Module 2: Single-Area OSPF 
2.1 Link-State Routing Protocol 
2.1.1 Overview of link-state routing  

Link-state routing algorithms maintain a complex database of topology information.


2.1.2 Link-state routing protocol features  

Link-state routing protocols collect route information from all other routers in the network or within a defined area of the network. Once all of the information is collected, each router calculates the best paths to all destinations in the network.


The following are some link-state routing protocol functions:


Each router multicasts hello packets to keep track of the state of the neighbor routers. Each router uses LSAs to keep track of all the routers in its area of the network. The hello packets contain information about the networks that are attached to the router. In Figure , P4 knows about its neighbors, P1 and P3, on the Perth3 network. The LSAs provide updates on the state of links that are interfaces on other routers in the network.

每個路由器群播hello封包來追蹤鄰近路由器的狀況。每顆路由器使用LSA來追蹤在它的網路區域內的所有路由器。hello封包包含與路由器連接的網路相關資訊。圖, P4 知道在Perth3網路上的鄰居P1和P3。LSA提供對在網路上其他路由器的介面的鏈路狀態的更新。

Routers that use link-state routing protocols have the following features:


2.1.3 How routing information is maintained  

When a failure occurs in the network, such as a neighbor becomes unreachable, link-state protocols flood LSAs with a special multicast address throughout an area. This process sends information out all ports, except the port on which the information was received. Each link-state router takes a copy of the LSA and updates its link-state, or topological database. The link-state router then forwards the LSA to all neighbor devices. LSAs cause every router within the area to recalculate routes. For this reason, the number of link-state routers within an area should be limited.


2.1.4 Link-state routing algorithms 

An LSA exchange is triggered by an event in the network instead of periodic updates. This speeds up the convergence process because there is no need to wait for a series of timers to expire before the routers can converge.


2.1.5 Advantages and disadvantages of link-state routing 

This page lists the advantages and disadvantages of link-state routing protocols. The following are advantages of link-state routing protocols:


The following are some disadvantages of link-state routing protocols:


2.1.6 Compare and contrast distance vector and link-state routing 
2.2 Single-Area OSPF Concepts 
2.2.1 OSPF overview 

OSPF can be used and configured as a single area for small networks. It can also be used for large networks.

OSPF 能被使用並組態成單一區域的小型網路。 它也能用在大型的網路。

As shown in Figure , large OSPF networks use a hierarchical design. Multiple areas connect to a distribution area, or area 0 which is also called the backbone. The design approach allows for extensive control of routing updates. Area definition reduces routing overhead, speeds up convergence, confines network instability to an area, and improves performance.

如同圖 顯示,大的OSPF網路使用階層式設計。多個區域連接到一個分區域,或也被稱為主幹的區域0。此設計方法使路由更新可廣泛控制。區域定義減少路由負擔、加快收歛、將網路不穩定性限制到一個區域和改進效能。

2.2.2 OSPF terminology 

OSPF gathers information from neighbor routers about the link status of each OSPF router. This information is flooded to all its neighbors. An OSPF router advertises its own link-states and passes on received link-states.

OSPF 收集從鄰近路由器來的關於每個OSPF 路由器的鏈結狀況的資訊。這資訊氾送給它所有的鄰居。OSPF 路由器通告它自己的鏈路狀態和傳遞收到的鏈路狀態。

The routers process the information about link-states and build a link-state database. Every router in the OSPF area will have the same link-state database. Therefore, every router has the same information about the state of the links and the neighbors of every other router.


Each router then applies the SPF algorithm on its own copy of the database. This calculation determines the best route to a destination. The SPF algorithm adds up the cost, which is a value that is usually based on bandwidth. The lowest cost path is added to the routing table, which is also known as the forwarding database.

每顆路由器隨後對它自己的資料庫複本使用SPF演算法。此計算決定到目的地的最佳路徑。SPF 演算法加總成本,成本通常依據頻寬的一個值。最小成本路徑會加到路由表,它也被稱為轉送資料庫。

Each router keeps a list of adjacent neighbors, called the adjacency database. The adjacency database is a list of all the neighbor routers to which a router has established bidirectional communication. This is unique to each router.


To reduce the number of exchanges of routing information among several neighbors on the same network, OSPF routers elect a designated router (DR) and a backup designated router (BDR) that serve as focal points for routing information exchange.


