OSPFv2 設定與實作

Single-Area OSPFv2 Concepts

陳禹璿

Published in

璿的筆記

24 min readApr 14, 2021

OSPF (Open Shortest Path First)是目前業界主流的Routing Protocol，也是一種開放給大家使用的路由協定，所以他可以在各種不同廠牌的設備間運作，OSPF可以分成兩種。

OSPF v2 (IPv4)
OSPF v3 (IPv6)

當路由器同時運作兩種OSPF時，他們是互不影響的。

這篇文章的資料來源都是來自於Cisco academy裡面的內容，這篇相當於我自己做的筆記

1. OSPF 的組成要素

其實大部分的路由協定都差不多，也就是交換一些路由資訊，宣告自己的網路

OSPF的 routing protocol message:

Hello packet
Database description packet
Link-state request packet
Link-state update packet
Link-state acknowledgment packet

OSPF Database

這個部分考試還蠻會考的，同時也很令人困惑

Adjacency database — This creates the neighbor table.
Link-state database (LSDB) — This creates the topology table.
Forwarding database — This creates the routing table.

OSPF algorithm:

OSPF使用Dijkstra shortest-path first (SPF) algorithm來計算最佳路徑，這種演算法使用累計的cost來算出最佳路徑。

2. Link-State 運作原理:

Establish Neighbor Adjacencies: 像有開OSPF的介面送出Hello封包，藉此來了解是否有鄰居的存在
Exchange Link-State Advertisements: 當鄰居建立起來後，Router會互相向新鄰居flood自己擁有的LSAs，並移植持續，直到大家的LSA都一樣為止
Build the Link State Database: 當Router收到LSA之後，就會利用剛剛收到的LSA建立LSDB
Execute the SPF Algorithm: Router會把LSDB的內容餵給SPF演算法吃，並計算出SPF tree
Choose the Best Route: 當SPF tree建立好了之後，就可以找出每一筆遠端網路的最佳路徑，並建立起Forwardind Database。

3. 單區OSPF與多區的OSPF

Single-Area:

也就是只有一個OSPF area。

Multiarea OSPF:

也就是一個以上的OSPF area，中間會有ABR擔任轉接服務，對於大型網路的話好處有很多

減少路由表的數量
減少LSA更新的沖刷面積
減少SPF計算，比如說更動area 10的路由，就只會讓Area 10 裡面的路由啟動SPF演算法，Area 0裡面的路由只需要等答案而已。

4. OSPF封包種類(Link-state packets, LSP)

OSPF利用LSP去執行尋找最佳路徑，鄰居的狀態或路由表更新等任務

Type 1: Hello packet:

Hello 封包是一種具有建立鄰居關係與選舉DR與BDR能力的封包，兩個路由器必須互相同意Hello封包內的資料才可以建立鄰居關係。

OSPFv2使用 224.0.0.5當群播地址，有開啟OSPF才會收到Hello封包。

Type 2: Database Description (DBD) packet:

這種封包會交換LSDB裡面的資訊目錄，以利同一個Area裡的各個路由器能有完全相同的LSDB，這樣計算出SPF tree才會精準。

Type 3: Link-State Request (LSR) packet

如果剛剛用DBD檢查到自己的LSDB不是最新的，那就送出LSR來獲取最新的LSA

其他路由器的 (DBDs) —我自己的 (LADB) = 我要去要求的 (LSR)

Type 4: Link-State Update (LSU) packet

回送給LSR的sender的更新訊息。

Type 5: Link-State Acknowledgment (LSAck) packet

給LSU sender的確認訊息。

5. OSPF 運作狀態

剛剛講了封包的運用，這邊要講OSPF形成的過程

1.Down State:

沒有Hello 封包
寄出去Hello封包，還沒有回音

2. Init State:

收到來自鄰居的Hello 封包，並從Hello封包內找到Router ID

3.Two-Way State:兩個路由器正在喬事情

選出DR，BDR

4. ExStart State: On point-to-point networks, the two routers decide which router will initiate the DBD packet exchange and decide upon the initial DBD packet sequence number.

