OSPF MTU Ignore 072

今回はOSPFのMTU Ignoreと呼ばれる機能について解説します。

 

座学

MTUの不一致

OSPFネイバー確立する際、ルータ間でLSDBが同期されます。この際、ルータ間ではDBD(DataBase Discription)パケットを交換し、お互いのLSDBに存在しているLSAのヘッダ情報をやり取りします。このDBDパケットには、DBDパケットの送信元インタフェースのMTUが格納されています。

 

もし、ネイバーの確立を試みているルータ間でインタフェースのMTUが異なる場合、ネイバーの状態がExStartやExChangeで止まってしまいます。

何かしらの事情で、ルータ間のインタフェースのMTUが異なり、かつ、MTUが変更できない状況でも、OSPFネイバーを確立し、LSDBを同期させたい場合は、このMTUの確認を無視させることが可能で、この機能のことをMTU Ignoreと呼びます。

 

実機での動作確認

検証内容

R1のg2、R2のg2でOSPF Area 0を有効にします。
Network TypeにはPoint-to-Pointを使用します。
R1のg2のMTUを1100 Byte、R2のg2のMTUを1200 Byteに設定します。

R1でMTU Ignoreを有効にした場合の動作を確認します。

 

初期設定


interface GigabitEthernet2
 ip address 10.1.1.1 255.255.255.0
 mtu 1100
 ip ospf network point-to-point
!
router ospf 1
 router-id 1.1.1.1
 network 10.1.1.0 0.0.0.255 area 0
            


interface GigabitEthernet2
 ip address 10.1.1.2 255.255.255.0
 mtu 1200
 ip ospf network point-to-point
!
router ospf 1
 router-id 2.2.2.2
 network 10.1.1.0 0.0.0.255 area 0

 

インタフェースのMTUの確認

R1のg2でMTUが1100 Byteであることが確認できます。

R1

R1#show ip interface GigabitEthernet2
GigabitEthernet2 is up, line protocol is up
  Internet address is 10.1.1.1/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1100 bytes
  .
  <一部省略>

 

R2のg2でMTUが1200 Byteであることが確認できます。

R2

R2#show ip interface GigabitEthernet2
GigabitEthernet2 is up, line protocol is up
  Internet address is 10.1.1.2/24
  Broadcast address is 255.255.255.255
  Address determined by setup command
  MTU is 1200 bytes
  .
  <一部省略>

 

DBDパケットの確認

R1が送信したDBDパケットのInterface MTUフィールドに1100が格納されていることが確認できます。

 

R2が送信したDBDパケットのInterface MTUフィールドに1200が格納されていることが確認できます。

 

MTU Ignore設定前

OSPFネイバーの状態

R1において、R2の状態がExStartで止まっていることが確認できます。

R1

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   EXSTART/  -     00:00:38    10.1.1.2        GigabitEthernet2

 

R2において、R1の状態がExStartで止まっていることが確認できます。

R2

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   EXSTART/  -     00:00:35    10.1.1.1        GigabitEthernet2

 

