chart: comparing HP EVI with Cisco OTV and VPLS


HP EVI is a MAC-over-GRE-over-IP solution. Ethernet frames are encapsulated into GRE/IP at ingress to the switch. The GRE/IP packets are then routed over the WAN connection between the data centers.

EVI adds a software process to act as control plane to distribute the MAC addresses in each VLAN between the EVI-enabled switch. Thus, the switch in data center A updates the MAC address table in data center B and vice versa. By contrast, in traditional use, Ethernet MAC addresses are auto-discovered as frames are received by the switch.

Because HP has chosen to use point-to-point GRE, the EVI edge switch must perform packet replication. Ethernet protocols such as ARP rely heavily on broadcasts to function. In a two-site network this isn’t problem, but for three sites or more, the EVI ingress switch needs to replicate a broadcast EVI frame to every site. HP assures me that this can be performed at line rate, for any speed, for any number of data centers. That may be so, but creating full mesh replication for n* (n-1) WAN circuits could result in poor bandwidth utilization in networks that have high volumes of Ethernet broadcasts.

Cisco’s OTV is also MAC-over-GRE-over-IP (using EoMPLS headers), but it adds a small OTV label into the IP header. The OTV control plane acts to propagate the MAC address routing table.

Like HP’s EVI, OTV can complicate load balancing. Cisco’s Virtual Port Channel (vPC) shares the control plane, while HP’s IRF shares the data plane. Although a vPC-enabled pair of Nexus 7000 switches run as autonomous control planes, NX-OS can load balance evenly using IP. OTV load balances by using a 5-tuple hash and will distribute traffic over multiple paths for the WAN.

OTV also supports the use of multicast routing in the WAN to deliver a much more efficient replication of Ethernet broadcasts in large-scale environments. Instead of meshing a large DCI core, a Source Specific Multicast (with good reasons) should be more efficient for multiple sites. Badly designed applications, such as Microsoft NLB, will be much more efficient using multicast.

For many enterprises, MPLS is not a consideration. MPLS is a relatively complex group of protocols that requires a fair amount of time to learn and comprehend. However, building mission-critical business services that aren’t MPLS is really hard. Service providers can offer L2 DCI using their MPLS networks with VPLS. Operationally, enterprise infrastructure is diverse and customised to each use case. Service provider networks tend toward homogeneity and simplicity because of the scale.

Some enterprises will buy managed VPLS services from service providers. They will also discover that such VPLS services are of variable quality, offer poor loop prevention, and can be expensive and inefficient. (For more, see the above-referenced report.) This is what drives Cisco and HP to deliver better options in OTV and EVI.


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