Technical data

DATA CENTER and CAMPUS NETWORKS DEPLOYMENT GUIDE
Deploying Brocade Networks with Microsoft Lync Server 2010 20 of 52
Table 1. DSCP Relative to Cost of Service.
802.1p
DSCP
Brocade FCX
0
0 – 7
QoS p0
1
8 – 15
QoS p1
2
16 – 23
QoS p2
3
24 – 31
QoS p3
4
32 – 39
QoS p4
5
40 – 47
QoS p5
6
49 – 55
QoS p6
7
56 – 63
QoS p7
Access Control Lists: Layer 2 Codes
This option is the most granular of the three methods for prioritizing and/or marking traffic coming into the switch. All
of this is done using extended access lists.
Prioritizing Traffic
Prioritizing traffic using an access list is generally used to force the switch to honor DSCP or CoS. By leveraging ACLs,
you assign traffic that matches the ACL to a hardware-forwarding queue and re-mark the packets that match the ACL
with the 802.1p priority. In order to prioritize traffic, the administrator should identify which traffic needs to be
prioritized based on source/destination IP, port number, and so on, just like a regular ACL. Next, the traffic must
match either the incoming CoS or DSCP value before it is sent to the desired priority queue.
By default, on all Brocade switches, all untagged traffic is placed in the best-effort queue (QoS p0), which is the
lowest priority. If the packet is tagged, it is queued according to the CoS value in the 802.1p header. In order to
honor DSCP values, which Microsoft Lync Server 2010 uses, the endpoint (clients and servers) ports need to be
tagged. Given that voice and video are susceptible to latency, packet loss, and jitter, assigning a higher priority is
critical in maintaining your SLAs. However, it is not sufficient merely to set QoS on the switch to which the laptop,
desktop, or IP phone is connected. QoS needs to be configured throughout the network to allow end-to-end QoS.
Microsoft uses DSCP values for voice and video, allowing network administrators to assign different priorities for
each type of traffic. By default, Microsoft Lync Server 2010 uses DSCP 40 for the
IP QoS value and 0 for 802.1p voice. A value of 0 means DSCP is disabled and will automatically be placed in
queue0, which is best effort. That means that for networks honoring the DSCP marking, voice is prioritized higher
than video by default. By referring to Figure 6, a DSCP value of 40 is placed into the Brocade QoS queue5. Any
other traffic that is assigned a higher DSCP value, such as 56, is given precedence over voice. A network
administrator can easily change the priority of voice and video by changing the DSCP value within Group Policy or
802.1p priority marking.
Marking is the process of changing the packet’s QoS information (the 802.1p and DSCP information in a packet) for
the next hop. For example, for traffic coming from a device that does not support DiffServ, you can change the
packet’s IP Precedence value into a DSCP value before forwarding the packet. For example, with a simple switch
configuration you can have an endpoint that is marked with a DSCP value of 40 and assign it to a CoS of 7, which is
the highest priority. If you left the defaults, then the switch would follow the default CoS Map and assign it to a
hardware queue of 5.
Marking is optional and is disabled by default on the Brocade switches. Marking is performed using ACLs. When
marking is not used, the device still performs the mappings for scheduling the packet but leaves the packet’s QoS
values unchanged when the device forwards the packet.