19-12 ip ospf mtu-ignore

19-12 ip ospf mtu-ignore

Syntax Description:

This command has no arguments.

Purpose: If there is a maximum transmission unit (MTU) mismatch between neighboring OSPF routers, then the routers will not form an OSPF adjacency .

Initial Cisco IOS Software Release: 12.0 (3)

Configuration Example: OSPF Adjacency and MTU Mismatch

In Figure 19-9, Routers A and B have an MTU of 1500 on their serial interfaces and Router C has an MTU of 1490. When Routers A and C reach the EXSTART state in the forming of the adjacency, they will attempt to exchange their OSPF database description packets. An OSPF router will ignore a database description packet if there is an MTU mismatch. When there is a mismatch, the database description packet will not be acknowledged and the sending router will continue to send the packets until they are acknowledged . The acknowledgement will never come and the routers will be stuck in the EXSTART state. To demonstrate this, Router C has been configured with an MTU of 1490 as shown in the following configurations.

Figure 19-9. OSPF Will Not Form a Neighbor Relationship if There Is an MTU Mismatch Between the Interfaces Connecting the Routers

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/0   bandwidth 64   no ip address   encapsulation frame-relay   frame-relay lmi-type ansi   !   interface Serial0/0.1 point-to-point   ip address 10.1.1.1 255.255.255.252   frame-relay interface-dlci 101   !   interface Serial0/0.2 point-to-point   ip address 10.1.1.5 255.255.255.252   frame-relay interface-dlci 102   !   router ospf 1   network 1.1.1.1 0.0.0.0 area 1   network 10.1.1.0 0.0.0.3 area 0   network 10.1.1.4 0.0.0.3 area 0  _________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   ip address 10.1.1.2 255.255.255.252   encapsulation frame-relay   ip ospf network point-to-point   no ip mroute-cache   bandwidth 64   frame-relay map ip 10.1.1.1 110 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi   !   router ospf 1   network 2.2.2.2 0.0.0.0 area 2   network 10.1.1.0 0.0.0.3 area 0  _________________________________________________________________ Router C  interface Loopback0   ip address 3.3.3.3 255.255.255.255   !   interface Serial0    mtu 1490    bandwidth 64   no ip address   encapsulation frame-relay   no ip mroute-cache   !   interface Serial0.1 point-to-point   ip address 10.1.1.6 255.255.255.252   frame-relay interface-dlci 120   !   router ospf 1   network 3.3.3.3 0.0.0.0 area 3   network 10.1.1.4 0.0.0.3 area 0  

Enable OSPF debugging on Router A and then reset the OSPF process on Router A.

 rtrA#  debug ip ospf events  OSPF events debugging is on rtrA#  clear ip ospf process  Reset ALL OSPF processes? [no]:  y  rtrA# . . . 05:37:58: OSPF: Rcv DBD from 3.3.3.3 on Serial0/0.2 seq 0xF91 opt 0x2 flag 0x7 l en 32  mtu 1490  state EXCHANGE 05:37:58: OSPF: Send DBD to 3.3.3.3 on Serial0/0.2 seq 0xF91 opt 0x42 flag 0x2 l en 152 05:38:03: OSPF: Rcv hello from 3.3.3.3 area 0 from Serial0/0.2 10.1.1.6 05:38:03: OSPF: End of hello processing 05:38:03: OSPF: Rcv DBD from 3.3.3.3 on Serial0/0.2 seq 0xF91 opt 0x2 flag 0x7 l en 32  mtu 1490  state EXCHANGE 05:38:03: OSPF: Send DBD to 3.3.3.3 on Serial0/0.2 seq 0xF91 opt 0x42 flag 0x2 l en 152 . . . 

You can see from the debug output on Router A that Routers A and C are stuck in the EXSTART state. This can also be seen by examining the OSPF neighbors as seen by Routers A and C.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 3.3.3.3           1  EXCHANGE  /  -    00:00:35    10.1.1.6        Serial0/0.2 2.2.2.2           1   FULL/  -        00:00:38    10.1.1.2        Serial0/0.1 _________________________________________________________________ rtrC#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1  EXSTART  /  -     00:00:38    10.1.1.5        Serial0.1 

This problem can be fixed by configuring the MTU to be the same on all of the serial interfaces. Cisco does not support the configuration of the MTU on Ethernet or Token Ring interfaces, so the only option is to use the ip ospf mtu-ignore interface command if a mismatch occurs on these network types. For this example, configure Routers A and C to ignore the MTU mismatch.

 Router A  interface Serial0/0   bandwidth 64   no ip address   encapsulation frame-relay   frame-relay lmi-type ansi   !   interface Serial0/0.1 point-to-point   ip address 10.1.1.1 255.255.255.252   frame-relay interface-dlci 101   !   interface Serial0/0.2 point-to-point   ip address 10.1.1.5 255.255.255.252    ip ospf mtu-ignore    frame-relay interface-dlci 102  _________________________________________________________________ Router C  interface Serial0   mtu 1490   bandwidth 64   no ip address   no ip directed-broadcast   encapsulation frame-relay   !   interface Serial0.1 point-to-point   ip address 10.1.1.6 255.255.255.252   no ip directed-broadcast    ip ospf mtu-ignore    frame-relay interface-dlci 120  

Verification

Verify that Routers A and C have formed an OSPF neighbor relationship.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 3.3.3.3           1   FULL/  -        00:00:36    10.1.1.6        Serial0/0.2 2.2.2.2           1   FULL/  -        00:00:36    10.1.1.2        Serial0/0.1 

Troubleshooting

Step 1. Verify the IP address and netmask assignments used on the network interfaces.

Step 2. Verify that the proper DLCIs and IP addresses are used in the frame relay map ip statements. These can be checked using the show frame-relay map command, as shown here.

 rtrA#  show frame-relay map  Serial0/0 (up): ip 10.1.1.2 dlci 101(0x65,0x1850), static,               broadcast,               CISCO, status defined, active Serial0/0 (up): ip 10.1.1.3 dlci 102(0x66,0x1860), static,               broadcast,               CISCO, status defined, active 

Step 3. Before configuring OSPF, check IP connectivity by pinging the other end of the link.

Step 4. If you are mixing network types (multipoint and point-to-point) verify that the Hello intervals match on all routers.

Step 5. If there is an MTU mismatch, either modify the MTU on one end of the link to match the other end or use the ip ospf mtu-ignore interface command.