Understanding the Impact of vPC Failures
Understanding the Impact of vPC Failures
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Before going to talk about the vPC failure scenarios, lets talk about the vPC in a brief and where we are using vPC in the Datacenter environment
Virtual Port Channel, or vPC for short, is a feature of Nexus switches that lets links physically linked to two different Nexus devices appear to a third device as a single port channel.
vPC offers various advantages, including higher bandwidth, load balancing, and redundancy.
⭐ vPC Failures Scenarios 👇
vPC Failure Scenarios occur when a vPC peer connection, vPC keepalive link, or one of the vPC peer switches fails, thereby causing disruption or inconsistency in the vPC domain. There are various kinds of vPC Failure Scenarios.
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| Fig 1.1- Understanding the Impact of vPC Failures |
Here are the different scenarios and what will be impacted you can get to know one by one
- vPC member port failure
- vPC peer-link failure
- vPC peer-keepalive link failure
- vPC primary switch failure
- vPC keepalive-link failure & later peer-link failure
- vPC peer-link and keepalive both fail but only keepalive returns
⭐ Scenario no 1: vPC member port failure 👇
When one vPC member port fails, the host MAC detects a link failure on one of the port channel members and redistributes the affected flows to the remaining port channel members. Prior to the failure, the MAC pointed to the primary port; after the failure, it points to the secondary port. This is one of the circumstances in which a vPC peer-link is utilized to transmit data traffic.
⭐ Scenario no 2: vPC peer-link failure 👇
Based on the configured role priority for the switch, one vPC peer switch is designated as the vPC primary switch and the other as the vPC secondary switch in a vPC topology. In a scenario where the vPC peer-link goes down, the vPC secondary switch shuts down all of its vPC member ports if it can still receive keepalive messages from the vPC primary switch (which indicates that the vPC primary switch is still alive). The vPC primary switch keeps all of its interfaces up.
⭐ Scenario no 3: vPC peer-keepalive link failure 👇
A vPC peer-keepalive link is a connection between two vPC peer switches that transports heartbeat messages. Failure of the vPC peer-keepalive link has no effect on vPC functionality or data forwarding. However, it is suggested that the keepalive link be repaired as quickly as possible in order to avoid a double failure scenario that could disrupt data transit.
If the vPC peer-keepalive link fails, the vPC peer-link will act as a backup keepalive link temporarily, using the CFS protocol to exchange information and know the presence of the peer switches. As a result, unless end users examine the vPC status or logs, they will be completely unaware of this failure.
⭐ Scenario no 4: vPC primary switch failure 👇
When the switch identified as the primary in the vPC domain fails or becomes inaccessible, this is referred to as a vPC primary switch failure. The consequences of this failure is determined by whether or not the vPC peer link and vPC keepalive link are still working. If both the vPC peer link and the vPC keepalive link are operational, the vPC secondary switch will notice the failure of the primary switch and take over as the operative primary.
All vPC member ports and orphan ports will continue to be forwarded by the vPC secondary switch. There will be no disruption or inconsistency in the vPC domain for end users.
⭐ Scenario no 5: vPC keepalive-link failure & later peer-link failure 👇
If the vPC keepalive connection fails first, followed by a peer-link failure, the vPC primary switch remains primary, but the vPC secondary switch becomes the operational primary switch, keeping its vPC member ports operational (this is also known as a dual active scenario). This can happen when both vPC switches are in good working condition but the failure happened due to a connectivity issue between the switches. This is referred to as a split-brain scenario.
Existing flows are not affected, however new flows may be affected because the peer-link is unavailable. Because the two vPC switches cannot synchronize the unicast MAC address and the IGMP groups, the whole unicast and multicast forwarding table cannot be maintained. There may also be some redundant packet forwarding.
⭐ Scenario no 6: vPC peer-link, keepalive fails but keepalive returns 👇
When both the vPC peer link and the vPC keepalive link fail, the vPC switches enter split-brain mode, with each switch acting as the operational primary and keeping its vPC member ports operational. In the vPC domain, this can result in traffic loops, duplicate frames, and inconsistent forwarding decisions.
When the vPC keepalive link returns, the vPC switches exchange heartbeat messages and detect the failure of the peer link. To avoid a split-brain scenario, the vPC secondary switch suspends all of its vPC member ports. All vPC member ports and orphan ports continue to be forwarded by the vPC primary switch.
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