OSPFのデバックの確認

R1において、R2の状態がExStartで止まっていることが確認できます。

R1

R1#debug ip ospf hello
R1#debug ip ospf adj
*May 29 14:00:08.984: OSPF-1 ADJ   Gi2: Interface going Up
*May 29 14:00:08.984: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.1
*May 29 14:00:08.985: OSPF-1 HELLO Gi2: Rcv hello from 2.2.2.2 area 0 10.1.1.2
*May 29 14:00:08.985: OSPF-1 ADJ   Gi2: 2 Way Communication to 2.2.2.2, state 2WAY
*May 29 14:00:08.985: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2: Prepare dbase exchange
*May 29 14:00:08.985: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0x1E58 opt 0x52 flag 0x7 len 32
*May 29 14:00:08.985: OSPF-1 HELLO Gi2: Send immediate hello to nbr 2.2.2.2, src address 10.1.1.2
*May 29 14:00:08.985: OSPF-1 HELLO Gi2: Send hello to 10.1.1.2 area 0 from 10.1.1.1
*May 29 14:00:08.986: OSPF-1 ADJ   Gi2: Rcv DBD from 2.2.2.2 seq 0x1A32 opt 0x52 flag 0x7 len 32  mtu 1200 state EXSTART
*May 29 14:00:08.986: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2 has larger interface MTU
*May 29 14:00:13.768: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0x1E58 opt 0x52 flag 0x7 len 32
*May 29 14:00:13.768: OSPF-1 ADJ   Gi2: Retransmitting DBD to 2.2.2.2 [1]
*May 29 14:00:13.898: OSPF-1 ADJ   Gi2: Rcv DBD from 2.2.2.2 seq 0x1A32 opt 0x52 flag 0x7 len 32  mtu 1200 state EXSTART
*May 29 14:00:13.898: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2 has larger interface MTU
*May 29 14:00:17.577: OSPF-1 HELLO Gi2: Rcv hello from 2.2.2.2 area 0 10.1.1.2
*May 29 14:00:18.400: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.1
*May 29 14:00:18.664: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0x1E58 opt 0x52 flag 0x7 len 32
*May 29 14:00:18.664: OSPF-1 ADJ   Gi2: Retransmitting DBD to 2.2.2.2 [2]
*May 29 14:00:18.682: OSPF-1 ADJ   Gi2: Rcv DBD from 2.2.2.2 seq 0x1A32 opt 0x52 flag 0x7 len 32  mtu 1200 state EXSTART
*May 29 14:00:18.682: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2 has larger interface MTU
*May 29 14:00:23.296: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0x1E58 opt 0x52 flag 0x7 len 32
*May 29 14:00:23.296: OSPF-1 ADJ   Gi2: Retransmitting DBD to 2.2.2.2 [3]
*May 29 14:00:23.325: OSPF-1 ADJ   Gi2: Rcv DBD from 2.2.2.2 seq 0x1A32 opt 0x52 flag 0x7 len 32  mtu 1200 state EXSTART
*May 29 14:00:23.325: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2 has larger interface MTU
*May 29 14:00:26.597: OSPF-1 HELLO Gi2: Rcv hello from 2.2.2.2 area 0 10.1.1.2

 

R2において、R1の状態がExStartで止まっていることが確認できます。

R2

R2#debug ip ospf hello
R2#debug ip ospf adj
*May 29 14:00:09.179: OSPF-1 ADJ   Gi2: Interface going Up
*May 29 14:00:09.179: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.2
*May 29 14:00:09.944: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1
*May 29 14:00:09.944: OSPF-1 HELLO Gi2: Send immediate hello to nbr 1.1.1.1, src address 10.1.1.1
*May 29 14:00:09.944: OSPF-1 HELLO Gi2: Send hello to 10.1.1.1 area 0 from 10.1.1.2
*May 29 14:00:09.946: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1
*May 29 14:00:09.946: OSPF-1 ADJ   Gi2: 2 Way Communication to 1.1.1.1, state 2WAY
*May 29 14:00:09.946: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1: Prepare dbase exchange
*May 29 14:00:09.946: OSPF-1 ADJ   Gi2: Send DBD to 1.1.1.1 seq 0x1A32 opt 0x52 flag 0x7 len 32
*May 29 14:00:09.946: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0x1E58 opt 0x52 flag 0x7 len 32  mtu 1100 state EXSTART
*May 29 14:00:09.946: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU
*May 29 14:00:09.946: OSPF-1 ADJ   Gi2: First DBD and we are not SLAVE
*May 29 14:00:14.729: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0x1E58 opt 0x52 flag 0x7 len 32  mtu 1100 state EXSTART
*May 29 14:00:14.729: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU
*May 29 14:00:14.729: OSPF-1 ADJ   Gi2: First DBD and we are not SLAVE
*May 29 14:00:14.858: OSPF-1 ADJ   Gi2: Send DBD to 1.1.1.1 seq 0x1A32 opt 0x52 flag 0x7 len 32
*May 29 14:00:14.858: OSPF-1 ADJ   Gi2: Retransmitting DBD to 1.1.1.1 [1]
*May 29 14:00:18.536: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.2
*May 29 14:00:19.361: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1
*May 29 14:00:19.625: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0x1E58 opt 0x52 flag 0x7 len 32  mtu 1100 state EXSTART
*May 29 14:00:19.625: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU
*May 29 14:00:19.625: OSPF-1 ADJ   Gi2: First DBD and we are not SLAVE
*May 29 14:00:19.642: OSPF-1 ADJ   Gi2: Send DBD to 1.1.1.1 seq 0x1A32 opt 0x52 flag 0x7 len 32
*May 29 14:00:19.642: OSPF-1 ADJ   Gi2: Retransmitting DBD to 1.1.1.1 [2]
*May 29 14:00:24.257: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0x1E58 opt 0x52 flag 0x7 len 32  mtu 1100 state EXSTART
*May 29 14:00:24.257: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU

