One of the main uses of per-VLAN network egress shapers is to enable load balancing across dual uplinks out of a spoke site. Figure: VLAN Shapers for Dual Uplinks represents a typical hub-and-spoke mobile backhaul topology. To ensure high availability through the use of redundancy, a mobile operator invests in dual 7750 SR nodes at the MTSO. Dual 7750 SR nodes at the MTSO offer equipment protection, as well as protection against backhaul link failures.
In this example, the cell site 7705 SAR is dual-homed to 7750 SR_1 and SR_2 at the MTSO, using two disjoint Ethernet virtual connections (EVCs) leased from a transport provider. Typically, the EVCs have the same capacity and operate in an forwarding/standby manner. One of the EVCs—the 7750 SR—transports all the user/mobile traffic to and from the cell site at any given time. The other EVC transports only minor volumes of control plane traffic between network elements (the 7705 SAR and the 7750 SR). Leasing two EVCs with the same capacity and using only one of them actively wastes bandwidth and is expensive (the mobile operator pays for two EVCs with the same capacity).
Mobile operators with increasing volumes of mobile traffic look for ways to utilize both of the EVCs simultaneously, in an active/active manner. In this case, using per-VLAN shapers would ensure that each EVC is loaded up to the leased capacity. Without per-VLAN shapers, the 7705 SAR supports a single per-port shaper, which does not meet the active/active requirement.
If the egress rate is set to twice the EVC capacity, then either one of the EVCs can end up with more traffic than its capacity.
If the egress rate is set to the EVC capacity, then half of the available bandwidth can never be consumed, which is similar to the 7705 SAR having no per-VLAN egress shapers.