2.2.3 Comparing OSPF with distance vector routing protocols 

OSPF guarantees loop-free routing. Distance vector protocols may cause routing loops.

OSPF 保證無迴圈路由,距離向量協定可能引起路由迴圈。

2.2.4 Shortest path algorithm 

In this algorithm, the best path is the lowest cost path. Edsger Wybe Dijkstra, a Dutch computer scientist, formulated the shortest path-algorithm, also known as Dijkstra's algorithm.

在此演算法,最佳路徑是最小成本的路徑。荷蘭的電腦科學家Edsger Wybe Dijkstra導出最短路徑演算法的公式,也被稱為Dijkstra演算法。

2.2.5 OSPF network types 

OSPF interfaces automatically recognize three types of networks:

  • Broadcast multi-access, such as Ethernet
  • Point-to-point networks
  • Nonbroadcast multi-access (NBMA), such as Frame Relay

OSPF 介面辨識三種網路的類型:

A fourth type, point-to-multipoint, can be manually configured on an interface by an administrator. 


In a multi-access network, it is not known in advance how many routers will be connected. In point-to-point networks, only two routers can be connected.


In a broadcast multi-access network segment, many routers may be connected. If every router had to establish full adjacency with every other router and exchange link-state information with every neighbor, there would be too much overhead. If there are 5 routers, 10 adjacency relationships would be needed and 10 link-states sent. If there are 10 routers then 45 adjacencies would be needed. In general, for n routers, n*(n-1)/2 adjacencies would need to be formed.

在廣播多重存取網路區段,可以連接許多路由器。假如每個路由器必須建立和每個其他路由器完全的鄰接且和每個鄰居交換鏈路狀態資訊,將會有太多的額外負擔。假如有5顆路由器,將需要10個鄰接關係和送出10鏈路狀態。假如有10顆路由器,將需要45個鄰接。一般而言,對n個路由器,將需要形成n*(n-1 )/2個鄰接。

The solution to this overhead is to hold an election for a designated router (DR). This router becomes adjacent to all other routers in the broadcast segment. All other routers on the segment send their link-state information to the DR. The DR in turn acts as the spokesperson for the segment. The DR sends link-state information to all other routers on the segment using the multicast address of for all OSPF routers.

對此額外負擔的解決方式是執行指定路由器(DR)的選舉。此路由器成為毗連到在廣播區段內所有其他路由器。在此區段內所有其他路由器發送它們的鏈路狀態資訊給DRDR隨後作用如同這區段的發言人。DR使用對所有OSPF路由器的多點廣播位址224 .0 .0 .5來發送鏈路狀態資訊給在區段內的所有其他路由器。

Despite the gain in efficiency that electing a DR provides, there is a disadvantage. The DR represents a single point of failure. A second router is elected as a backup designated router (BDR) to take over the duties of the DR if it should fail. To ensure that both the DR and the BDR see the link-states all routers send on the segment, the multicast address for all designated routers,, is used.


On point-to-point networks only two nodes exist and no DR or BDR is elected. Both routers become fully adjacent with each other.


2.2.6 OSPF Hello protocol 

When a router starts an OSPF routing process on an interface, it sends a hello packet and continues to send hellos at regular intervals. The rules that govern the exchange of OSPF hello packets are called the Hello protocol.

當路由器要在某介面開始OSPF路由程序時,它送出一個hello封包且定期持續送出hello。管理支配OSPF hello封包交換的規則稱為Hello協定。

At Layer 3 of the OSI model, the hello packets are addressed to the multicast address This address is “all OSPF routers”. OSPF routers use hello packets to initiate new adjacencies and to ensure that neighbor routers are still functioning. Hellos are sent every 10 seconds by default on broadcast multi-access and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, the default time is 30 seconds.

在OSI模式的第3層,hello封包的群播位址是224 .0 .0 .5。此位址代表“所有OSPF路由器 ”。OSPF 路由器使用hello封包來起始新的鄰接和確保鄰近路由器仍然在運作。在廣播多重存取和點對點網路,預設每10秒送出Hello。在連接到諸如訊框中繼的NBMA網路上的介面預設時間是30秒。

On multi-access networks the Hello protocol elects a designated router (DR) and a backup designated router (BDR).