在這邊兩個Router會用higher router ID來決定誰先送DBD的資訊

5.Exchange State:

交換DBD的資訊

當R1的Router比較高時，R1先送給R2(這個範例R1跟R2的DBD剛好一樣，所以不需要送出LSA，LSU)

R1: (LSDB)R1的DBD →R2

R2: (LSAck) →R1

R2:(LSDB)R2的DBD →R1

R1:(LSAck) →R2

6.Loading State:

如果發現兩邊的DBD不一致，則交換LAS，LAU封包
因為有更新，所以啟動SPF 演算法

7. Full State:

大功告成

6. DR的重要性:

DR(Designated Router)跟BDR(Backup Designated Router)是一個協助發送LSA的路由器，試想，如果今天有十台路由器戶相連接，並且同時發送LSA訊息，那會導致正個網路被LSA塞爆，所以當路由器戶相連接，他們就會選出一台DR與一台BDR來作為發送LSA的路由器，就是統一由他們來處理更新資訊的部分就是了。

7 .實際操作 — Signal OSPF v2

enabled OSPF: router ospf process-id
Router ID
宣告網路
Configure Passive Interfaces

(1). enabled OSPF: router ospf process-id

這個process-id的數字跟整個ospf運作沒有關係，所以想設多少就設多少

Router(config)#router ospf 1
Router(config-router)#?
  area                   OSPF area parameters
  auto-cost              Calculate OSPF interface cost according to bandwidth
  default-information    Control distribution of default information
  distance               Define an administrative distance
  exit                   Exit from routing protocol configuration mode
  log-adjacency-changes  Log changes in adjacency state
  neighbor               Specify a neighbor router
  network                Enable routing on an IP network
  no                     Negate a command or set its defaults
  passive-interface      Suppress routing updates on an interface
  redistribute           Redistribute information from another routing protocol
  router-id              router-id for this OSPF process

(2). Router ID

Router ID就是OSPF唯一的識別碼了，所以一定要謹慎小心的設定

Router ID可以決定Router在Exstart state裡面誰先寄出DBD的順序
具有較高的Router ID可以當選DR的身分
Router ID可以手動設定，如果沒有設定，Router會先選用Loopback的IP，如果沒有，就用最大的ip地址
設定方法

Router(config-router)#router-id 1.1.1.1

如果必須修改Router-id，必須輸入下列指令

R1# clear ip ospf process

(3). 宣告網路

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

Router(config)#router ospf 1
Router(config-router)#network 192.168.1.0 0.0.0.255 area 0
Router(config-router)#network 172.32.108.0 0.0.255.255 area 0

(4). Configure Passive Interfaces:

當我的介面沒有連接上任何路由器的時候，我其實是不需要像這個介面送出 OSPF Package的，但是我仍需要再OSPF裡面宣告我擁有這段網路。

比如說拓樸圖-1裡面，192.168.1.0那邊是沒有連接到其他路由器，那就不需要朝那裏發送OSPF封包。

Router(config-router)#passive-interface f0/0

8. 實際操作 — Multiaccess OSPF

OSPF Designated Router

(1). OSPF Designated Router

在OSPF得multiaccess networks裡面，路由器們有三種腳色

DR(designated Router)
BDR(backup designated Router)
DROTHER

當路DROTHER要通知自己得LSA給其他Router時，他不會直接沖刷整個網路，他會把LSA送給224.0.0.6這個群播ip，當然，這個ip只有DR跟BDR在監聽，DR收到之後會把LSA送到 224.0.0.5給其他路由

(2). OSPF neighbor

這邊就是看adjacency database

Router#show ip ospf neighborR1# show ip ospf neighbor 
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:31    192.168.1.2     GigabitEthernet0/0/0
3.3.3.3           1   FULL/DR         00:00:39    192.168.1.3     GigabitEthernet0/0/0

完成收斂: FULL/BDR，FULL/DR，FULL/DROTHER

正在交換hello封包: 2-WAY/DROTHER

(3). DR/BDR Election

前面講過DR可以用Router-id來選，這邊補充完整過程

先用 highest interface priority來選
如果都一樣，就用router ID最高的來選(前提是要手動設定)
如果沒有手動設定，就先用最大的loopback
如果沒有loopback，就用最大的ipv4

改OSPF Priority(拓樸圖2)

R1 route-id:1.1.1.1

R2 route-id: 2.2.2.2

R2#show ip ospf interface f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.2/24, Area 0
  Process ID 10, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 192.168.0.2
  Backup Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:06
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

R2的Priority是預設的 1，但因為router-id比較大，所以是RD

現在我們試著更改Priority來改變DR

R1(config)#int f0/0
R1(config-if)#ip os
R1(config-if)#ip ospf pr
R1(config-if)#ip ospf priority 100
R1(config-if)#endR1#clear ip ospf process 
Reset ALL OSPF processes? [no]: yR1#show ip ospf interface f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.0.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:02
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 2.2.2.2  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

接著再看一次R2，已經變成BDR了

R2#show ip ospf interface f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.2/24, Area 0
  Process ID 10, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State BDR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.0.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:07
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1
    Adjacent with neighbor 1.1.1.1  (Designated Router)
  Suppress hello for 0 neighbor(s)

9 .Metric 與 Cost

之前有提到說要OSPF是用cost來決定最佳路徑，這邊要來說明如何更改最佳路徑，或是利用調整cost來達到

先說如何決定cost

(1). OSPF Cost

Cost = reference bandwidth / interface bandwidth

reference bandwidth: 10⁸ (100,000,000);

interface bandwidth: 跟port有關

R1#show interfaces f0/0
FastEthernet0/0 is up, line protocol is up (connected)
  Hardware is Lance, address is 00d0.ff66.d701 (bia 00d0.ff66.d701)
  Internet address is 192.168.0.1/24
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1

BW: 100000Kbit = 100,000,000bps

cost = 100,000,000 / 100,000,000 = 1

知道怎麼算之後，那要怎麼改變cost呢?

