10-24-2008 01:37 AM - edited 03-06-2019 02:07 AM
Hello Learned Friends. Some help required here!! :-)
1. Whats Layer 3 Precedence mean/do &
what does Layer 2 COS mean(I know
what it does)?
2. When configuring Voice VLAN:-
Whats the trusted mode/ untrusted
mode by CDP request from the switch
access port & how is it achieved?
Does it have any link to the cmd "mls
qos trust cos"? What actually happens
at these steps?
3. Has anyone any clearer steps on this
whole configuration process than what
Mr. Lammle(SYBEX) shows? Maybe with
schematics,flowcharts & diagrams? I'm
MANY THNX
BS =)
10-24-2008 04:07 AM
Hello Baljit,
1)
with the introduction of Quality of Service in the IP networks and the differentiated Services models packets (OSI L3) or ethernet Frames (OSI L2) can be treated in a different manner according to their marking:
TOS or DSCP is a form to signal that an IP packet has to treated according to the value: for example in the case of VoIP packets that are part of the voice conversation are marked with TOS 5 or DSCF EF = expedit forwarding that means that these packets are placed in a prioritizated queue
802.1Q frames are ethernet frames with Vlan tagging information (that tells to which Vlan the frame belong) in the 4 bytes 802.1Q header there are 3 bits called CoS = Class of Service specified in 802.1p that allows to implement differentiated services in a LAN switched environment.
So an IP packet traveling inside an ethernet frame out a 802.1Q trunk port has:
a CoS 3 bits field in the L2 802.1Q header
a TOS or DSCP settings inside the IP header (L3).
Usually CoS is a copy of TOS 3leftmost bits of DSCP.
2)
Multilayer switches implement QoS using the trust boundary concept:
mls qos trust instructs the switch to accept the CoS settings on received frames (accept=don't modify)
the switch associate to the frame an internal DSCP that reflects the CoS settings seen.
If the port is not trusted all incoming frames will have their CoS and ToS /DSCP set to a single value (0 by default or the CoS value set for the port with mls qos cos x)
CDP version 2 is used to communicate with a Cisco IP Phone: what is the voice vlan (the voice vlan tag to be used), the power to be provided by the switch if the port supports PoE and is configured to provide it, the QoS behaviour that the IP phone has to use on downstream connected frames.
So the switch can even extend the trust boundary to the IP phone port to the PC.
The phone will send VoIP packets with CoS 5 and DSCP = EF.
Again frames coming from PC can be:
untrusted with CoS reset to 0
trusted = accepted
3)
on modern IOS switches
int fas0/1
switchport
switchport access vlan
switchport mode access
switchport voice vlan
from a L2 point of view this is an implicit 802.1Q trunk with untagged frames in data-vlan and frames tagged with voice-vlan
Older switches may require
int f0/1
switchport
switchport trunk enc dot1q
switchport mode trunk
switchport native vlan
switchport voice vlan
switchport trunk allowed vlan
Hope to help
Giuseppe
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