What happens if I use the default and trust dot1p? Cisco IP phones only marks the DSCP field in the IP packet but they don't mark the 802.1p field in the frame? If that is the case, it makes sense to trust DSCP. But if they mark also the 802.1p field, I could also use the dot1p, right?
The Cisco Phones also mark the traffic with 802.1p. However, this only works when the traffic is tagged and not untagged because the 802.1p value in the the VLAN header.
There is another command:
vlan x
voice
tagged 1-10
This only enabled the switch to transmit LLDP-MED to tell the phone to use VLAN x as the voice VLAN.
Isn't that enough to prioritize the voice traffic that is received inbound on the port? Should I use the two commands all together to prioritize the voice traffic (qos trust dscp and voice)?
The voice command is to send LLDP-MED in the LLDP data. This has nothing to do with QoS. The default of the switch is to use 802.1p. So, if the only requirerement is to prioritize the VOICE traffic of the Cisco phones than the default are ok. In this case make sure that the voice traffic is send tagged.
I prefer to use DSCP because 802.1p is in the VLAN header. This gives the possibility to use the DSCP field of a client is sending untagged data.