Neighbor state machine after the MTU value of H3C OSPF interface is changed

The neighbor state machine after the mtu value of the H3C OSPF interface has been studying the OSPF principle and configuration of H3C over the past few days. Now there are two routers R3 and R4 connected through their respective Ethernet ports E0/0, after configuring the IP address and OSPF correctly, modify the mtu value of the R3 E0/0 interface to 1000 (1500 by default) and restart the OSPF process of R3, then R3 and R4 can still establish an adjacent relationship (that is, FULL ). After the two routers establish the OSPF neighbor relationship (2-way), they will send the Digest (DD) of their respective LSDB packets. The DD packet contains the MTU parameter, in this way, the MTU parameters of the two routers cannot be negotiated, so they will stay in the Exstart state, but they will still be in the FULL state, which is strange! Enable debug ospf packet of R3 and restart the OSPF process of R3. Based on the debug information, it is found that the MTU parameter in the DD packet sent by R3 to R4 is 0, the MTU parameter in the DD message received from R4 is also 0. After checking the relevant information, I suddenly realized. Www.2cto.com by default, the MTU value is not specified when the interface sends the DD message, that is, the MTU value in the DD message is 0, so even if the MTU of the interface is modified, in the DD packet, the MTU value is still 0 and enters the E0/0 interface of R3 and R4 respectively. Use the ospf mtu-enable command to configure the interface to fill in the MTU value of the local end in the sent DD packet, after this modification, restart the ospf process of the R3 router. the MTU parameter in the debug information is set by R3. The DD packet in the debug information is as follows: * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: OSPF 1: SEND Packet. * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: Source Address: 34.34.34.3 * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: destination Address: 34.34.34.4 * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: Ver #2, Type: 2, Length: 32. * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: Router: 3.3.3.3, Area: 0.0.0.2, Checksum: 0. * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: AuType: 02, Key (ascii): 0 0 1 10 0 0 2 9e. * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: MTU: 1000, Option: _ E _, R_ I _M_MS Bit: _ I _M_MS _. * May 4 05: 32: 41: 172 2010 RT3 RM/6/RMDEBUG: DD SeqNumber: 5990. * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: OSPF 1: RECV Packet. * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: Source Address: 34.34.34.4 * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: destination Address: 34.34.34.3 * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: Ver #2, Type: 2, Length: 32. * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: Router: 4.4.4.4, Area: 0.0.0.2, Checksum: 0. * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: AuType: 02, Key (ascii): 0 0 1 10 0 3 fb. * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: MTU: 1500, Option: _ E _, R_ I _M_MS Bit: _ I _M_MS _. www.2cto.com * May 4 05: 32: 41: 188 2010 RT3 RM/6/RMDEBUG: DD SeqNumber: 59ad.
Display ospf peer on R3 to view the Neighbor status as follows: OSPF Process 1 with Router ID 3.3.3.3 Neighbor Brief Information Area: 0.0.0.2 Router ID Address Pri Dead-Time Interface State 4.4.4.4 34.34.34.4 1 36 Eth0/1/1 ExStart/BDR
The neighbor state machine between R3 and R4 stays in the Exstart state. At this time, the routers of both parties are constantly sending DD packets. In the actual environment, if an H3C router is connected to another manufacturer's router to enable OSPF, the MTU value of the interface may be different, at the same time, the H3C router enables the option to enter the MTU value when the interface sends the DD message, so the above situation may occur, that is, the OSPF neighbor state machine of both parties stays in the ExStart State and cannot establish a FULL relationship)