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OSPF
OSPF CCNA
| Question | Answer |
|---|---|
| in the OSPF packet header the protocol field is set to __ and the destination ip address is either ___ or ___ multicast addresses | the protocol field is set to 89 to indicate OSPF, and the destination address is set to one of two multicast addresses: 224.0.0.5 or 224.0.0.6 |
| If the OSPF packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address: ________-05 or ______-06. | If the OSPF packet is encapsulated in an Ethernet frame, the destination MAC address is also a multicast address: 01-00-5E-00-00-05 or 01-00-5E-00-00-06. |
| What are the five different types of OSPF LSPs | HELLO, DBD, LSR, LSU, LSAck |
| HELLO LSP: OSPF packet Type 1 is the OSPF Hello packet. Hello packets are used to: | Discover OSPF neighbors and establish neighbor adjacencies. Advertise parameters on which two routers must agree to become neighbors. Elect the(DR) and(BDR) on multiaccess networks like Ethernet and Frame Relay. |
| OSPF packet header contains | Network Mask of sending interface, Hello Interval: number of secs between sending router's hellos Router Priority for DR/BDR election, (DR)Router ID, BDR ID of the BDR, if any List of Neighbors: lists the OSPF Router ID of the neighboring router(s) |
| OSPF uses a metric called cost. Calculated by . . . | 10^8/bandwidth or 100,000,000/bandwidth. If you had a 100Mbps link, the cost would be 1 because 100,000,000/100,000,000 |
| DBD Database Description packet is TYPE 2 | The Database Description (DBD) packet contains an abbreviated list of the sending router's link-state database and is used by receiving routers to check against the local link-state database |
| Router ID | (RID) is an IP used to ID the router. Cisco chooses Router ID by using highest IP address of all configured loopback interfaces. If no loopback interfaces have an address, OSPF will choose the highest IP address of all active physical interfaces |
| Neighbor | Neighbors are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link. |
| Adjacency | a relationship between 2 OSPF routers that permits direct exchange of route updates. OSPF is really picky. EIGRP, which shares routes with all of its neighbors. OSPF only shares routes with neighbors that have also established adjacencies. |
| OSPF Hello packets are addressed to | 224.0.0.5 |
| Topological database | information from all of the Link State Advertisement packets that have been received for an area. The router uses the information from the topology database as input into the Dijkstra algorithm that computes the shortest path to every network. |
| Link State Advertisement | OSPF data packet containing link-state and routing information that’s shared among OSPF routers.An OSPF router will exchange LSA packets only with routers to which it has established adjacencies. |
| Designated router | (DR) is elected whenever OSPF routers are connected to the same multi-access network. Cisco likes to call these “broadcast” networks, but really, they are networks that have multiple recipients. Try not to confuse multi-access with multipoint. |
| How is the DR election won? | The election is won by the router with the highest priority, and the Router ID is used as a tiebreaker if the priority of more than one router turns out to be the same. |
| BDR | A Backup Designated Router (BDR) is a hot standby for the DR on multi-access links (remember that Cisco sometimes likes to call these “broadcast” networks). The BDR receives all routing updates from OSPF adjacent routers but doesn’t flood LSA updates. |
| OSPF areas An OSPF area is a grouping of contiguous networks and routers. All routers in the same area share a common Area ID. | a router can be a member of more than one area at a time, the Area ID is associated with specific interfaces on the router. |
| a router can be a member of more than one area at a time, This would allow some interfaces to belong to area 1 while the remaining interfaces can belong to area 0. All of the routers within the same area have the same topology table. | When configuring OSPF, you’ve got to remember that there must be an area 0 and that this is typically configured on the routers that connect to the backbone of the network. Areas also play a role in establishing a hierarchical network organization. |
| Broadcast (multi-access) | BMA nets like Ethernet allow multiple devices to connect to the same network as & provide a broadcast ability in which a single packet is delivered to all nodes on the network. In OSPF, a DR & BDR must be elected for each broadcast multi-access network. |
| Non-broadcast multi-access | (NBMA) networks are types like Frame Relay, X.25, & Asynchronous Transfer Mode (ATM). They allow multi-access but have no broadcast ability like Ethernet. NBMA networks require special OSPF configuration to work & neighbor relationships must be defined. |
| The OSPF network command uses a combination of network-address & __ | The OSPF network command uses a combination of network-address and wildcard-mask similar to that which can be used by EIGRP. Unlike EIGRP, however, OSPF requires the wildcard mask. |
| If the OSPF router-id command or the loopback address is configured after the OSPF network command, the router ID will be derived from | the interface with the highest active IP address. |
| The router ID can be modified with the IP address from a subsequent OSPF router-id command by reloading the router or by using the following command: | Router#clear ip ospf process Note: Modifying a router ID with a new loopback or physical interface IP address may require reloading the router. |
| If the router ID is the same on two neighboring routers, the neighbor establishment may not occur & routing may not function. When duplicate OSPF router IDs occur, IOS will display a message similar to: | %OSPF-4-DUP_RTRID1: Detected router with duplicate router ID |
| An IP address from a loopback interface will usually only replace a current OSPF router ID by __________ | reloading the router. |
| how to propogate a default route in OSPF | default-information originate |
| The show ip ospf neighbor command can be used to verify and troubleshoot OSPF neighbor relationships. For each neighbor, this command displays the following output: | Neighbor ID, Pri(OSPF priority of interface), State(FULL=R1 & Neigh have same OSPF link-state Database), Dead Time(how long left without HELLO till Neigh is declared down), Address=IP of neigh's interface, Interface(the int this router has made adjacency |
