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What is recommended MINT MTU size?

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TitleWhat is recommended MINT MTU size?
Question
When your networks spans over WAN you shall consider global MINT MTU size to help avoid fragmentation.
What is the best MTU size for your deployment?
Environment
  • All Summit WM3000 Series Controllers
  • ExtremeWiNG Controllers
  • WirelessWiNG Controllers
  • ExtremeWiNG Access Points
  • WirelessWiNG Acess Points
  • WiNG 5 Software
Answer

So this is the default WiNG global policy configuration:

VX(config-mint-policy-global-default)# show context include-factory
 mint-policy global-default
 udp port 24576
 mtu 1500
 no level 2 area-id
 router packet priority 0
 no lsp checksum 
 no lsp disable-suspect-lsp-checks
When customer does any kind of WAN crossing which affects maximum transmission unit size you shall confirm MINT will also handle this. To verify the general MTU size that will pass check with don't fragment option when pinging the remote site:
VX#ping 88.88.88.88 dont-fragment size 1472
PING 88.88.88.88 (88.88.88.88) 1472(1500) bytes of data.
1480 bytes from 88.88.88.88: icmp_seq=1 ttl=63 time=0.782 ms

VX#ping 88.88.88.88 dont-fragment size 1473
PING 88.88.88.88 (88.88.88.88) 1473(1501) bytes of data.
From 192.168.7.205 icmp_seq=1 Frag needed and DF set (mtu = 1500)
When you know what is the MTU for general traffic use following approximate formula 
Highest DF PING MTU - 100 bytes = WAN crossing MINT MTU
If you want to confirm this size is enough you may do simple test MINT ping test
Showing fragmented MINT ping
VX#mint ping 4D.87.1A.F0 size 1472
MiNT ping 4D.87.1A.F0 with 1472 bytes of data.
 Response from 4D.87.1A.F0: id=16777216 time=0.751 ms
 
AP#service pktcap on bridge filter mint
Capturing up to 50 packets. Use Ctrl-C to abort.
1 9:15:50.894618 UDP: 192.168.7.205 > 88.88.88.88 ports 24576 > 24576, data length 1534, IPv4 fragment id 53959, offset 0, DSCP 0| DGRAM 12.34.04.07/63569 > 4D.87.1A.F0/15 ping
2 9:15:50.894618 UDP: 192.168.7.205 > 88.88.88.88 IPv4 fragment, IPv4 fragment id 53959, offset 1448, IPv4 length 106, DSCP 0
3 9:15:50.894755 UDP: 88.88.88.88 > 192.168.7.205 ports 24576 > 24576, data length 1534, IPv4 fragment id 23669, offset 0, DSCP 0| DGRAM 4D.87.1A.F0/15 > 12.34.04.07/63569 ping
4 9:15:50.894755 UDP: 88.88.88.88 > 192.168.7.205 IPv4 fragment, IPv4 fragment id 23669, offset 1448, IPv4 length 106, DSCP 0
Showing non-fragmented MINT ping
VX#mint ping 4D.87.1A.F0 size 1386
MiNT ping 4D.87.1A.F0 with 1386 bytes of data.
 Response from 4D.87.1A.F0: id=16777216 time=0.699 ms

AP#service pktcap on bridge filter mint
Capturing up to 50 packets. Use Ctrl-C to abort.
1 9:24:02.688107 UDP: 192.168.7.205 > 88.88.88.88 ports 24576 > 24576, data length 1448, DSCP 0| DGRAM 12.34.04.07/62395 > 4D.87.1A.F0/15 ping
2 9:24:02.688238 UDP: 88.88.88.88 > 192.168.7.205 ports 24576 > 24576, data length 1448, DSCP 0| DGRAM 4D.87.1A.F0/15 > 12.34.04.07/62395 ping
Be careful not to set too small MINT MTU as this might affect proper adoption / configuration of adopted devices.
MINT usually does not use big packets, but can go as high as 1000 bytes when AP gets adopted or configured
ROUTER#service pktcap on bridge filter mint and host 88.88.88.88
<omitted>
7 9:21:18.149772 UDP: 88.88.88.88 > 192.168.7.205 ports 24576 > 24576, data length 72, DSCP 0| RDP/STREAM 4D.87.1A.F0/58143 > 12.34.04.07/11 adoption/config, AF, seq 55626, ack 8144, win 10, data 0
6 9:21:18.149758 UDP: 88.88.88.88 > 192.168.7.205 ports 24576 > 24576, data length 877, DSCP 0| RDP/STREAM 4D.87.1A.F0/58143 > 12.34.04.07/11 adoption/config, AP, seq 55625, ack 8144, win 10, data 805
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