Although the hello packet is small, it consists of the OSPF packet header. For the hello packet the type field is set to 1.

雖然hello封包是小的,它包含OSPF 封包標頭。hello封包的類型欄位是被設為1。

The hello packet carries information that all neighbors must agree upon before an adjacency is formed, and link-state information is exchanged.


2.2.7 Steps in the operation of OSPF 

When a router starts an OSPF routing process on an interface, it sends a Hello packet and continues to send Hellos at regular intervals. The set of rules that govern the exchange of OSPF Hello packets is called the Hello protocol. On multi-access networks, the Hello protocol elects a designated router (DR) and a backup designated router (BDR). The Hello carries information about which all neighbors must agree to form an adjacency and exchange link-state information. On multi-access networks the DR and BDR maintain adjacencies with all other OSPF routers on the network.  

當路由器在某一介面要開始OSPF路由程序時,它送出一個Hello封包且定期持續送出Hello。管理支配OSPF hello封包交換的這組規則稱為Hello協定。在多重存取網路,Hello協定選舉指定路由器(DR)和備份指定路由器(BDR)。Hello載送所有鄰居必須同意而形成鄰接性的資訊且交換鏈路狀態資訊。在多重存取網路, DR和BDR 與網路上的所有其他OSPF路由器保持鄰接。

Adjacent routers go through a sequence of states. Adjacent routers must be in the full state before routing tables are created and traffic routed. Each router sends link-state advertisements (LSA) in link-state update (LSU) packets. These LSAs describe all of the routers links. Each router that receives an LSA from its neighbor records the LSA in the link-state database. This process is repeated for all routers in the OSPF network.

鄰近路由器經歷了一序列的狀況。在路由表格被產生和訊務交通被繞送之前,鄰近路由器必需經此全部的狀況。每個路由器發送在鏈路狀態更新(LSU)封包內的鏈路狀態廣播(LSA)。這些LSA描述所有路由器鏈路。從它的鄰居收到LSA的每個路由器記錄LSA在鏈路狀態資料庫。此程序在OSPF 網路的所有路由器中重複進行。

When the databases are complete, each router uses the SPF algorithm to calculate a loop free logical topology to every known network. The shortest path with the lowest cost is used in building this topology, therefore the best route is selected.

當資料庫建置完成,每個路由器使用SPF演算法來計算到每個已知網路的無迴圈的邏輯拓撲。使用最小 成本的最短路徑來建立此拓撲,因此最佳路徑被選擇了。

When there is a change in a link-state, routers use a flooding process to notify other routers on the network about the change. The Hello protocol dead interval provides a simple mechanism for determining that an adjacent neighbor is down. -

當一鏈路狀態有改變,時,路由器使用氾送程序來通知網路上的其他路由器關於此改變。Hello協定的停效(死 )間隔提供一個簡單的機制來決定鄰近鄰居是否當機。

2.3 Single-Area OSPF Configuration 
2.3.1 Configuring OSPF routing process  

OSPF routing uses the concept of areas. Each router contains a complete database of link-states in a specific area. An area in the OSPF network may be assigned any number from 0 to 65,535. However a single area is assigned the number 0 and is known as area 0. In multi-area OSPF networks, all areas are required to connect to area 0. Area 0 is also called the backbone area.

OSPF路由使用區域的觀念。每個路由器包含一個在一特定區域內完整的鏈路狀態資料庫。在OSPF 網路內的一個區域可以指定為從0到65 ,535的任何编號,然而被指定為編號0的單一區域,即所謂的區域0。在多區域的OSPF網路,所有區域都要連接到區域0。區域0也被稱為主幹區域。

OSPF configuration requires that the OSPF routing process be enabled on the router with network addresses and area information specified. Network addresses are configured with a wildcard mask and not a subnet mask. The wildcard mask represents the links or host addresses that can be present in this segment. Area IDs can be written as a whole number or dotted decimal notation.

