Part 6 - OSPF
Differences in 'show ip ospf interface' Output with OSPFv2 Interface Configuration
'show ip ospf interface g0/0/0' with the new interface configuration - Shows "Attached via Interface Enable" 'show ip ospf interface g0/0/0' with the use of the OSPF 'network' command - Show "Attached via Network Statement"
How a Cisco router chooses its RID
1. If the 'router-id <rid>' OSPF subcommand is configured, this value is used as the RID. 2. If any loopback interfaces have an IP address configured, and the interface has an interface status of up, the router picks the highest numeric IP address among these loopback interfaces. 3. 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 stat will be included by OSPF when choosing its router ID.)
Routing Protocol Functions
1. Learn routing information about IP subnets from neighboring routers. 2. Advertise routing information about IP subnets to neighboring routers. 3. If more than one possible route exists to reach one subnet, pick the best route based on a metric. 4. If the network topology changes-for example, a link fails- react by advertising that some routes have failed and pick a new currently best route. (This process is called convergence.)
Rules for How a Router Sets its OSPF Interface Costs:
1. Set the cost explicitly, using the 'ip ospf cost <x>' interface subcommand, to a value between 1 and 65,535, inclusive. 2. Although it should be avoided, change the interface bandwidth with the 'bandwidth <speed>' command, with <speed> being a number in kilobits per second (Kbps). 3. Change the reference bandwidth, using router OSPF subcommand 'auto-cost reference-bandwidth <ref-bw>', with a unit of megabits per second (Mbps).
Stable OSPF Neighbor States and Their Meanings: Neighbor State - Term for Neighbor - Term for Relationship
2-way - Neighbor - Neighbor Relationship Full - Adjacent Neighbor/Fully Adjacent Neighbor - Adjacency
link-state
A classification of the underlying algorithm used in some routing protocols. Link-state protocols build a detailed database that lists links (subnets) and their state (up, down), from which the best routes can then be calculated.
Area Border Router (ABR)
A router using OSPF in which the router has interfaces in multiple OSPF areas.
Interior Gateway Protocol (IGP)
A routing protocol designed to be used to exchange routing information inside a single autonomous system.
metric
A unit of measure used by routing protocol algorithms to determine the best route for traffic to use to reach a particular destination.
[no] shutdown
An OSPF configuration mode command to disable (shutdown) or enable (no shutdown) the OSPF process
link-state update
An OSPF packet used to send an LSA to a neighboring router.
backup designated router (BDR)
An OSPF router connected to a multiaccess network that monitors the work of the designated router (DR) and takes over the work of the DR if the DR fails.
OSPF Design Terminology: Term - Description
Area Border Router (ABR) - An OSPF router with interfaces connected to the backbone area and to at least one other area. Backbone router - A router connected to the backbone area (includes ABRs) Internal router - A router in one area (not the backbone area) Area - A set of routers and links that shares 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. 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.
Three main branches of routing protocol algorithms exist for IGP routing protocols:
Distance vector (sometimes called Bellman-Ford after its creators) Advanced distance vector (sometimes called "balanced hybrid") Link-state
Normal working states OSPF
FULL/ -: The neighbor state is full, with the "-" instead of letters meaning that the link does not use a DR/BDR. FULL/DR: The neighbor state is full, and the neighbor is the DR. FULL/BDR: The neighbor state is full, and the neighbor is the backup DR (BDR). FULL/DROTHER: The neighbor state is full, and the neighbor is neither the DR nor BDR. (It also implies that the local router is a DR or BDR because the state would reach a full state.) 2WAY/DROTHER: The neighbor state is 2-way, and the neighbor is neither the DR nor BDR-that is, a DROther router. (It also implies that the local router is also a DROther router because otherwise the state would reach a full state.)
interface loopback <number>
Global command to create a loopback interface and to navigate to interface configuration mode for that interface.
OSPF Neighbor Requirements and the Best 'show/debug' commands?
Hello and dead timers must have - 'show ip ospf interface' They must be in the same area. - 'show ip ospf 'interface brief RIDS must be unique. - show ip ospf They must pass any neighbor authentication. show ip ospf interface OSPF process must be done by shut down. show ip ospf, show ip ospf interface
Interior and Exterior Routing Protocols
IGP: A routing protocol that was designed and intended for use inside a single autonomous system (AS) EGP: A routing protocol that was designed and intended for use between different autonomous systems.
maximum paths
In Cisco IOS, a reference to the number of equal cost routes (paths) to reach a single subnet that IOS will add to the IP routing table at the same time.
router ID (router ID)
In EIGRP and OSPF, a 32-bit number, written in dotted-decimal notation, that uniquely identifies each router.
fully adjacent
In OSPF, a characterization of the state of a neighbor in which the two neighbors have reached the full state.
reference bandwidth
In OSPF, a configurable value for the OSPF routing process, used by OSPF when calculating an interface's default OSPF cost metric, calculated as the interface's bandwidth divided by the reference bandwidth.
