OSPF Network Types
- The OSPF Broadcast Network Type
  - Verifying Operations with Network Type Broadcast
  - Configuring to Influence the DR/BDR Election
- The OSPF Point-to-Point Network Type
OSPF Neighbor Relationships
英語

OSPF Network Types

OSPFのinterfaceごとの設定

Network Type Keyword	Dynamically Discovers Neighbors	Use a DR/BDR
broadcast	yes	yes
point-to-point	yes	no

The OSPF Broadcast Network Type

デフォルトでbroadcast
- ch20の設定はこれに基づいている

R3(config)#router ospf 1
OSPF process 1 cannot start. There must be at least one "up" IP interface
R3(config-router)#router-id 3.3.3.3
R3(config-router)#passive-interface g0/0/1
R3(config-router)#exit
R3(config)#interface g0/0/0
R3(config-if)#no shutdown
R3(config-if)#ip address 10.1.1.3 255.255.255.0
R3(config-if)#ip ospf 1 area 0
R3(config-if)#exit
R3(config)#interface g0/0/1
R3(config-if)#no shutdown
R3(config-if)#ip address 10.1.33.3 255.255.255.0
R3(config-if)#ip ospf 1 area 0
R3(config-if)#end

R1,2,4もいい感じに設定する
neighbor確認

R1#show ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           1   FULL/BDR        00:00:38    10.1.1.3        GigabitEthernet0/0/0
4.4.4.4           1   FULL/DR         00:00:38    10.1.1.4        GigabitEthernet0/0/0
2.2.2.2           1   2WAY/DROTHER    00:00:38    10.1.1.2        GigabitEthernet0/0/0
R1#

4.4.4.4がDR、3.3.3.3がBDR、になった
1.1.1.1と2.2.2.2はDROthersなので2way stateどまり
何が起きているか
1. 224.0.0.5マルチキャストアドレスにOSPF Hello送信してneighbor発見
2. DR/BDR interfaceの選出
3. 他にrouterのいないネットワークに面しているinterfaceは無条件にDR
  - R1のg0/0/1 (左)など
  - 【補】今回はpassive interfaceを設定したので関係ないかも
4. 中央のネットワークでは、今回は R4 g0/0/1がDR、R3 g0/0/1がBDR
5. DR/BDRにメッセージを送信するときは、224.0.0.6マルチキャストアドレスを用いる

Verifying Operations with Network Type Broadcast

R4#show ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/DROTHER    00:00:34    10.1.1.2        GigabitEthernet0/0/0
1.1.1.1           1   FULL/DROTHER    00:00:34    10.1.1.1        GigabitEthernet0/0/0
3.3.3.3           1   FULL/BDR        00:00:34    10.1.1.3        GigabitEthernet0/0/0

f:id:wand_ta:20200510192007p:plain

R3#show ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface
4.4.4.4           1   FULL/DR         00:00:31    10.1.1.4        GigabitEthernet0/0/0
2.2.2.2           1   FULL/DROTHER    00:00:31    10.1.1.2        GigabitEthernet0/0/0
1.1.1.1           1   FULL/DROTHER    00:00:31    10.1.1.1        GigabitEthernet0/0/0

f:id:wand_ta:20200510192018p:plain

本当は show ip ospf interface briefが見たいのだがPacket Tracerだと動かないので代用:

R1#show ip ospf interface

GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 10.1.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DROTHER, Priority 1
  Designated Router (ID) 4.4.4.4, Interface address 10.1.1.4
  Backup Designated Router (ID) 3.3.3.3, Interface address 10.1.1.3
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:08
  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 3, Adjacent neighbor count is 2
    Adjacent with neighbor 4.4.4.4  (Designated Router)
    Adjacent with neighbor 3.3.3.3  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)
GigabitEthernet0/0/1 is up, line protocol is up
  Internet address is 10.1.11.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 10.1.11.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    No Hellos (Passive interface)
  Index 2/2, 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 0, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s)

g0/0/0 抜粋

GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 10.1.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DROTHER, Priority 1
  Designated Router (ID) 4.4.4.4, Interface address 10.1.1.4
  Backup Designated Router (ID) 3.3.3.3, Interface address 10.1.1.3
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:08
  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 3, Adjacent neighbor count is 2
    Adjacent with neighbor 4.4.4.4  (Designated Router)
    Adjacent with neighbor 3.3.3.3  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

State DROTHER
- DRでもBDRでもない
Neighbor Count is 3, Adjacent neighbor count is 2
- briefでNbrs F/Cとして表示されるはずの部分
Network Type BROADCAST
- network typeがデフォルトのbroadcast
  - 昔の機種だと違ったりする
  - その場合はip ospf network broadcast interface subcommand で設定する

Configuring to Influence the DR/BDR Election

DR/BDR選出後の挙動
- DR障害時、BDRが引き継いで、新たにBDRが選出される
- より適任なrouterがsubnetに参入しても、DR/BDRの差し替えはおきない
つまり、新しいrouterの優先度を高くしてDR/BDRに選出させようとしても、現DR/BDRに障害が発生しない限り、新しく追加したrouterがDR/BDRになることはない
STPのroot選出との比較