OSPF組態要求OSPF路由程序在路由器上啟用而具有網路位址和特定區域資訊。 網路位址和萬用遮罩組態在一起,而不是子網路遮罩。萬用遮罩代表有可能出現在這區段的鏈路或主機位址。區域ID可寫成整個數字或寫成點十進制記數。

To enable OSPF routing, use the global configuration command syntax:

Router(config)#router ospfprocess-id


          Router(config)#router ospf 程序ID

The process ID is a number that is used to identify an OSPF routing process on the router. Multiple OSPF processes can be started on the same router. The number can be any value between 1 and 65,535. Most network administrators keep the same process ID throughout an autonomous system, but this is not a requirement. It is rarely necessary to run more than one OSPF process on a router. IP networks are advertised as follows in OSPF:

Router(config-router)#network address wildcard-mask area area-id

程序ID是用來辨識路由器上一個OSPF路由程序的號碼。一顆路由器可啟動多個OSPF程序。此號碼可為1到65 ,535間的任意值。大部分網路管理者對一個自主系統,保持相同程序ID, 但這不是必要的。在一個路由器上很少需要執行超過一個OSPF程序。IP網路在OSPF以下列方式廣播:

           Router(config-router)# network 位址  萬用遮罩  area 區域ID

Each network must be identified with the area to which it belongs. The network address can be a whole network, a subnet, or the address of the interface. The wildcard mask represents the set of host addresses that the segment supports. This is different than a subnet mask, which is used when configuring IP addresses on interfaces.


Lab Exercise: Configuring the OSPF Routing Process

In this lab, students will setup an IP addressing scheme for OSPF area 0 and configure and verify OSPF routing.

2.3.2 Configuring OSPF loopback address and router priority 

When the OSPF process starts, the Cisco IOS uses the highest local active IP address as its OSPF router ID. If there is no active interface, the OSPF process will not start. If the active interface goes down, the OSPF process has no router ID and therefore ceases to function until the interface comes up again

當OSPF程序開始,Cisco IOS使用本機最高運作IP位址當它的OSPF路由器ID。假如沒有活動介面,OSPF程序將不會開始。假如活動介面停止,OSPF程序因沒有路由器ID而終止作用,一直到介面又起效用。.

To ensure OSPF stability there should be an active interface for the OSPF process at all times. A loopback interface, which is a logical interface, can be configured for this purpose. When a loopback interface is configured, OSPF uses this address as the router ID, regardless of the value. On a router that has more than one loopback interface, OSPF takes the highest loopback IP address as its router ID.


To create and assign an IP address to a loopback interface use the following commands:

Router(config)#interface loopback number

Router(config-if)#ip address ip-address subnet-mask



          Router(config)#interface loopback 編號

          Router(config)#ip address ip位址 子網路遮罩

It is considered good practice to use loopback interfaces for all routers running OSPF. This loopback interface should be configured with an address using a 32-bit subnet mask of A 32-bit subnet mask is called a host mask because the subnet mask specifies a network of one host. When OSPF is requested to advertise a loopback network, OSPF always advertises the loopback as a host route with a 32-bit mask.


In broadcast multi-access networks there may be more than two routers. OSPF elects a designated router (DR) to be the focal point of all link-state updates and link-state advertisements. Because the DR role is critical, a backup designated router (BDR) is elected to take over if the DR fails.


If the network type of an interface is broadcast, the default OSPF priority is 1. When OSPF priorities are the same, the OSPF election for DR is decided on the router ID. The highest router ID is selected.


The election result can be determined by ensuring that the ballots, the hello packets, contain a priority for that router interface. The interface reporting the highest priority for a router will ensure that it becomes the DR.


The priorities can be set to any value from 0 to 255. A value of 0 prevents that router from being elected. A router with the highest OSPF priority will be selected as the DR. A router with the second highest priority will be the BDR. After the election process, the DR and BDR retain their roles even if routers are added to the network with higher OSPF priority values.


Modify the OSPF priority by entering global interface configuration ip ospf priority command on an interface that is participating in OSPF. The command show ip ospf interface will display the interface priority value as well as other key information.

Router(config-if)#ip ospf prioritynumber

Router#show ip ospf interfacetype number

在參與OSPF的一個介面上以進入全域介面組態 ip ospf priority指令來更改OSPF優先權。 指令show ip ospf interface 將顯示介面優先權值和其他的關鍵資訊。

          Router(config-if)#ip ospf priority 數值

          Router#show ip ospf interface 型態 數值

Lab Exercise: Configuring OSPF with Loopback Addresses

In this lab, students will configure OSPF Loopback addresses and observe the election process.