2-way state
In OSPF, a neighbor state that implies that the router has exchanged Hellos with the neighbor and that all required parameters match.
full state
In OSPF, a neighbor state that implies that the two routers have exchanged the complete (full) contents of their respective LSDBs.
internal router
In OSPF, a router with all interfaces in the same non-backbone area.
Dead Interval
In OSPF, a timer used for each neighbor. A router considers the neighbor to have failed if no Hellos are received from that neighbor in the time defined by the timer.
designated router (DR)
In OSPF, on a multiaccess network, the router that wins an election and is therefore responsible for managing a streamlined process for exchanging OSPF topology information between all routers attached to that network.
link-state database (LSDB)
In OSPF, the data structure in RAM of a router that holds the various LSAs, with the collective LSAs representing the entire topology of the network.
link-state advertisement (LSA)
In OSPF, the name of the data structure that resides inside the LSDB and describes in detail the various components in a network, including routers and links (subnets).
interface bandwidth
In OSPF, the numerator in the calculation of an interface's default OSPF cost metric, calculated as the interface bandwidth divided by the reference bandwidth.
backbone area
In OSPFv2 and OSPFv3, the special area in a multiarea design, with all nonbackbone areas needing to connect to the backbone area, area 0.
neighbor
In routing protocols, another router with which a router decides to exchange routing information.
bandwidth <bandwidth>
Interface subcommand that directly sets the interface bandwidth (Kbps).
ip ospf cost <interface-cost>
Interface subcommand that sets the OSPF cost associated with the interface.
ip ospf dead-interval <number>
Interface subcommand that sets the OSPF dead timer
ip ospf priority <value>
Interface subcommand that sets the OSPF priority, used when electing a new DR or BDR
ip ospf hello-interval <seconds>
Interface subcommand that sets the interval for periodic Hellos
ip ospf <process-id> area <area-number>
Interface subcommand to enable OSPF on the interface and to assign the interface to a specific OSPF area.
ip ospf network {broadcast | point-to-point}
Interface subcommand used to set the OSPF network type of the interface
Neighbor Requirements for OSPF: Requirement - Required for OSPF - Neighbor Missing if Incorrect
Interfaces must be in an up/up state. - Yes - Yes Access Control lists (ACL) must not filter routing protocol messages. - Yes - Yes Interfaces must be in the same subnet - Yes Yes They must pass routing protocol neighbor authentication (if configured). - Yes - Yes Hello and hold/dead timers must match - Yes - Yes Router IDs (RID) must be unique. - Yes - Yes They must be in the same area. - Yes - Yes OSPF process must not be shut down - Yes - Yes Neighboring interfaces must use same MTU settings. - Yes - No Neighboring interfaces must use same OSPF network type - Yes - No
show ip ospf interface [<type> <number>]
Lists a long section of settings, status and counters for OSPF operation on all interfaces, or on the list interface, including the Hello and Dead Timers.
show interfaces
Lists a long set of messages, per interface, that lists configuration, state, and counter information
show ip ospf database
Lists a summary of the LSAs in the database, with one line of output per LSA. It is organized by LSA type (first type 1, then type 2, and so on).
show ip route
Lists all IPv4 routes.
show ip ospf neighbor [<type> <number>]
Lists brief output about neighbors, identified by neighbor router ID, including current state, with one line per neighbor; optionally limits the output to neighbors on the listed interface.
show ip ospf
Lists information about the OSPF process running on the router, including the OSPF router ID, areas to which the router connects, and the number of interfaces in each area.
show ip ospf neighbor
Lists neighbors and current status with neighbors, per interface
show ip route ospf
Lists routers in the routing table learned by OSPF.
show ip ospf interface brief
Lists the interfaces on which the OSPF protocol is enabled (based on the 'network' commands), including passive interfaces.
show ip ospf neighbor <neighbor-id>
Lists the same output as the 'show ip ospf neighbor' detail command, but only for the listed neighbor (by neighbor RID).
