OSPF mtu mismatch question

[taroyamada9999]

Hello,

I have following cisco catalyst switches and running OSPF.

3750G (C3750-ADVIPSERVICESK 12.2(25)SED1)

3750X (C3750E-UNIVERSALK9-M 15.0(1)SE3)

3850 (cat3k_caa-universalk9 03.02.01.SE)

They are all connected through L2 WAN (192.168.40.x/24 star topology)

The link between 3750X and 3850 doesn't reach FULL state.

I think this is because MTU mismatch.

3750G VLAN60 SVI mtu 1500

3750X VLAN60 SVI mtu 1500

3850 VLAN60 SVI mtu 9198

But 3750G doesn't complain anything and staying FULL state with 3750X and 3850.

The output of show ip ospf neigbors on each switch is like the following.

3750G (ID: 10.10.11.1 IP address:192.168.40.101)

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           1   FULL/DROTHER    00:00:36    192.168.40.102  Vlan60

3.3.3.3           1   FULL/BDR        00:00:33    192.168.40.103  Vlan60

3750X (ID: 2.2.2.2 IP address:192.168.40.102)

Neighbor ID     Pri   State           Dead Time   Address         Interface

3.3.3.3           1   EXSTART/BDR     00:00:36    192.168.40.103  Vlan60 (repeat EXSTART > DOWN )

10.10.11.1        1   FULL/DR         00:00:30    192.168.40.101  Vlan60

3850 (ID:3.3.3.3 IP address: 192.168.40.103)

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           1   EXSTART/DROTHER 00:00:38    192.168.40.102  Vlan60 (repeat EXSTART > DOWN )

10.10.11.1        1   FULL/DR         00:00:32    192.168.40.101  Vlan60

I think if I changed MTU on 3850 to 1500 (ip mtu 1500), all seem to get OK.

But the question is why 3750G doesn't complain now.

Thanks in advance,

Taro

[Edison Ortiz]

If indeed it's a MTU issue, the 3750G could become neighbors with the other 2 devices if the 3750G has a higher system MTU than the received OSPF packet or you have OSPF MTU-IGNORE configured in the interface.

[taroyamada9999]

Thanks, Edison.

Does the behavior make sense if "ip ospf mtu-ignore" is enabled on 3750G?

As far as i see the config, i don't see the command.

I checked if "ip ospf mtu-ignore" is enabled by default and not show in the config on specific IOS version,

but i was not able to find any post/article.

[Daniel Boling]

Here is a really good discussion I remember reading a while back:

https://learningnetwork.cisco.com/thread/37638

[taroyamada9999]

Thanks, Daniel.

The link you gave me doesn't answer my question why 3750G doesn't complain about mtu mismatch.

or the issue is something different than mtu mismatch in the first place.

[taroyamada9999]

Does anyone know why??

[get_rthym]

Can you plz post sh run and sh int vlan x.

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[taroyamada9999]

Thanks for your reply.

Here is the information.

3750X

show run

interface Vlan60
ip address 192.168.40.102 255.255.255.0
ip ospf network broadcast

interface GigabitEthernet1/0/39
switchport access vlan 60
switchport mode access
speed 100
duplex full
priority-queue out
mls qos trust dscp
spanning-tree portfast

router ospf 1
router-id 2.2.2.2
passive-interface default
no passive-interface Vlan60
network 10.2.0.0 0.0.255.255 area 0
network 192.168.40.102 0.0.0.0 area 0

sh interface

sh int vlan 60
Vlan60 is up, line protocol is up
  Hardware is EtherSVI, address is 6073.5c5e.a755 (bia 6073.5c5e.a755)
  Internet address is 192.168.40.102/24
  MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive not supported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 3000 bits/sec, 4 packets/sec
     1709621 packets input, 160456050 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     5682192 packets output, 694571645 bytes, 0 underruns
     0 output errors, 1 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out
    
sh int gigabitEthernet 1/0/39
GigabitEthernet1/0/39 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 6073.5c5e.a727 (bia 6073.5c5e.a727)
  MTU 9000 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 77/255, rxload 5/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:08, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 137106466
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 2215000 bits/sec, 2226 packets/sec
  5 minute output rate 30233000 bits/sec, 3004 packets/sec
     6170441449 packets input, 1505330005415 bytes, 0 no buffer
     Received 2647635 broadcasts (2647634 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 2647634 multicast, 0 pause input
     0 input packets with dribble condition detected
     8025568328 packets output, 9506017028037 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

sh system mtu

System MTU size is 1500 bytes
System Jumbo MTU size is 9000 bytes
System Alternate MTU size is 1500 bytes
Routing MTU size is 1500 bytes