| two routers have not formed an OSPF adjacency if ___ | show ip ospf neighbor - If the router ID of the neighboring router is not displayed, or if it does not show as a state of FULL, the two routers have not formed an OSPF adjacency |
| Four reasons two routers may not form an OSPF adjacency | The subnet masks do not match, causing the routers to be on separate networks. OSPF Hello or Dead Timers do not match. OSPF Network Types do not match. There is a missing or incorrect OSPF network command. |
| show ip protocols--command is a quick way to verify vital OSPF configuration information | , including the OSPF process ID, the router ID, networks the router is advertising, the neighbors the router is receiving updates from, and the default administrative distance, which is 110 for OSPF. |
| show ip ospf | the show ip ospf displays OSPF process ID and router ID. Additionally, this command displays the OSPF area information as well as the last time the SPF algorithm was calculated |
| A network that cycles between an up state and a down state is referred to as a flapping link. A flapping link can cause OSPF routers in an area to constantly recalculate the SPF algorithm, preventing proper convergence.To minimize problem | , Router waits 5 seconds after receiving an LSU before running SPF algorithm. known as SPF schedule delay.to prevent constantly running algorithmThe router waits 10 seconds after running the SPF algorithm before rerunning algorithm. |
| the reference bandwidth can be modified to accommodate networks with links faster than 100,000,000 bps (100 Mbps) using the OSPF command . . . | "auto-cost reference-bandwidth". When this command is necessary, it is recommended that it is used on all routers so the OSPF routing metric remains consistent. |
| OSPF cost values for FA & FASTER,E,T1,128 kbps,64 kbps,56 kbps | FA=10^8/10^8 =1, E=10^8/10^7=10,T1=64,128=781,64=1562,56=1785 |
| What command is used to view the bandwidth value used for an interface | show interface |
| The figure displays the routing table for R1. R1 believes that both of its serial interfaces are connected to T1 links, although one of the links is a 64 kbps link and the other one is a 256 kbps link. This results in | This results in R1's routing table having two equal-cost paths to the 192.168.8.0/30 network, when Serial 0/0/1 is actually the better path. |
| An alternative method to using the bandwidth command is to use the | "R1(config-if)#ip ospf cost" command, which allows you to directly specify the cost of an interface. |
| DROthers only send their LSAs to the DR and BDR using the multicast address ____ | 224.0.0.6 |
| The DR uses the multicast address ________ | 224.0.0.5 |
| How do the DR and BDR get elected? The following criteria are applied: | 1. DR: Router with the highest OSPF interface priority. 2. BDR: Router with the second highest OSPF interface priority. 3. If OSPF interface priorities are equal, the highest router ID is used to break the tie. |
| show ip ospf neighbor | The command output in the figure displays the neighbor adjacency of each router on the multiaccess network |
| show ip ospf interface fastethernet 0/0 | This command will show the DR, BDR, or DROTHER state of this router, along with the router ID of the DR and BDR on this multiaccess network |
| If all of the routers on the multiaccess network have not finished booting, it is possible that a router with a lower router ID will become the DR. This could be a lower-end router that took less time to boot. | hen the DR is elected, it remains the DR until one of the following conditions occurs: |
| So, how do you make sure that the routers you want to be DR and BDR win the election? Without further configurations, the solution is to either: | Boot up the DR first, followed by the BDR, and then boot all other routers, or Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers. |
| However, as you may have already guessed, we can change the OSPF interface priority to better control our DR/BDR elections. | "ip ospf priority" from interface. Router(config-if)#ip ospf priority {0 - 255} |
| Because priorities are an interface-specific value, they provide better control of the OSPF multiaccess networks. They also allow a router to be . . . | the DR in one network and a DROther in another. |
| verify the priority on a Router | "show ip ospf interface" ----- default priority is 1 |
| Like RIP, OSPF requires the use of the -----command to advertise the 0.0.0.0/0 static default route to the other routers in the area. | default-information originate ........from the router config mode, i.e. router rip or router eigrp |
| O*E2 0.0.0.0/0 [110/1] via 192.168.10.10, 00:05:34, Serial0/0/1 | OSPF external routes fall in two categories: E1 or E2. OSPF accumulates cost for an E1 route as the route is being propagated through the OSPF area. cost of an E2 route is always the external cost, irrespective of the interior cost to reach that route. |
| In order to obtain more accurate cost calculations, it may be necessary to adjust the reference bandwidth value. The reference bandwidth can be modified to accommodate these faster links by using the OSPF command | R1(config-router)#auto-cost reference-bandwidth ? 1-4294967 The reference bandwidth in terms of Mbits per second Notice the value is in Mbps. the default value is equivalent to 100. To increase it to 10GigE speeds, change reference bandwidth to 10000. |
| OSPF Hello and Dead intervals can be modified manually using the following interface commands: | Router(config-if)#ip ospf hello-interval(seconds) Router(config-if)#ip ospf dead-interval (seconds) |
| The mismatching Hello and Dead intervals can be verified on R2 using | how ip ospf interface serial 0/0/0 |
| What command can be used to identify the bandwidth value of an interface used by OSPF metric. | show interface |
| Command used to modify OSPF cost of interface without modifying the bandwidth value of that interface? | The interface command "ip ospf cost ###" |
| What is the default HELLO interval on Ethernet & point-to-point. | HELLO interval is 10 seconds on multiaccess & point-to-point |
| What's the default HELLO interval on NBMA such as Frame Relay, ATM, X.25? | HELLO interval is 30 seconds on NBMA |
| What values must match before OSPF routers form an adjacency? | HELLO & DEAD intervals, Network Type, Subnet Mask |
| In which type of network will DR not be elected | ptp and ptmp |
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