Neighbor Maintenance Tasks Once Neighbors Have Been Established With OSPF
Maintain neighbor state by sending Hello messages based on the Hello Interval and listening for Hellos before the Dead Interval expires. Flood any changed LSAs to each neighbor. Reflood unchanged LSAs as their lifetime expires (default 30 minutes).
router-id <id>
OSPF command that statically sets the router ID
OSPF Passive Interfaces
OSPF continues to advertise about the subnet that is connected to the interface. OSPF no longer sends OSPF Hellos on the interface. OSPF no longer processes any received Hellos on the interface.
passive-interface <default>
OSPF subcommand that changes the OSPF default for interfaces to be passive instead of active (not passive).
no passive-interface <type> <number>
OSPF subcommand that tells OSPF to be active (not passive) on that interface or subinterface.
default-information originate [always]
OSPF subcommand to tell OSPF to create and advertise an OSPF default route, as long as the router has some default route (or to always advertise a default, if the 'always' option is configured).
OSPF area design follows a couple of basic rules. To apply the rules, start with a clean drawing of the internetwork, with routers, and all interfaces. Then choose the area for each router interface, as follows:
Put all interfaces connected to the same subnet inside the same area. An area should be contiguous. Some routers may be internal to an area, with all interfaces assigned to that single area. Some routers may be Area Border Routers (ABR) because some interfaces connect to the backbone area, and some connect to nonbackbone areas. All nonbackbone areas must have a path to reach the backbone area (area 0) by having at least one ABR connected to both the backbone area and the nonbackbone area.
IP IGP Metrics: IGP - Metric - Description
RIPv2 - Hop count - The number of routers (hops) between a router and the destination subnet OSPF - Cost - The sum of all interface cost settings for all links in a route, with the cost defaulting to be based on interface bandwidth. EIGRP - Calculation based on bandwidth and delay - Calculated based on the route's slowest link and the cumulative delay associated with each interface in the route.
clear ip ospf process
Resets the OSPF process, resetting all neighbor relationships and also causing the process to make a choice of OSPF RID.
maximum-paths <number-of-paths>
Router subcommand that defines the maximum number of equal-cost routes that can be added to the routing table.
netwrok <ip-address> <wildcard-mask> <area> <area-id>
Router subcommand that enables OSPF on interfaces matching the address/wildcard combination and sets the OSPF area.
router ospf <process-id>
Router subcommand that enters OSPF configuration mode for the listed process.
passive-interface <type> <number>
Router subcommand that makes the interface passive to OSPF, meaning that the OSPF process will not form neighbor relationships with neighbors reachable on that interface.
auto-cost reference-bandwidth <number>
Router subcommand that tells OSPF the numerator in the Reference_bandwidth / Interface_bandwidth formula used to calculate the OSPF cost based on the interface bandwidth.
Key points arguing for the use of multiple areas in larger OSPF networks:
Routers require fewer CPU cycles to process the smaller per-area LSDB with the SPF algorithm, reducing CPU overhead and improving convergence time. The smaller per-area LSDB requires less memory. Changes in the network (for example, links failing and recovering) require SPF calculations only on routers in the area where the link changed state, reducing the number of routers that must rerun SPF. Less information must be advertised between areas, reducing the bandwidth required to send LSAs.
show ip route <ip-address> <mask>
Shows a detailed description of the route for the listed subnet/mask.
show ip protocols
Shows routing protocols parameters and current timer values.
OSPF Interface Configuration
Step 1. Use the 'no network <network-id> area <area-id>' subcommands in OSPF configuration mode to remove the 'network' commands. Step 2. Add one 'ip ospf <process-id> area <area-id>' command in interface configuration mode under each interface on which OSPF should operate, with the correct OSPF process (<process-id>) and the correct OSPF area number.
Configure Single-Area OSPFv2
Step 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 router ID by doing the following: A. Use the 'router-id <id-value>' router subcommand to define the router ID, or 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 working loopbacks, or 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 interface for the listed area.
Configurable Settings to Influence the DR/BDR Election
The highest OSPF interface priority: The highest value wins during an election, with values ranging from 0 to 255. The highest OSPF Router ID: If the priority ties, the election chooses the router with the highest OSPF RID.
distance vector
The logic behind the behavior of some interior routing protocols, such as RIP. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops but are computationally simpler than link-state routing algorithms.
Shortest Path First (SPF) algorithm
The name of the algorithm used by link-state routing protocols to analyze the LSDB and find the least-cost routes from that router to each subnet.
