MTU Settings

MTU mismatch issue between Cisco IOS XR and NXOS

The MTU is set to 1600 on the ASR 9K bundle-ether interface.

RP/0/RSP0/CPU0:PE1#show running-config interface bundle-ether 4
Thu Aug 11 11:33:56.117 DST
interface Bundle-Ether4
 description to OTT-Nexus9k-CE3 eth-trunk2
 mtu 1600
!

RP/0/RSP0/CPU0:PE1#show interfaces bundle-ether 4
Thu Aug 11 11:34:08.349 DST
Bundle-Ether4 is up, line protocol is up 
  Interface state transitions: 7
  Hardware is Aggregated Ethernet interface(s), address is d46d.5018.4da0
  Description: to OTT-Nexus9k-CE3 eth-trunk2
  Internet address is Unknown
  MTU 1600 bytes, BW 10000000 Kbit (Max: 10000000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 10000Mb/s
  loopback not set,
    No. of members in this bundle: 1
      TenGigE0/0/1/1               Full-duplex  10000Mb/s    Active          
  Last input 00:00:00, output 00:00:00
  Last clearing of "show interface" counters never
  5 minute input rate 1000 bits/sec, 3 packets/sec
  5 minute output rate 1000 bits/sec, 2 packets/sec
     2515143 packets input, 211121472 bytes, 12 total input drops
     19400 drops for unrecognized upper-level protocol
     Received 175 broadcast packets, 1860630 multicast packets
              0 runts, 0 giants, 0 throttles, 0 parity
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     1168288 packets output, 104691344 bytes, 0 total output drops
     Output 44 broadcast packets, 567016 multicast packets
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions

Any sub-interfaces would inherit the MTU settings of the parent interface and increment it with the 4bytes vlan tag.

RP/0/RSP0/CPU0:PE1#show interfaces bundle-ether 4.44
Thu Aug 11 11:38:47.620 DST
Bundle-Ether4.44 is up, line protocol is up 
  Interface state transitions: 7
  Hardware is VLAN sub-interface(s), address is d46d.5018.4da0
  Description: Nexus9k-Signal_VRF
  Internet address is 10.10.208.48/31
  MTU 1604 bytes, BW 10000000 Kbit (Max: 10000000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation 802.1Q Virtual LAN, VLAN Id 44,  loopback not set,
  ARP type ARPA, ARP timeout 04:00:00
  Last input 00:00:00, output 00:00:00
  Last clearing of "show interface" counters never
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
     198480 packets input, 15654506 bytes, 0 total input drops
     0 drops for unrecognized upper-level protocol
     Received 23 broadcast packets, 125887 multicast packets
     125416 packets output, 11084348 bytes, 0 total output drops
     Output 4 broadcast packets, 58754 multicast packets

On the Nexus 9K port-channel is connected to ASR9K and the parent interface is configured for mtu value 1600. But still the OSPF neighbor is in EXSTART state.

Nexus9k-CE3# show ip ospf neighbors vrf Signal_VRF
 OSPF Process ID CoreOSPF VRF Signal_VRF
 Total number of neighbors: 2
 Neighbor ID     Pri State            Up Time  Address         Interface
 10.10.208.53      1 FULL/BDR         1w5d     10.10.208.53    Vlan46 
 10.10.100.9       1 EXSTART/DR       00:00:05 10.10.208.48    Po2.44

Now a work around is to configure ip ospf mtu-ignore which I eventually did, but is there a way to configure this without the need for ignoring mtu?

MTU settings in Windows PC

In Windows 10 or 11 you can check the MTU for every interface through Powershell.

Windows PowerShell
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Install the latest PowerShell for new features and improvements! https://aka.ms/PSWindows

PS C:\Users\Kaisar Mirza> netsh interface ipv4 show interface

Idx     Met         MTU          State                Name
---  ----------  ----------  ------------  ---------------------------
  1          75  4294967295  connected     Loopback Pseudo-Interface 1
 16           5       65535  disconnected  OpenVPN Wintun
  7          50        1500  connected     Wi-Fi
 20           5        1500  disconnected  Ethernet
 17          25        1500  disconnected  Local Area Connection* 1
 25           1        1500  connected     Local Area Connection
 13          25        1500  disconnected  Local Area Connection* 2
 22          25        1500  disconnected  OpenVPN Connect DCO Adapter
  3          65        1500  disconnected  Bluetooth Network Connection
  9          35        1500  connected     VMware Network Adapter VMnet1
 10          35        1500  connected     VMware Network Adapter VMnet8
 12          25        1500  connected     Ethernet 2