2.3.3 Modifying OSPF cost metric  

OSPF uses cost as the metric for determining the best route. A cost is associated with the output side of each router interface. Costs are also associated with externally derived routing data. In general, the path cost is calculated using the formula 10^8/ bandwidth, where bandwidth is expressed in bps. The system administrator can also configure cost by other methods. The lower the cost, the more likely the interface is to be used to forward data traffic. The Cisco IOS automatically determines cost based on the bandwidth of the interface. It is essential for proper OSPF operation that the correct interface bandwidth is set.

OSPF使用成本當決定最佳路由的權值。成本是和每個路由器介面的輸出端有關,成本也和外部衍生的路由資料有關。大致而言,路徑成本使用公式10^8 / 頻寬加以計算,此處頻寬以bps表示。系統管理員也能以其他的方法組態成本。成本愈低,介面被使用來轉送資料訊務愈可能。Cisco IOS依據介面的頻寬自動決定成本。為了合適的OSPF運作,設定正確介面頻寬是必要的。

Router(config)#interface serial 0/0

Router(config-if)#bandwidth 56

Cost can be changed to influence the outcome of the OSPF cost calculation. A common situation requiring a cost change is in a multi-vendor routing environment. A cost change would ensure that one vendor’s cost value would match another vendor’s cost value. Another situation is when Gigabit Ethernet is being used. The default cost assigns the lowest cost value of 1 to a 100 Mbps link. In a 100-Mbps and Gigabit Ethernet situation, the default cost values could cause routing to take a less desirable path unless they are adjusted. The cost number can be between 1 and 65,535.


Use the following interface configuration command to set the link cost:

Router(config-if)#ip ospf costnumber


           Router(config-if)#ip ospf cost 數值  

Lab Exercise: Modifying OSPF Cost Metric

In this lab, students will setup an Open Shortest Path First (OSPF) area

2.3.4 Configuring OSPF authentication 

By default, a router trusts that routing information is coming from a router that should be sending the information. A router also trusts that the information has not been tampered with along the route.


To guarantee this trust, routers in a specific area can be configured to authenticate each other.


Each OSPF interface can present an authentication key for use by routers sending OSPF information to other routers on the segment. The authentication key, known as a password, is a shared secret between the routers. This key is used to generate the authentication data in the OSPF packet header. The password can be up to eight characters. Use the following command syntax to configure OSPF authentication:

Router(config-if)#ip ospf authentication-keypassword

由路由器發送OSPF資訊給在區段內的其他路由器,每個OSPF 介面存有一認證鍵供使用。這認證鍵,稱為密碼,是路由器間共享的秘密。這鍵值被使用來產生OSPF封包標頭內的認證資料。密碼可達八字元。使用下面指令語法來組態OSPF認證:

             Router(config-if)#ip ospf authentication-key 密碼

After the password is configured, authentication must be enabled:

Router(config-router)#areaarea-number authentication


             Router(config-router)#area 區域編號 authentication

With simple authentication, the password is sent as plain text. This means that it can be easily decoded if a packet sniffer captures an OSPF packet.

憑著簡單的認證,密碼以明文傳送。此意味假如封包欄截者擷取一個OSPF 封包,它可能容易被解碼

It is recommended that authentication information be encrypted. To send encrypted authentication information and to ensure greater security, the message-digest keyword is used. The MD5 keyword specifies the type of message-digest hashing algorithm to use, and the encryption type field refers to the type of encryption, where 0 means none and 7 means proprietary.


Use the interface configuration command mode syntax:

Router(config-if)#ip ospf message-digest-keykey-id encryption-type md5key


             Router(config-if)#ip ospf message-digest-key 鍵id  加密型態 md5

The key-id is an identifier and takes the value in the range of 1 through 255. The key is an alphanumeric password up to sixteen characters. Neighbor routers must use the same key identifier with the same key value.


The following is configured in router configuration mode:

Router(config-router)#areaarea-id authentication message-digest


         Router(config-router)#area 區域ID authentication message-digest

MD5 authentication creates a message digest. A message digest is scrambled data that is based on the password and the packet contents. The receiving router uses the shared password and the packet to re-calculate the digest. If the digests match, the router believes that the source and contents of the packet have not been tampered with. The authentication type identifies which authentication, if any, is being used. In the case of message-digest authentication, the authentication data field contains the key-id and the length of the message digest that is appended to the packet. The message digest is like a watermark that cannot be counterfeited.