 

 

MTU Ignore設定後

R1のg2でMTU Ignoreを有効にします。

追加した設定

MTU Ignoreはip ospf mtu-ignoreコマンドで有効化可能で、MTUが小さいルータに対してのみ設定すれば、ルータ間でOSPFネイバーが確立可能です。また、インタフェースにMTU Ignoreが設定されているかは、show ip ospf interfaceコマンドでは確認できません。

R1

interface GigabitEthernet2
 ip ospf mtu-ignore

 

OSPFネイバーの状態

R1において、R2の状態がFullまで進んでことが確認できます。

R1

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  -        00:00:32    10.1.1.2        GigabitEthernet2

 

R2において、R1の状態がFullまで進んでことが確認できます。

R2

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   FULL/  -        00:00:39    10.1.1.1        GigabitEthernet2

 

OSPFのデバックの確認

R1において、R2の状態がFullまで進んでことが確認できます。

R1

R1#debug ip ospf hello
R1#debug ip ospf adj
*May 29 14:11:16.657: OSPF-1 ADJ   Gi2: Interface going Up
*May 29 14:11:16.657: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.1
*May 29 14:11:16.658: OSPF-1 HELLO Gi2: Rcv hello from 2.2.2.2 area 0 10.1.1.2
*May 29 14:11:16.658: OSPF-1 ADJ   Gi2: 2 Way Communication to 2.2.2.2, state 2WAY
*May 29 14:11:16.658: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2: Prepare dbase exchange
*May 29 14:11:16.658: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0xE88 opt 0x52 flag 0x7 len 32
*May 29 14:11:16.658: OSPF-1 HELLO Gi2: Send immediate hello to nbr 2.2.2.2, src address 10.1.1.2
*May 29 14:11:16.658: OSPF-1 HELLO Gi2: Send hello to 10.1.1.2 area 0 from 10.1.1.1
*May 29 14:11:16.659: OSPF-1 ADJ   Gi2: Rcv DBD from 2.2.2.2 seq 0x2267 opt 0x52 flag 0x7 len 32  mtu 1200 state EXSTART
*May 29 14:11:16.659: OSPF-1 ADJ   Gi2: NBR Negotiation Done. We are the SLAVE
*May 29 14:11:16.659: OSPF-1 ADJ   Gi2: Nbr 2.2.2.2: Summary list built, size 1
*May 29 14:11:16.659: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0x2267 opt 0x52 flag 0x2 len 52
*May 29 14:11:16.660: OSPF-1 ADJ   Gi2: Rcv DBD from 2.2.2.2 seq 0x2268 opt 0x52 flag 0x1 len 52  mtu 1200 state EXCHANGE
*May 29 14:11:16.660: OSPF-1 ADJ   Gi2: Exchange Done with 2.2.2.2
*May 29 14:11:16.660: OSPF-1 ADJ   Gi2: Send LS REQ to 2.2.2.2 length 36
*May 29 14:11:16.660: OSPF-1 ADJ   Gi2: Send DBD to 2.2.2.2 seq 0x2268 opt 0x52 flag 0x0 len 32
*May 29 14:11:16.661: OSPF-1 ADJ   Gi2: Rcv LS UPD from Nbr ID 2.2.2.2 length 64 LSA count 1
*May 29 14:11:16.661: OSPF-1 ADJ   Gi2: Synchronized with 2.2.2.2, state FULL
*May 29 14:11:16.661: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet2 from LOADING to FULL, Loading Done
*May 29 14:11:16.661: OSPF-1 ADJ   Gi2: Rcv LS REQ from 2.2.2.2 length 36 LSA count 1
*May 29 14:11:24.791: OSPF-1 HELLO Gi2: Rcv hello from 2.2.2.2 area 0 10.1.1.2
*May 29 14:11:25.936: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.1

