原理概述

原因

在OSPF的廣播和NBMA類型網(wǎng)絡(luò)中，如果有臺(tái)路由器，若任意兩臺(tái)路由器之間都要建立鄰接關(guān)系，則需要建立n*(n-1)/2個(gè)鄰接關(guān)系，即當(dāng)路由器很多時(shí)，則需要建立和維護(hù)的鄰接關(guān)系就很多，兩兩之間需要發(fā)送的報(bào)文也就很多，這會(huì)造成內(nèi)容重復(fù)的報(bào)文在網(wǎng)絡(luò)中傳遞，浪費(fèi)了設(shè)備的帶寬資源。

定義：

DR（Designated Router，指定路由器），即所有其他路由器都只將各自的鏈路狀態(tài)信息發(fā)送給DR，再由DR以組播方式發(fā)送至所有路由器。
BDR（Backup Designated Router）的概念，作為DR路由器的備份，當(dāng)DR路由器失效時(shí)，BDR成為DR，并再選擇新的BDR路由器。其他非DR/BDR路由器都稱為DR Other 路由器。

選舉DR/BDR的規(guī)則：

• DR優(yōu)先級(jí)，如果DR優(yōu)先級(jí)為0，則不參與選舉
• Router-ID

需要注意的是，DR是在某個(gè)廣播或者NBMA網(wǎng)段內(nèi)進(jìn)行選舉的，是針對(duì)路由器的接口而言的。某個(gè)路由器在一個(gè)接口上可能是DR，在另一個(gè)接口上可能是BDR，或者是DR Other。

實(shí)驗(yàn)內(nèi)容

實(shí)驗(yàn)拓?fù)?/h2>

實(shí)驗(yàn)步驟

1.配置路由器接口地址和環(huán)回接口地址
2.配置ospf網(wǎng)絡(luò)

[R1]router id 1.1.1.1 [R1]ospf 1 [R1-ospf-1]area 0 [R1-ospf-1-area-0.0.0.0]net 172.16.1.0 0.0.0.255[R2]router id 2.2.2.2 [R2]ospf 1 [R2-ospf-1]area 0 [R2-ospf-1-area-0.0.0.0]net 172.16.1.0 0.0.0.255[R3]router id 3.3.3.3 [R3]ospf 1 [R3-ospf-1]area 0 [R3-ospf-1-area-0.0.0.0]net 172.16.1.0 0.0.0.255[R4]router id 4.4.4.4 [R4]ospf 1 [R4-ospf-1]area 0 [R4-ospf-1-area-0.0.0.0]net 172.16.1.0 0.0.0.255

配置完成后，同時(shí)重啟4臺(tái)路由器上的OSPF進(jìn)程，或者直接同時(shí)重啟路由器。

<R1>reset ospf process <R2>reset ospf process <R3>reset ospf process <R4>reset ospf process

使用display ospf peer brief查看OSPF鄰居建立情況

[R1]dis ospf peer brief OSPF Process 1 with Router ID 1.1.1.1Peer Statistic Information----------------------------------------------------------------------------Area Id Interface Neighbor id State 0.0.0.0 GigabitEthernet0/0/0 2.2.2.2 2-Way 0.0.0.0 GigabitEthernet0/0/0 3.3.3.3 Full 0.0.0.0 GigabitEthernet0/0/0 4.4.4.4 Full ----------------------------------------------------------------------------可以觀察到，R1已經(jīng)和其他路由器成功建立起OSPF鄰居關(guān)系。

查看DR/BDR狀態(tài)

[R1]dis ospf peer OSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2 Address: 172.16.1.2 State: 2-Way Mode:Nbr is Master Priority: 1DR: 172.16.1.4 BDR: 172.16.1.3 MTU: 0 Dead timer due in 31 sec Retrans timer interval: 0 Neighbor is up for 00:00:00 Authentication Sequence: [ 0 ] Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1DR: 172.16.1.4 BDR: 172.16.1.3 MTU: 0 Dead timer due in 35 sec Retrans timer interval: 5 Neighbor is up for 00:05:13 Authentication Sequence: [ 0 ] Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 1DR: 172.16.1.4 BDR: 172.16.1.3 MTU: 0 Dead timer due in 36 sec Retrans timer interval: 0 Neighbor is up for 00:05:13 Authentication Sequence: [ 0 ] 可以觀察到，R4為DR，R3為BDR。這是由于在默認(rèn)情況下，每臺(tái)路由器上的DR優(yōu)先級(jí)都為1，此時(shí)是通過Router-ID的數(shù)值高低比較的。

3.驗(yàn)證網(wǎng)絡(luò)類型點(diǎn)到多點(diǎn)的選舉情況

[R1]int g0/0/0 [R1-GigabitEthernet0/0/0]ospf network-type p2mp[R2]int g0/0/0 [R2-GigabitEthernet0/0/0]ospf network-type p2mp[R3]int g0/0/0 [R3-GigabitEthernet0/0/0]ospf network-type p2mp[R4]int g0/0/0 [R4-GigabitEthernet0/0/0]ospf network-type p2mp

