SAI L3 Intf 0 MTU size about sai HOT 3 OPEN

I think this MTU value can be fixed as 16536 (max value) when Vlan 1 router interface is created. This cannot be configured from Sonic. Note, In Sonic, only CPU port is part of Vlan 1

from sai.

I think this is not a SAI related question. Since SAI doesn't have the definition of the MTU of packets sent back to CPU. Could you move the issue towards SONiC websites?

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Thanks for your follow-up Prince, ShuoTian!

The issue is about switch front-panel ports.

We are testing ip mtu with SONiC 201803 branch image on our Broadcom Tomahawk (bcm56960) d7032 switch. After changing ip_mtu to 9100, we saw SONiC "show interface status", e.g. Ethernet0, has 9100 ip_mtu size and "bcmcmd ps" shows 9122. The problem we hit is that only able to route and forward up to 1532 bytes packet. Refer to the following “* MTU of Routing Interface doesn't work” on test details.

The Broadcom ASIC L3 forwarding datapath mtu is capped by L3 Intf 0 1514 bytes mtu size. SAI creates this Intf 0.

Thanks!

Wilson

• MTU of Routing Interface doesn't work

[Test step and config]

Test Center TCA port sends L3 IP packets to switch d7032 front-panel port Ethernet0 and TCB receives from Ethernet4.

/etc/sonic/config_db.json
"INTERFACE": {
"Ethernet0|20.20.20.100/24": {},
"Ethernet4|10.10.10.100/24": {}
},

[Test Result]

• Test 1, ip mtu: 1500, Passed:

root@SONiC-Inventec-d7032-100:~# vi /etc/sonic/config_db.json

"PORT": {
"Ethernet0": {
"alias": "Ethernet0",
"lanes": "1,2,3,4",
"mtu":1500
},
"Ethernet4": {
"alias": "Ethernet4",
"lanes": "5,6,7,8",
"mtu":1500
},

root@SONiC-Inventec-d7032-100:~# show interfaces status
Command: intfutil status
Interface Lanes Speed MTU Alias Oper Admin

Ethernet0 1,2,3,4 N/A 1500 Ethernet0 up up
Ethernet4 5,6,7,8 N/A 1500 Ethernet4 up up

root@SONiC-Inventec-d7032-100:~# bcmcmd "ps"
ps
ena/ speed/ link auto STP lrn inter max cut loop
port link Lns duplex scan neg? state pause discrd ops face frame thru? back
ce0( 1) up 4 100G FD SW No Forward TX RX None F KR4 1522 No
ce1( 2) up 4 100G FD SW No Forward TX RX None F KR4 1522 No

TCA sends traffic to TCB:
size
63 drop
64 pass
1526 pass (correct)
1527 drop (correct)

• Test 2, ip mtu: 9100, failed:

root@SONiC-Inventec-d7032-100:~# vi /etc/sonic/config_db.json

"PORT": {
"Ethernet0": {
"alias": "Ethernet0",
"lanes": "1,2,3,4",
"mtu":9100
},
"Ethernet4": {
"alias": "Ethernet4",
"lanes": "5,6,7,8",
"mtu":9100
},

root@SONiC-Inventec-d7032-100:~# show interfaces status
Command: intfutil status
Interface Lanes Speed MTU Alias Oper Admin

Ethernet0 1,2,3,4 N/A 9100 Ethernet0 up up
Ethernet4 5,6,7,8 N/A 9100 Ethernet4 up up

root@SONiC-Inventec-d7032-100:~# bcmcmd "ps"
ps
ena/ speed/ link auto STP lrn inter max cut loop
port link Lns duplex scan neg? state pause discrd ops face frame thru? back
ce0( 1) up 4 100G FD SW No Forward TX RX None F KR4 9122 No
ce1( 2) up 4 100G FD SW No Forward TX RX None F KR4 9122 No

TCA sends traffic to TCB:
size
1532 pass
1533 drop (failed!!)
9100 drop (failed!!)

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