The 2-way state is a particularly important OSPF state. At that point, the following major facts are true:
The router received a Hello from the neighbor, with that router's own RID listed as being seen by the neighbor. The router has checked all the parameters in the Hello received from the neighbor, with no problems. The router is willing to become an OSPF neighbor. If both routers reach a 2-way state with each other, it means that both routers meet all OSPF configuration requirements to become neighbors. Effectively, at that point, they are neighbors and ready to exchange their LSDB with each other.
topology database
The structured data that describes the network topology to a routing protocol. Link-state and balanced hybrid routing protocols use topology tables, from which they build the entries in the routing table.
convergence
The time required for routing protocols to react to changes in the network, removing bad routes and adding new, better routes so that the current best routes are in all the routers' routing tables.
If you misconfigure network type settings such that one router uses broadcast, and the other uses point-to-point, the following occurs:
The two routers become fully adjacent neighbors (that is, they reach a full state). They exchange their LSDBs. They do not add IP routes to the IP routing table.
OSPF Wildcard Matching
Wildcard 0.0.0.0: Compare all four octets. In other words, the numbers must exactly match. Wildcard 0.0.0.255: Compare the first three octets only. Ignore the last octet when comparing the numbers. Wildcard 0.0.255.255: Compare the first two octets only. Ignore the last two octets when comparing the numbers. Wildcard 0.255.255.255: Compare the first octet only. Ignore the last three octets when comparing the numbers. Wildcard 255.255.255.255: Compare nothing; this wildcard mask means that all addresses will match the network command.
Hello Interval
With OSPF and EIGRP, an interface timer that dictates how often the router should send Hello messages.
An engineer connects routers R11 and R12 to the same Ethernet LAN and configures them to use OSPFv2. Which answers describe a combination of settings that would prevent the two routers from becoming OSPF neighbors? (Choose two answers.) a. R11's interface uses area 11 while R12's interface uses area 12. b. R11's OSPF process uses process ID 11 while R12 uses process ID 12. c. R11's interface uses OSPF priority 11 while R12's uses OSPF priority 12. d. R11's interface uses an OSPF Hello timer value of 11 while R12's uses 12.
a. R11's interface uses area 11 while R12's interface uses area 12. d. R11's interface uses an OSPF Hello timer value of 11 while R12's uses 12.
OSPF interface configuration uses the 'ip ospf <process-id> area <area-number> configuration command. In which modes do you configure the following settings when using this command? (Choose two answers.) a. The router ID is configured explicitly in router mode. b. The router ID is configured explicitly in interface mode. c. An interface's area number is configured in router mode. d. An interface's area number is configured in interface mode.
a. The router ID is configured explicitly in router mode. d. An interface's area number is configured in interface mode.
Which of the following 'network' commands, following the command 'router ospf 1', tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1? a. network 10.1.0.0 0.0.255.255 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 0 d. network 10.1.1.0 255.0.0.0 area 0 e. network 10.0.0.0 255.0.0.0 area 0
a. network 10.1.0.0 0.0.255.255 area 0
Which of the following commands list the OSPF neighbors off interfaces serial 0/0? (Choose two answers.) a. show ip ospf neighbor b. show ip ospf interface brief c. show ip neighbor d. show ip interface e. show ip ospf neighbor serial 0/0
a. show ip ospf neighbor e. show ip ospf neighbor serial 0/0
Which of the following configuration settings on a router does not influence which IPv4 route a router chooses to add to its IPv4 routing table when using OSPFv2? a. 'auto-cost reference-bandwidth' b. 'delay' c. 'bandwidth' d. 'ip ospf cost'
b. 'delay'
Which of the following OSPF neighbor states is expected when the exchange of topology information is complete between two OSPF neighbors? a. 2-way b. Full c. Up/up d. Final
b. Full
Which of the following interior routing protocols support VLSM? (Choose three answers.) a. RIPv1 b. RIPv2 c. EIGRP d. OSPF
b. RIPv2 c. EIGRP d. OSPF
Routers R1 and R2, with router IDs 1.1.1.1 and 2.2.2.2, connect over an Ethernet WAN link. The configuration uses all defaults, except giving R1 and interface priority of 11 and changing both routers to use OSPF network type point-to-point. If the WAN link initializes for both routers on the same time, which of the following answers are true? (Choose two answers.) a. Router R1 will become the DR. b. Router R1 will dynamically discover the existence of router R2. c. Router R2 will be neither the DR nor the BDR. d. Router R2's 'show ip ospf' neighbor command will list R1 with a status of "FULL/DR."
b. Router R1 will dynamically discover the existence of router R2. c. Router R2 will be neither the DR nor the BDR.
