General
cEdge requires a 4 Byte overhead when using a dot1Q subinterface. There is no such requirement in legacy IOS-XE code. Would like that requirement dropped from cEdge SD-WAN code.
Symptom
Drop cEdge requirement for dot1Q subinterface MTU to be 4 Bytes less than main interface
Conditions
Drop cEdge requirement for dot1Q subinterface MTU to be 4 Bytes less than main interface
Further Problem Description
cEdge dot1Q subinterface should not require the dot1Q sub-interface be configured with ip MTU 1496 Bytes if main interface is 1500 Bytes.
In cEdge code, if main interface is set to 1500 Bytes, the dot1Q subinterface under that main interface must be set to 1496 Bytes:
svla1-wan-gw1#config-t
admin connected from 127.0.0.1 using console on svla1-wan-gw1
svla1-wan-gw1(config)# interface GigabitEthernet0/0/2.700
svla1-wan-gw1(config-if)# no ip mtu 1496
svla1-wan-gw1(config-if)# commit
Aborted: 'vpn 0 interface GigabitEthernet0/0/2.700 mtu': Tagged interface MTU needs to be at least 4 bytes less than parent interface (GigabitEthernet0/0/2) mtu (1500)
In legacy IOS-XE code on 4451x and ASR, we don't have that requirement:
frkl01-wan-gw1#sh int gig0/0/2
GigabitEthernet0/0/2 is up, line protocol is up
Hardware is ISR4451-X-4x1GE, address is 002a.1091.ded2 (bia 002a.1091.ded2)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 52/255, rxload 40/255
Encapsulation 802.1Q Virtual LAN, Vlan ID 1., loopback not set
Keepalive not supported
Full Duplex, 100Mbps, media type is 100BaseFX
The subinterface under that main interface that faces the telco has 1500 Byte MTU by default:
frkl01-wan-gw1#sh int gig0/0/2.160
GigabitEthernet0/0/2.160 is up, line protocol is up
Hardware is ISR4451-X-4x1GE, address is 002a.1091.ded2 (bia 002a.1091.ded2)
Description: rtp10-wag-gw1-ten0-2-0-160 : Verizon
Internet address is 10.150.57.17/30
MTU 1500 bytes, BW 50000 Kbit/sec, DLY 500 usec,
frkl01-wan-gw1#sh run int gig0/0/2.160 | incl mtu
frkl01-wan-gw1#