3850

sh run

interface Vlan60
ip address 192.168.40.103 255.255.255.0
ip ospf network broadcast

interface GigabitEthernet1/0/24
switchport access vlan 60
switchport mode access
speed 100
duplex full
spanning-tree portfast

router ospf 1
router-id 3.3.3.3
passive-interface default
no passive-interface Vlan60
network 10.3.0.0 0.0.255.255 area 0
network 192.168.40.103 0.0.0.0 area 0

sh interface

  sh interfaces vlan 60
Vlan60 is up, line protocol is up
  Hardware is Ethernet SVI, address is 44ad.d96d.4e44 (bia 44ad.d96d.4e44)
  Internet address is 192.168.40.103/24
  MTU 9198 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 7/255
  Encapsulation ARPA, loopback not set
  Keepalive not supported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 29344000 bits/sec, 2565 packets/sec
  5 minute output rate 1733000 bits/sec, 1860 packets/sec
     2106300027 packets input, 3681780117 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     3994823471 packets output, 849352157 bytes, 0 underruns
     0 output errors, 2 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out
    
sh interfaces gigabitEthernet 1/0/24
GigabitEthernet1/0/24 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 44ad.d96d.4e18 (bia 44ad.d96d.4e18)
  MTU 9198 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 4/255, rxload 70/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 27499000 bits/sec, 2413 packets/sec
  5 minute output rate 1651000 bits/sec, 1752 packets/sec
     2095007493 packets input, 2494707744 bytes, 0 no buffer
     Received 11698507 broadcasts (11698501 multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 11698501 multicast, 0 pause input
     0 input packets with dribble condition detected
     3997144835 packets output, 1186738596 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

sh system mtu
Global Ethernet MTU is 9198 bytes.

3750G

sh run

interface Vlan60
ip address 192.168.40.101 255.255.255.0
ip ospf network broadcast

interface GigabitEthernet1/0/24
switchport access vlan 60
switchport mode access
speed 100
duplex full
priority-queue out
mls qos trust dscp
spanning-tree portfast

router ospf 1
log-adjacency-changes
passive-interface default
no passive-interface Vlan60
network 10.1.0.0 0.0.255.255 area 0
network 192.168.40.101 0.0.0.0 area 0

sh interface

sh int vlan 60
Vlan60 is up, line protocol is up
  Hardware is EtherSVI, address is 0024.5081.c5d5 (bia 0024.5081.c5d5)
  Internet address is 192.168.40.101/24
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 1000 bits/sec, 1 packets/sec
  5 minute output rate 1000 bits/sec, 1 packets/sec
     6634505 packets input, 569546795 bytes, 0 no buffer
     Received 0 broadcasts (0 IP multicast)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     3649759 packets output, 416839158 bytes, 0 underruns
     0 output errors, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
    
sh interfaces gigabitEthernet 1/0/24
GigabitEthernet1/0/24 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet, address is 0024.5081.c598 (bia 0024.5081.c598)
  MTU 9000 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 2/255, rxload 5/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX
  input flow-control is off, output flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:25, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 2303000 bits/sec, 741 packets/sec
  5 minute output rate 860000 bits/sec, 642 packets/sec
     592388221 packets input, 101014036 bytes, 0 no buffer
     Received 2702855 broadcasts (0 multicast)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 2702839 multicast, 0 pause input
     0 input packets with dribble condition detected
     1131577290 packets output, 4108045582 bytes, 0 underruns
     0 output errors, 0 collisions, 5 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

sh system mtu

System MTU size is 1500 bytes
System Jumbo MTU size is 9000 bytes
Routing MTU size is 1500 bytes

[get_rthym]

Can u make the neighbors point -to-point n see if all becomes neighbor.

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