Routers R1 and R2, with router IDs 1.1.1.1 and 2.2.2.2, connect over an Ethernet WAN link. If using all default OSPF settings, if the WAN link initializes for both routers at the same time, which of the following answers are true? (Choose two answers.) a. Router R1 will become the DR b. Router R1 will dynamically discover the existence of router R2. c. Router R2 will be neither the DR nor the BDR. d. Router R1's 'show ip ospf neighbor' command will list R2 with a statue of "FULL/DR"
b. Router R1 will dynamically discover the existence of router R2. d. Router R1's 'show ip ospf neighbor' command will list R2 with a statue of "FULL/DR"
Which of the following 'network' commands, following the command 'router ospf 1', tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1? a. network 10.0.0.0 255.0.0.0 area 0 b. network 10.0.0.0 0.255.255.255 area 0 c. network 10.0.0.1 0.0.0.255 area 0 d. network 10.0.0.1 0.0.255.255 area 0
b. network 10.0.0.0 0.255.255.255 area 0
An engineer migrates from a more traditional OSPFv2 configuration that uses 'network' commands in OSPF configuration mode to instead use OSPFv2 interface configuration. Which of the following commands configures the area number assigned to an interface in this new configuration? a. The 'area' command in interface configuration mode b. The 'ip ospf' command in interface configuration mode c. The 'router ospf' command in interface configuration mode d. The 'network' command in interface configuration mode
b. the 'ip ospf command' in interface configuration mode
Two OSPF Network Types and Key Behaviors: Network Type Keyword - Dynamically Discovers Neighbors - Uses a DR/BDR
broadcast - Yes - Yes point-to-point - Yes - No
An engineer connects router R13 and R14 to the same Ethernet LAN and configures them to use OSPFv2. Which answers describe a combination of settings that would prevent the two routers from becoming OSPF neighbors? a. Both routers' interface IP addresses reside in the same subnet. b. Both routers' OSPF process uses process ID 13. c. Both routers' OSPF process uses router ID 13.13.13.13. d. Both routers' interfaces use an OSPF Dead interval of 40.
c. Both routers' OSPF process uses router ID 13.13.13.13.
Which of the following routing protocols use a metric that is, by default, at least partially affected by link bandwidth? (Choose two answers.) a. RIPv1 b. RIPv2 c. EIGRP d. OSPF
c. EIGRP d. OSPF
A company has a small/medium-sized network with 15 routers and 40 subnets and uses OSPFv2. Which of the following is considered an advantage of using a single-area design as opposed to a multiarea design? a. It reduces the processing overhead on most routers. b. Status changes to one link may not require SPF to run on all other routers. c. It allows for simpler planning and operations. d. It allows for route summarization, reducing the size of IP routing tables.
c. It allows for simpler planning and operations.
Two routers using OSPFv2 have become neighbors and exchanged all LSAs. As a result, Router R1 now lists some OSPF-learned routes in its routing table. Which of the following best describes how R1 uses those recently learned LSAs to choose which IP routes to add to its IP routing table? a. Each LSA lists a route to be copied to the routing table. b. Some LSAs list a route that can be copied to the routing table. c. Run some SPF math against the LSAs to calculate the routes. d. R1 does not use the LSAs at all when choosing what routes to add.
c. Run some SPF math against the LSAs to calculate the routes.
Per the command output, with how many routers is router R9 full adjacent over its Gi0/0 interface? R9# show ip ospf interface brief Interface PID Area IP Address/Mask Cost State Nbrs FC Gi0/0 1 0 10.1.1.1/24 1 DROTH 2/5 a. 7 b. 0 c. 5 d. 2
d. 2
Which of the following routing protocols is considered to use link-state logic? a. RIPv1 b. RIPv2 c. EIGRP d. OSPF
d. OSPF
Router R15 has been a working part of a network that uses OSPFv2. An engineer then issues the 'shutdown' command in OSPF configuration mode on R15. Which of the following occurs? a. R15 empties its IP routing table of all OSPF routes but keeps its LSDB intact. b. R15 empties its LSDB but keeps OSPF neighbor relationships active. c. R15 keeps OSPF neighbors open but does not accept new OSPF neighbors. d. R15 keeps all OSPF configuration but ceases all OSPF activities (routes, LSDB, neighbors).
d. R15 keeps all OSPF configuration but ceases all OSPF activities (routes, LSDB, neighbors).
