networks

    Best path is based on OSPF cost

    Pg 356

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    OSPF and Large Networks A larger topology database requires more

    memory on each router.

    Processing the larger-topology databasewith the SPF algorithm requiresprocessing power that grows exponentiallywith the size of the topology database.

    A single interface status change (up todown or down to up) forces every router torun SPF again!

    Pg 358

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    OSPF Areas Divide larger networks into Areas

    Limit how much detail each router needs to

    know Routers in different areas just need to know to

    send a packet to an area, not the details of

    the area

    Pg 358

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    OSPF Areas

    Pg 359

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    OSPF Terminology

    Pg 359

    Term DescriptionArea Border Router (ABR) An OSPF router with interfaces connected to the backbone area and

    to at least one other area.

    Autonomous System

    Border Router (ASBR)

    An OSPF router that connects to routers that do not use OSPF for

    the purpose of exchanging external routes into and out of the

    OSPF domain.

    Backbone router A router in one area, the backbone area.

    Internal router A router in a single nonbackbone area.

    Area A set of routers and links that share the same detailed LSDB

    information, but not with routers in other areas, for better

    efficiency.

    Backbone area A special OSPF area to which all other areas must connect. Area 0.External route A route learned from outside the OSPF domain and then advertised

    into the OSPF domain.

    Intra-area route A route to a subnet inside the same area as the router.

    Interarea route A route to a subnet in an area of which the router is not a part.

    Autonomous system In OSPF, a reference to a set of routers that use OSPF.

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    Configuring OSPF Enabling OSPF

    Router OSPF [process #]

    Process number doesnt matter-between 1 and 65,535

    Configure Router ID (optional)

    Router-id [id value]

    Configuring OSPF Areas Network 10.0.0.0 0.255.255.255 area #

    Wildcards

    Pg 361

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    Wildcards Used in OSPF and Access Lists

    0 means that the corresponding octet must

    match exactly 255 means that corresponding octet can beanything

    10.0.0.0 0.255.255.255

    Anything starting with 10.

    192.168.10.0 0.0.255.255

    Anything starting with 192.168

    Pg 361

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    OSPF Configuration Change interval times (optional)

    Ip ospf hello-interval time

    ip ospf dead-interval time interface

    Change Costs or Bandwidth

    ip ospf cost value interface subcommand

    bandwidth value interface subcommand auto-cost reference-bandwidth value router

    subcommand

    Pg 361

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    OSPF Configuration OSPF Authentication (Optional)

    ip ospf authentication interface

    subcommand. area authentication router subcommand.

    Configure support for multiple equal-cost

    routes using maximum-paths number router

    subcommand

    Pg 362

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    OSPF In action

    interface ethernet 0/0

    ip address 10.1.1.1 255.255.255.0

    interface serial 0/0

    ip address 10.1.4.1 255.255.255.0

    interface serial 0/1

    ip address 10.1.6.1 255.255.255.0

    !

    router ospf 1

    network 10.0.0.0 0.255.255.255 area 0

    Pg 362

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    OSPF With Multiple Areas Must have an area 0

    Make good choices

    on areas Decide which subnets

    are on which areas

    Look at area border

    routers

    Pg 364

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    How to program a subnet into a different

    area router ospf 1

    network 10.1.1.1 0.0.0.0 area 0

    network 10.1.4.1 0.0.0.0 area 1

    network 10.1.6.1 0.0.0.0 area 0

    Pg 365

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    OSPF Router ID Setting the router ID can affect DR election

    1. If the router-id rid OSPF subcommand isconfigured, this value is used as the RID.

    2. If any loopback interfaces have an IP addressconfigured and the interface has a line andprotocol status of up/up, the router picks thehighest numeric IP address among the up/up

    loopback interfaces. 3. The router picks the highest numeric IPaddress from all other working (up/up)interfaces.

    Pg 366

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    Loopback Interfaces Virtual interfaces on a router not tied toa

    physical interface

    Always up up

    Useful for setting an ID that will not be

    affected by the state of the physical

    interfaces

    Pg 367

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    Hello and Dead Timers Can be altered to change amount of traffic

    Need to be the same among routers that

    should become neighbors

    Pg 368

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    OSPF Metric (COST)

    1. The cost can be explicitly set using the ip ospf costxinterface subcommand, to a value between 1 and65,535, inclusive.

    2. IOS can calculate a value based on the genericformula Ref-BW / Int-BW, where Ref- BW is a referencebandwidth that defaults to 100 Mbps, and Int-BW is theinterface's bandwidth setting.

    3. The reference bandwidth can be configured from its

    default setting of 100 (100 Mbps) using the router OSPFsubcommand auto-cost reference-bandwidth ref-bw,which in turn affects the calculation of the defaultinterface cost.

    Pg 369

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    OSPF Authentication

    Without Authentication, routers can

    receive false updates

    Null No password

    Text password

    Easy to sniff

    MD5 hash

    Most secure and should be used if any

    Pg 370

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    OSPF Authentication

    Pg 372

    Type Meaning Command to Enable

    Authentication

    What the Password Is Configured

    With

    0 None ip ospf authentication null

    1 Clear text ip ospf authentication ip ospf authentication-key key-value

    2 MD5 ip ospf authentication

    messagedigest

    ip ospf message-digest-key key-number

    md5 key-value

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    Load Balancing

    Ability to share load among paths of equal

    cost

    Default is 4 Up to 16

    Maximum paths # command

    Pg 372

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    Any Questions?