Mtu Setting

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?

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