	OSPFのDR/BDR選出	cf. STPのroot選出
priority	0-255	0-65535の範囲で4096の倍数
priorityの解釈	highest-is-best	lowest-is-best
priorityがタイのとき	Router IDが最も大きいもの	System IDが最も小さいもの
preemption (差し替え)	ない	ある

コスト設定

R1(config)#interface g0/0/0
R1(config-if)#ip ospf priority 99
R1(config-if)#
R1(config-if)#^Z
R1#
%SYS-5-CONFIG_I: Configured from console by console

確かに設定されている

R1#show ip ospf interface g0/0/0

GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 10.1.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DROTHER, Priority 99
  Designated Router (ID) 4.4.4.4, Interface address 10.1.1.4
  Backup Designated Router (ID) 3.3.3.3, Interface address 10.1.1.3
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:05
  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 3, Adjacent neighbor count is 2
    Adjacent with neighbor 4.4.4.4  (Designated Router)
    Adjacent with neighbor 3.3.3.3  (Backup Designated Router)
  Suppress hello for 0 neighbor(s)

show ip ospf neighborの情報にも反映されるが、DR/BDRは変わらない

R4#show ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           1   FULL/BDR        00:00:36    10.1.1.3        GigabitEthernet0/0/0
1.1.1.1          99   FULL/DROTHER    00:00:36    10.1.1.1        GigabitEthernet0/0/0
2.2.2.2           1   FULL/DROTHER    00:00:36    10.1.1.2        GigabitEthernet0/0/0

R4のg0/0/0 (DR)をshutdownしてみる

R4(config-if)#shutdown
R4(config-if)#no shutdown

R4#show ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1          99   FULL/BDR        00:00:31    10.1.1.1        GigabitEthernet0/0/0
2.2.2.2           1   2WAY/DROTHER    00:00:31    10.1.1.2        GigabitEthernet0/0/0
3.3.3.3           1   FULL/DR         00:00:31    10.1.1.3        GigabitEthernet0/0/0

BDRだったR3がDRになった
新たなBDRとして、priority99のR1が選出された

The OSPF Point-to-Point Network Type

f:id:wand_ta:20200510192139p:plain

リンクに2ルータしかないときに有用
- シリアル通信
- WAN
  - HDLC
  - PPPoE

Router(config)#interface Serial9/0
Router(config-if)#ip address 10.1.14.1 255.255.255.0
Router(config-if)#ip ospf 1 area 4
Router(config-if)#ip ospf network point-to-point

show ip ospf interface s9/0

Serial9/0 is up, line protocol is up
  Internet address is 10.1.14.1/24, Area 4
  Process ID 1, Router ID 10.1.14.1, Network Type POINT-TO-POINT, Cost: 64
  Transmit Delay is 1 sec, State POINT-TO-POINT,
  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 10.1.14.4
  Suppress hello for 0 neighbor(s)

これ

...
Network Type POINT-TO-POINT
...
State POINT-TO-POINT
...

DRとかBDRとかいう概念もない

OSPF Neighbor Relationships

OSPF Neighbor Requirements

requirement	required for ospf	neighbor missing if incorrect
up/up	yes	yes
aclでフィルターされない	yes	yes
同一subnet	yes	yes
routing protocol neighbor authenticationが通ること	yes	yes
hello/dead timerの一致	yes	yes
RIDの重複がなきこと	yes	yes
同一エリア	yes	yes
OSPFプロセスがshutdownでなきこと	yes	yes
MTU一致	yes	no
OSPF Network Type一致	yes	no

トラブルは2種類
- neighborになれない
- neighborになれるが正しくroutingされない
トラブルシューティング用コマンド
show ip ospf
- RID確認
- OSPF processがshutdownしてないこと

R2#show ip ospf
 Routing Process "ospf 1" with ID 2.2.2.2
 Supports only single TOS(TOS0) routes
 Supports opaque LSA
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
 Number of external LSA 0. Checksum Sum 0x000000
 Number of opaque AS LSA 0. Checksum Sum 0x000000
 Number of DCbitless external and opaque AS LSA 0
 Number of DoNotAge external and opaque AS LSA 0
 Number of areas in this router is 1. 1 normal 0 stub 0 nssa
 External flood list length 0

show ip ospf interface gigabitEthernet 0/0/0
- neighbor authentication
- hello/dead timer
- OSPF processがshutdownしてないこと

R2#show ip ospf interface gigabitEthernet 0/0/0

GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 10.1.12.2/24, Area 23
  Process ID 1, 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 10.1.12.2
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:04
  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 0, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s)

show ip ospf interface brief
- area一致

Issues That Prevent Neighbor Adjacencies

area不一致
RID重複
hello/dead interval不一致
routerサブコマンドでshutdownされている

Issues That Allow Adjacencies but Prevent IP Routes

MTU不一致
OSPF network type不一致

英語

preemption
- 先買権、優先買取権
- 差し替え
  - 本文の文脈ではたぶんこっち

勉強日記

チラ裏

CCNA試験対策 ch21 OSPF Network Types and Neighbors

OSPF Network Types

The OSPF Broadcast Network Type

Verifying Operations with Network Type Broadcast

Configuring to Influence the DR/BDR Election

The OSPF Point-to-Point Network Type

OSPF Neighbor Relationships

OSPF Neighbor Requirements

Issues That Prevent Neighbor Adjacencies

Issues That Allow Adjacencies but Prevent IP Routes

英語