有兩種方式

直接改cost

R1(config)#int f0/0
R1(config-if)#ip os
R1(config-if)#ip ospf c
R1(config-if)#ip ospf cost 50
R1(config-if)#end
R1#
%SYS-5-CONFIG_I: Configured from console by consoleR1#show ip ospf interface f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 50
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.0.2

改reference bandwidth

interface bandwidth是固定的，但是reference bandwidth可以改。

Router(config-router)# auto-cost reference-bandwidth Mbps

這邊我們嘗試讓cost改成10，因為interface bandwidth不能動，所以把reference bandwidth改成1000Mbps(原本是100Mbps)

R1(config)#router ospf 10
R1(config-router)#auto-cost reference-bandwidth 1000
% OSPF: Reference bandwidth is changed.
        Please ensure reference bandwidth is consistent across all routers.
R1(config-router)#end
R1#
%SYS-5-CONFIG_I: Configured from console by consoleR1#show ip ospf int f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1

(2). Cost 計算

Cost的計算只要計算出去的孔就好了，比如說我們要從R1到10.0.0.0這個遠端網路，我們會經過R1的Fa0/0，R2的loopback

R1的Fa0/0:(剛剛有調整)

FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10

R2的loopback:(為預設值)

Loopback0 is up, line protocol is up
  Internet address is 10.0.0.1/8, Area 0
  Process ID 10, Router ID 2.2.2.2, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host

R1到10.0.0.0的Metric就是10+1 = 11

R1#show ip route 10.0.0.1
Routing entry for 10.0.0.1/32
Known via "ospf 10", distance 110, metric 11, type intra area
  Last update from 192.168.0.2 on FastEthernet0/0, 00:04:11 ago
  Routing Descriptor Blocks:
  * 192.168.0.2, from 2.2.2.2, 00:04:11 ago, via FastEthernet0/0
      Route metric is 11, traffic share count is 1

10 . Hello 封包的 Intervals

Hello封包有兩種主要的參數

Hello (預設10秒一次，使用 multicast address 224.0.0.5)
Dead (預設40秒一次)

Dead interval是用來管制LSDB裡面的內容，一般情況各Router都會利用Hello封包來確認對方的死活，如果傳了4(預設40秒一次)次給對方，對方都沒有回應，那我們就認定對方已經死亡了。

確認死亡後，我方會先更新自己的LSDB，把該路由刪除，並把LSDB的資訊沖刷出去。

R1#show ip ospf interface f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 50
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1
  Backup Designated Router (ID) 2.2.2.2, Interface address 192.168.0.2
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5R1#show ip ospf neighborNeighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:34(管制剩餘的dead interval) 192.168.0.2     FastEthernet0/0

修改方法

Router(config-if)# ip ospf hello-interval seconds
Router(config-if)# ip ospf dead-interval seconds

R1(config)#int f0/0
R1(config-if)#ip ospf hello-interval 30
R1(config-if)#ip ospf dead-interval 60
R1(config-if)#endR1#show ip ospf interface f0/0FastEthernet0/0 is up, line protocol is up
  Internet address is 192.168.0.1/24, Area 0
  Process ID 10, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 50
  Transmit Delay is 1 sec, State DR, Priority 100
  Designated Router (ID) 1.1.1.1, Interface address 192.168.0.1
  No backup designated router on this network
  Timer intervals configured, Hello 30, Dead 60, Wait 60, Retransmit 5

11. OSPF的default-gateway

步驟:

在連接isp的Router上宣告一筆靜態路由
使用default-information originate把它撒出去

12. OSPF常用指令

驗證Routing:

show ip interface brief — This verifies that the desired interfaces are active with correct IP addressing.
show ip route- This verifies that the routing table contains all the expected routes.

驗證OSPF訊息:

show ip ospf neighbor
show ip protocols
show ip ospf
show ip ospf interface

13. 參考資料

Open Shortest Path First (OSPF) 開放最短路由優先協定

Open Shortest Path First (OSPF) 是屬於 Link-State Routing Protocol，每隻在 OSPF 裡的 Router 都會向 Neighbor 交換自己的 Link-State，當…

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