在R1上再次觀察此時(shí)OSPF的選舉情況

[R1]dis ospf peer OSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2 Address: 172.16.1.2 State: Full Mode:Nbr is Master Priority: 1DR: None BDR: None MTU: 0 Dead timer due in 112 sec Retrans timer interval: 5 Neighbor is up for 00:01:50 Authentication Sequence: [ 0 ] Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1DR: None BDR: None MTU: 0 Dead timer due in 101 sec Retrans timer interval: 5 Neighbor is up for 00:00:47 Authentication Sequence: [ 0 ] Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 1DR: None BDR: None MTU: 0 Dead timer due in 103 sec Retrans timer interval: 0 Neighbor is up for 00:00:47 Authentication Sequence: [ 0 ] 可以觀察到，DR/BDR都為None，驗(yàn)證了在點(diǎn)到多點(diǎn)的網(wǎng)絡(luò)類型中不選舉DR/BDR。在點(diǎn)到點(diǎn)的網(wǎng)絡(luò)中也是。

4.根據(jù)網(wǎng)絡(luò)需求影響DR/BDR選舉
根據(jù)需求，需要使性能較好、處理能力較強(qiáng)的R1成為DR，性能次之的R2成為BDR，而性能最差的R4不能參加選舉。

首先將OSPF網(wǎng)絡(luò)類型還原為默認(rèn)的廣播網(wǎng)絡(luò)類型

[R1]int g0/0/0 [R1-GigabitEthernet0/0/0]ospf network-type broadcast[R2]int g0/0/0 [R2-GigabitEthernet0/0/0]ospf network-type broadcast[R3]int g0/0/0 [R3-GigabitEthernet0/0/0]ospf network-type broadcast[R4]int g0/0/0 [R4-GigabitEthernet0/0/0]ospf network-type broadcast

配置完成后，修改R1上的接口的DR優(yōu)先級(jí)為100，R2為50，R4為0，R3保持默認(rèn)不變

[R1]int g0/0/0 [R1-GigabitEthernet0/0/0]ospf dr-priority 100[R2]int g0/0/0 [R2-GigabitEthernet0/0/0]ospf dr-priority 50[R4]int g0/0/0 [R4-GigabitEthernet0/0/0]ospf dr-priority 0

配置完成后，查看路由器的DR/BDR選舉情況

[R1]dis ospf peer OSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2 Address: 172.16.1.2 State: Full Mode:Nbr is Master Priority: 50DR: 172.16.1.3 BDR: 172.16.1.1 MTU: 0 Dead timer due in 34 sec Retrans timer interval: 5 Neighbor is up for 00:01:08 Authentication Sequence: [ 0 ] Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1DR: 172.16.1.3 BDR: 172.16.1.1 MTU: 0 Dead timer due in 34 sec Retrans timer interval: 0 Neighbor is up for 00:03:11 Authentication Sequence: [ 0 ] Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 0DR: 172.16.1.3 BDR: 172.16.1.1 MTU: 0 Dead timer due in 29 sec Retrans timer interval: 5 Neighbor is up for 00:01:03 Authentication Sequence: [ 0 ] 發(fā)現(xiàn)此時(shí)的DR與BDR都沒有改變，即驗(yàn)證了OSPF的選舉是非搶占性的。

同時(shí)重啟4臺(tái)路由器的OSPF進(jìn)程，或者直接同時(shí)重啟路由器。

[R1]dis ospf peerOSPF Process 1 with Router ID 1.1.1.1Neighbors Area 0.0.0.0 interface 172.16.1.1(GigabitEthernet0/0/0)'s neighborsRouter ID: 2.2.2.2 Address: 172.16.1.2 State: Full Mode:Nbr is Master Priority: 50DR: 172.16.1.1 BDR: 172.16.1.2 MTU: 0 Dead timer due in 31 sec Retrans timer interval: 4 Neighbor is up for 00:00:23 Authentication Sequence: [ 0 ] Router ID: 3.3.3.3 Address: 172.16.1.3 State: Full Mode:Nbr is Master Priority: 1DR: 172.16.1.1 BDR: 172.16.1.2 MTU: 0 Dead timer due in 33 sec Retrans timer interval: 5 Neighbor is up for 00:00:23 Authentication Sequence: [ 0 ] Router ID: 4.4.4.4 Address: 172.16.1.4 State: Full Mode:Nbr is Master Priority: 0DR: 172.16.1.1 BDR: 172.16.1.2 MTU: 0 Dead timer due in 33 sec Retrans timer interval: 5 Neighbor is up for 00:00:13 Authentication Sequence: [ 0 ] 此時(shí)在該廣播網(wǎng)絡(luò)中，R1為DR，R2為BDR。

實(shí)驗(yàn)?zāi)康?/h2>

• 理解OSPF在哪種網(wǎng)絡(luò)類型中會(huì)選舉DR/BDR
• 掌握OSPF DR/BDR的選舉規(guī)則
• 掌握如何更改設(shè)備接口上的DR優(yōu)先級(jí)
• 理解OSPF DR/BDR選舉的非搶占性特性

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