Enterprise QoS Solution Reference Network Design Guide

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Branch Router LAN Edge QoS Design DSCP-to-CoS Remapping 4-8 Enterprise QoS Solution Reference Network Design Guide Chapter 4 Branch Router QoS Design In the ingress direction, the branch router might be required to perform classification and marking of branch-to-campus traffic. This might be because the branch switch lacks the capability to classify and mark traffic, or because the traffic consists of stateful flows that require NBAR for classification. NBAR classification also is required at branch LAN ingress edges to identify (and immediately drop) known worm traffic. Each of these types of polices is discussed in more detail in the following sections. DSCP-to-CoS remapping is optional. Newer Catalyst switches perform QoS based on internal DSCP values that are generated either by trusted DSCP markings or by trusted CoS markings (coupled with CoS-to-DSCP mappings). In the case of legacy switches at the branch that perform QoS strictly by preset CoS values, CoS might need to be remapped on the branch router’s LAN edge. Enhanced packet marking (Cisco IOS Release 12.2[13]T or higher) is the optimal tool for resetting CoS values because it uses a table. In this manner, a default DSCP-to-CoS mapping can be used without having to configure explicitly a class-based marking policy that matches every DiffServ class and performs a corresponding set cos function. In both cases, keep in mind that only Ethernet trunking protocols, such as 802.1Q, carry CoS information. Therefore, the policy works correctly only when applied to a trunked subinterface, not to a main Ethernet interface. Example 14-2 presents an enhanced packet marking DSCP-to-CoS configuration for a branch LAN edge. Note that this DSCP-to-CoS remapping policy requires application to both the voice VLAN (VVLAN) subinterface and the data VLAN (DVLAN) subinterface on the branch router’s LAN edge. Example 4-2 Branch LAN Edge Enhanced Packet Marking for DSCP-to-CoS Remapping Example ! ip cef ! IP CEF is Required for Packet Marking ! policy-map BRANCH-LAN-EDGE-OUT class class-default set cos dscp ! Enables default DSCP-to-CoS Mapping ! ! interface FastEthernet0/0 no ip address speed auto duplex auto ! interface FastEthernet0/0.60 description DVLAN SUBNET 10.1.60.0 encapsulation dot1Q 60 ip address 10.1.60.1 255.255.255.0 service-policy output BRANCH-LAN-EDGE-OUT ! Restores CoS for Data VLAN ! interface FastEthernet0/0.160 description VVLAN SUBNET 10.1.160.0 encapsulation dot1Q 160 ip address 10.1.160.1 255.255.255.0 service-policy output BRANCH-LAN-EDGE-OUT ! Restores CoS on Voice VLAN ! Verification commands: show policy Version 3.3
Chapter 4 Branch Router QoS Design Version 3.3 show policy interface Branch-to-Campus Classification and Marking Branch Router LAN Edge QoS Design In keeping with the unofficial Differentiated Services design principle of marking traffic as close to its source as possible, IP phones mark voice-bearer traffic (to DSCP EF) and Call-Signaling traffic (currently, to DSCP AF31, but this will soon change to DSCP CS3) on the phones themselves. Some IP/VC devices mark Interactive-Video traffic to AF41 on their network interface cards (NICs). However, as has already been discussed, it is generally not recommended that end-user PCs be trusted to set their CoS/DSCP markings correctly because users easily can abuse this (either unintentionally or deliberately). Note There may be some exceptions to this general rule, such as PCs running applications like Cisco VT Advantage in controlled environments. A network administrator must make these policy decisions. Therefore, application traffic that originates from untrusted hosts should be marked on branch access switches. However, in some circumstances, this might not be possible: The branch access switch does not have Layer 3 or Layer 4 awareness for traffic classification or does not support marking. Classification needs to be performed at the application layer (through NBAR). In such cases, DSCP classification must be performed at the ingress interface of the branch router. Administrators can identify and then mark application traffic based on the following criteria: Source or destination IP address (or subnet)—Typically, destination subnets are used when setting branch QoS policies. For example, the destination subnet for a group of application servers can be used to identify a particular type of client-to-server application traffic. Well-known TCP/UDP ports—It is important to know whether the well-known port is a source port or a destination port from the branch router’s perspective. NBAR protocol (for example, Citrix or KaZaa) or application subparameter (for example, HTTP URL)—NBAR Packet Description Language Modules (PDLMs) also can be configured to identify stateful or proprietary applications, as well as worms. Regardless of the method used to identify the application traffic, the inbound classification and marking policy needs to be applied only to the DVLAN subinterface. This is because only trusted IP Telephony applications, which mark their voice traffic correctly, are admitted onto the voice VLAN. Note If the branch access switches support access-edge policers (as described in Chapter 2, “Campus QoS Design”), these likewise should be enabled on them. This adds another layer of defense to the network, mitigating DoS/worm traffic that originates from the branch through Scavenger-class QoS. If the branch access switches do not support such policing, compatible policers could be placed at the branch router access edge. This is in harmony with the principle of policing as close to the source as possible. Whenever possible, though, such policing should be done in Catalyst hardware rather than Cisco IOS Software. Enterprise QoS Solution Reference Network Design Guide 4-9
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