High Availability inside the Datacenter: In Leaf/Spine VXLAN based data centers, everyone likes to provide HA with Active/Active in it, so choices are different. There are two types of HA in data centers, Layer 3 and Layer 2. For layer 3 HA, always there is more than one spine that can provide ECMP and HA at same time. However, Layer 2 redundancy for hosts and l4-l7 services that connected to leafs are more than an easy choice. As Cisco provided vPC for nearly 10 years ago, almost this was the first (and only) choice of network engineers. Also, other vendors have their own technologies. For example, Arista provided Multi-chassis Link Aggregation (MLAG) for L2 HA in leafs. But, there is always a problem in implementation of them. One example in vPC is “peer-link” that is an important component in the vPC feature. However, it can be a tough one in most cases like Dynamic Layer-3 routing over vPC or Orphan members that may cause local traffic switching between vPC peers without using Fabric links. To address the “peer-link” issue, there is a “fabric-peering” solution that uses Fabric links instead of “peer-link” and convert it to “virtual peer-link”. With this solution there is no concern about local switching in specific cases.
This solution works better, but it cannot solve Dynamic Layer-3 routing over vPC or other issues (PIP, VIP, virtual-rmac) without enhancements. There is another HA solution that you can find it below.
Designated Forwarder
MAC Aliasing
MAC Mass Withdrawal
Regardless of which HA solution is better, there are some differences between HA solutions that some of them are listed below on Cisco technologies. You must keep in mind that this is Data Center comparison. Because vPC does not supported on Cisco routers but EVPN does.vPC/vPC2 | EVPN Multihoming | |
Hardware | All Nexus platforms | Nexus 9300 Only (until now) |
FEX supported | Yes | No |
Same OS version | Yes | Not mentioned |
Multiple components | Yes | No |
QoS needed | For vPC2(fabric peering) | No |
ISSU | Yes | No |
Maximum peers | 2 | 2+ |
In All active mode, each leaf can forward traffic. In this mode, load balancing method is per-flow. This is regular load balancing mechanism that share traffic between leafs as below figure. This method is better for providing more bandwidth based on end hosts.
Port Active is a mechanism that brings Active-Standby scenario to EVPN multi-homing and only one leaf forwards traffic. However, in failure scenarios with fast convergence, traffic will switch to standby leaf. This method is your choice when you want to force traffic on a specific link that is cheaper or you want to use only one link.
This is important that every EVPN feature in this post is not implemented on all platforms and vendors. To recapitulate, both solutions have cons and pros. Depend on Data Center design and requirement, you can choose one of solutions. Keep in mind that you can not enable both feature on a switch at a same time. Also, LACP is an additional tool to improve these features functionality and it helps avoiding misconfigurations. note: all figures are taken from ciscolive presentations
Orhan Ergun, CCIE/CCDE Trainer, Author of Many Networking Books, Network Design Advisor, and Cisco Champion 2019/2020/2021
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