Implementing QoS with MPLS is like building a circuit-switched network that can create end-to-end circuits over any type of Layer 2 transport medium. In an MPLS network, a LER (Label Edge Router) assigns incoming packets a "label." Packets then travel along an LSP (Label Switch Path) that makes forwarding decisions based on the label's contents. At each hop along the path, an LSR (Label Switch Router) strips off the existing label and applies one with new forwarding instructions. Network operators establish LSPs to route around network congestion or create IP tunnels for network-based VPNs.
MPLS standards are still advancing through the IETF, and implementations have evolved differently, depending on the vendor. For example, Cisco Systems implements MPLS as Tag Switching. If you standardize on routers and switches from Cisco or another vendor that supports MPLS, your voice and video traffic will get priority without RSVP's heavy overhead. In addition, some private network providers, such as Masergy Communications, employ MPLS to move converged traffic to your remote offices.
If your enterprise is not a Cisco or Extreme Networks shop and you have heterogeneous networking equipment, you can use a QoS option such as DiffServ. Layer 3 switches (ASIC-based routers) can now perform route lookups at sufficient speeds to obviate label switching. Rather than tag traffic the way MPLS does, DiffServ modifies bits in the IP header to indicate QoS. DiffServ-compliant switches, such as Extreme Networks' Summit and BlackDiamond devices, read the value stored in the IPv4 packet header's ToS (Type of Service) or IPv6 packet's Traffic Class octet.
The value or DSCP (Differentiated Services Code Point) is 6 bits wide and capable of 64 classifications. The default code point (000000) represents a common IP packet and maintains IP's best effort in forwarding. Code points for preferential treatment (11x000) are given priority queuing and can be forwarded using a variety of mechanisms (such as strict priority queuing, weighted fair queuing and class-based queuing).
Although DiffServ may appear to be the QoS of choice for large enterprises, you need to test this solution in your environment. For example, vendors may use different forwarding mechanisms for the same DSCP. Other environments with Gigabit Ethernet backbones may have little need for QoS on the LAN, but may need to prioritize traffic over a saturated T1 WAN link. In that case, both a QoS strategy and a packet shaper, such as Packeteer's PacketShaper, may provide the requisite assurances that voice and video traffic will receive priority throughout the enterprise.
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