MD5認證產生訊息摘要。訊息摘要是依據密碼和封包內容的混合資料。接收的路由器使用共享密碼和封包來重新計算摘要。假如摘要相符,路由器相信封包的來源和內容沒有被偽造。 假如有的話,認證型態識別那種認證被使用。在訊息摘要認證狀況,認證資料欄位包含附加到封包的鍵識別子和訊息摘要的長度。訊息摘要如同不能被仿冒的浮水印。

Lab Exercise: Configuring OSPF Authentication

In this lab, students will introduce OSPF authentication into the area.

2.3.5 Configuring OSPF timers  

OSPF routers must have the same hello intervals and the same dead intervals to exchange information. By default, the dead interval is four times the value of the hello interval. This means that a router has four chances to send a hello packet before being declared dead.


On broadcast OSPF networks, the default hello interval is 10 seconds and the default dead interval is 40 seconds. On nonbroadcast networks, the default hello interval is 30 seconds and the default dead interval is 120 seconds. These default values result in efficient OSPF operation and seldom need to be modified.


A network administrator is allowed to choose these timer values. A justification that OSPF network performance will be improved is needed prior to changing the timers. These timers must be configured to match those of any neighboring router.


To configure the hello and dead intervals on an interface, use the following commands:

Router(config-if)#ip ospf hello-intervalseconds

Router(config-if)#ip ospf dead-intervalseconds


         Router(config-if)#ip ospf hello-interval 秒數

         Router(config-if)#ip ospf dead-interval 秒數

2.3.6 OSPF, propagating a default route  

To reach networks outside the domain, either OSPF must know about the network or OSPF must have a default route.


A practical alternative is to add a default route to the OSPF router connected to the outside network. This route can be redistributed to each router in  the AS through normal OSPF updates.

實際的替代方式是 加入預設路徑給要連到外面網路OSPF 路由器。以經正常OSPF 更新重新分配AS內的每個路由器。

A configured default route is used by a router to generate a gateway of last resort. The static default route configuration syntax uses the network address and a subnet mask

Router(config)#ip route [interface | next-hop address ]

組態預設路徑被路由器用來產生最後依據的閘道。靜態預設路由的組態語法使用網路0 .0 .0 .0位址和子網路遮罩0 .0 .0 .0 :

         Router(config)#ip route [介面 | 下個跳躍位址 ]

This is referred to as the quad-zero route, and any network address is matched using the following rule. The network gateway is determined by ANDing the packet destination with the subnet mask.


The following configuration statement will propagate this route to all the routers in a normal OSPF area:

Router(config-router)#default-information originate


          Router(config-router)#default-information originate

All routers in the OSPF area will learn a default route provided that the interface of the border router to the default gateway is active.


Lab Exercise: Propagating Default Routes in an OSPF Domain

In this lab, students will configure the OSPF network so that all hosts in the OSPF area can connect to outside networks.

2.3.7 Common OSPF configuration issues 

An OSPF router must establish a neighbor or adjacency relationship with another OSPF router to exchange routing information. Failure to establish a neighbor relationship is caused by any of the following reasons:

  • Hellos are not sent from both neighbors.
  • Hello and dead interval timers are not the same.
  • Interfaces are on different network types.
  • Authentication passwords or keys are different.

OSPF 路由器必須建立和其他OSPF路由器的鄰居或鄰近關係以交換路由資訊。無法建立鄰居關係是由於任何下列理由:

  • Hello沒有從鄰居兩端傳送。

  • Hello和停效間隔計時器不相同。

  • 介面屬於不同的網路類型。

  • 認證密碼或鍵值不同。

In OSPF routing it is also important to ensure the following:

  • All interfaces have the correct addresses and subnet mask.
  • network area statements have the correct wildcard masks.
  • network area statements put interfaces into the correct area.


  • 所有介面有正確位址和子網路遮罩。

  • network area敘述有正確的萬用遮罩。

  • network area敘述將介面放到正確的區域。

2.3.8 Verifying the OSPF configuration  

To verify the OSPF configuration a number of show commands are available. Figure lists these commands. Figure shows commands useful for troubleshooting OSPF.

要驗證OSPF組態,有一些show命令可用。圖列示這些命令。圖顯示對診斷OSPF有用的 命令。