CCNA 200-301, Volume I Chapter 20 Implementing OSPF

1、CCNA 200-301, Volume IChapter 20Implementing OSPFObjectivesImplementing Single-Area OSPFv2OSPFv2 Interface ConfigurationAdditional OSPFv2 FeaturesStep 1. Use the router ospf process-id global command to enter OSPF configuration mode for a particular OSPF process.Step 2. (Optional) Configure the OSPF

2、 router ID by doing the following:A. Use the router-id id-value router subcommand to define the router ID.B. Use the interface loopback number global command, along with an ip address address mask command, to configure an IP address on a loopback interface (chooses the highest IP address of all work

3、ing loopbacks).C. Rely on an interface IP address (chooses the highest IP address of all working nonloopbacks).Step 3. Use one or more network ip-address wildcard-mask area area-id router subcommands to enable OSPFv2 on any interfaces matched by the configured address and mask, enabling OSPF on the

4、interface for the listed area.Step 4. (Optional) Use the passive-interface type number router subcommand to configure any OSPF interfaces as passive if no neighbors can or should be discovered on the interface.Implementing Single-Area OSPFImplementing Single-Area OSPFSample Network for OSPF Single-A

5、rea ConfigurationIPv4 Address Configuration on R1 (Including VLAN Trunking)The router ospf 1 global command puts the users into OSPF configuration mode and sets the OSPF process-id to 1.This process-id just needs to be unique on the local router and be between 1 and 65,535.The OSPF network command t

6、ells the router to find its local interfaces that match the first two parameters in the command. Those interfaces then discover neighbors, create neighbor relationships and assign the interface to the area listed.Implementing Single-Area OSPFThe wildcard mask gives the local router its rules for mat

7、ching its own interfaces:Wildcard 0.0.0.0: Compare all 4 octets. In other words, the numbers must exactly match.Wildcard 0.0.0.255: Compare the first 3 octets only. Ignore the last octet when comparing the numbers.Wildcard 0.0.255.255: Compare the first 2 octets only. Ignore the last 2 octets when c

8、omparing the numbers.Wildcard 0.255.255.255: Compare the first octet only. Ignore the last 3 octets when comparing the numbers.Wildcard 255.255.255.255: Compare nothingthis wildcard mask means that all addresses will match the network command.Matching with the OSPF network CommandMatching with the O

9、SPF network CommandMatching with the OSPF network CommandOSPF Verification CommandsInterface: This is the local routers interface connected to the neighbor. For example, the first neighbor in the list is reachable through R3s S0/0/0 interface.Address: This is the neighbors IP address on that link. A

10、gain, for this first neighbor, the neighbor, which is R1, uses IP address 10.1.13.1.State: While many possible states exist, for the details discussed in this chapter, FULL is the correct and fully working state in this case.Neighbor ID: This is the router ID of the neighbor.Verifying OSPFv2 Single

11、AreaWhen OSPF is working correctly in the internetwork with a single-area design, all of the routers will have the same LSDB contents.For the purposes of the CCNA knowledge of the specifics about the output of this command are not required.Verifying OSPFv2 Single Area IPv4 Routes Added by OSPF Route

12、r R3 Configuration and the show ip protocols Commandshow ip ospf interface brief CommandOSPF speaking routers must have a router-ID (RID) for proper operation.By default, routers will choose an interface IP address to use as the RID.Typically, engineers prefer to choose each routers RID.A Cisco rout

13、er uses the following process when the router reloads and brings up the OSPF process to find the RID:If the router-id rid OSPF subcommand is configured, this value is used as the RID.If any loopback interfaces have an IP address configured, and the interface has an interface status of up, the router

14、 picks the highest numeric IP address among these loopback interfaces.The router picks the highest numeric IP address from all other interfaces whose interface status code (first status code) is up. (In other words, an interface in up/down state will be included by OSPF when choosing its router ID.)

15、Configuring the OSPF Router IDOSPF Router ID Configuration ExamplesImplementing Multiarea OSPFOSPF Single-Area Configuration OSPF Single-Area ConfigurationAn OSPF interface configured as passive will do the following:Quit sending OSPF Hellos on the interface.Ignore received Hellos on the interface.D

16、o not form neighbor relationships over the interface.OSPF does not form neighbor relationships over interfaces configured as passive, but does still advertise about the subnet connected to that interface.OSPF Passive InterfacesTo configure an interface as passive there are two options:Adding the pas

17、sive-interface type number command under router configuration mode.Configuring interfaces as passive by default with the passive-interface default command under global configuration mode and using the no passive-interface type number command to disable passive of specific interfaces.OSPF Passive Int

18、erfacesOSPF makes it interesting to figure out which interfaces are passive:The show ip ospf interface brief command lists all interfaces on which OSPF is enabled including passive interfaces.The show ip ospf interface command lists a single line that mentions that the interface is passive.OSPF Pass

19、ive InterfacesThe most classic case for using a routing protocol to advertise a default route has to do with an enterprises connection to the Internet.The enterprise engineer uses the following design goals:All routers learn specific routes for subnets inside the company; a default route is not need

20、ed when forwarding packets to these destinations.One router connects to the Internet, and it has a default route that points toward the Internet.All routers should dynamically learn a default route, used for all traffic going to the Internet, so that all packets destined to locations in the Internet

21、 go to the one router connected to the Internet.OSPF Default RoutesR1 has a static default route with a next-hop address of the ISP router.R1 uses the OSPF default-information originate command to advertise a default route using OSPF to B1 and B2.R1 needs a default route, either defined as a static

22、default route, learned from the ISP with DHCP or learned from the ISP with a routing protocol like eBGP unless the always parameter is used.OSPF Default RoutesThe branch routers then place the learned OSPF default route into their routing tables.OSPF Default RoutesR1 uses DHCP to learn its IP addres

23、s on its Gi0/3 interface from the ISP.R1 then creates a static default route with the ISP routers IP address of 192.0.2.1 as the next-hop address.OSPF Default RoutesCisco routers allow three different ways to change the OSPF interface cost:Directly, using the interface subcommand ip ospf cost x.Usin

24、g the default calculation per interface, and changing the interface bandwidth setting, which changes the calculated value.Using the default calculation per interface, and changing the OSPF reference bandwidth setting, which changes the calculated value.IOS uses the Reference_bandwidth / Interface_bandwidth formula to choose an interfaces cost.With this formula the following sequence of logic happens:A higher interface bandwidththat is, a faster bandwidthresults in a lower number in the calculation.A lower number