 

R2において、R1の状態がFullまで進んでことが確認できます。

R2

R2#debug ip ospf hello
R2#debug ip ospf adj
*May 29 14:11:16.572: OSPF-1 ADJ   Gi2: Interface going Up
*May 29 14:11:16.572: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.2
*May 29 14:11:17.618: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1
*May 29 14:11:17.618: OSPF-1 HELLO Gi2: Send immediate hello to nbr 1.1.1.1, src address 10.1.1.1
*May 29 14:11:17.618: OSPF-1 HELLO Gi2: Send hello to 10.1.1.1 area 0 from 10.1.1.2
*May 29 14:11:17.619: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1
*May 29 14:11:17.619: OSPF-1 ADJ   Gi2: 2 Way Communication to 1.1.1.1, state 2WAY
*May 29 14:11:17.619: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1: Prepare dbase exchange
*May 29 14:11:17.619: OSPF-1 ADJ   Gi2: Send DBD to 1.1.1.1 seq 0x2267 opt 0x52 flag 0x7 len 32
*May 29 14:11:17.619: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0xE88 opt 0x52 flag 0x7 len 32  mtu 1100 state EXSTART
*May 29 14:11:17.619: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU
*May 29 14:11:17.619: OSPF-1 ADJ   Gi2: First DBD and we are not SLAVE
*May 29 14:11:17.620: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0x2267 opt 0x52 flag 0x2 len 52  mtu 1100 state EXSTART
*May 29 14:11:17.620: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU
*May 29 14:11:17.620: OSPF-1 ADJ   Gi2: NBR Negotiation Done. We are the MASTER
*May 29 14:11:17.620: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1: Summary list built, size 1
*May 29 14:11:17.620: OSPF-1 ADJ   Gi2: Send DBD to 1.1.1.1 seq 0x2268 opt 0x52 flag 0x1 len 52
*May 29 14:11:17.621: OSPF-1 ADJ   Gi2: Rcv LS REQ from 1.1.1.1 length 36 LSA count 1
*May 29 14:11:17.621: OSPF-1 ADJ   Gi2: Send LS UPD to 10.1.1.1 length 64 LSA count 1
*May 29 14:11:17.621: OSPF-1 ADJ   Gi2: Rcv DBD from 1.1.1.1 seq 0x2268 opt 0x52 flag 0x0 len 32  mtu 1100 state EXCHANGE
*May 29 14:11:17.621: OSPF-1 ADJ   Gi2: Nbr 1.1.1.1 has smaller interface MTU
*May 29 14:11:17.621: OSPF-1 ADJ   Gi2: Exchange Done with 1.1.1.1
*May 29 14:11:17.621: OSPF-1 ADJ   Gi2: Send LS REQ to 1.1.1.1 length 36
*May 29 14:11:17.622: OSPF-1 ADJ   Gi2: Rcv LS UPD from Nbr ID 1.1.1.1 length 64 LSA count 1
*May 29 14:11:17.622: OSPF-1 ADJ   Gi2: Synchronized with 1.1.1.1, state FULL
*May 29 14:11:17.622: %OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1 on GigabitEthernet2 from LOADING to FULL, Loading Done
*May 29 14:11:25.751: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.2
*May 29 14:11:26.897: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1
*May 29 14:11:35.051: OSPF-1 HELLO Gi2: Send hello to 224.0.0.5 area 0 from 10.1.1.2
*May 29 14:11:36.833: OSPF-1 HELLO Gi2: Rcv hello from 1.1.1.1 area 0 10.1.1.1

 

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