BACKGROUND

The present disclosure relates to optimization of network routing for systems that include devices that provide multihoming in the Ethernet Virtual Private Network (EVPN) environment. In particular, multihoming redundant devices for an ethernet segment may store backup paths in a forwarding table for immediate use in case of ethernet segment link interruption.

SUMMARY

In some computer networks, network devices (e.g., routers, switches, etc.) are configured in multihoming topologies, where two or more network devices provide an active redundant connection to the same host. In an ethernet virtual private network (EVPN), the various direct connections between a multihomed host and the redundant network devices (e.g., Provider Edge devices) are referred to as an ethernet segment (ES) and is assigned an ethernet segment identifier (ESI).

The redundant network devices may be configured in an ‘all-active’ configuration or a ‘single-active’ configuration. In the ‘all-active’ configuration any one of the redundant network devices may send and receive traffic in for the ES in its EVPN Instance (EVI). On the other hand, in ‘single-active’ active configuration, only a single redundant network device (that is marked as ‘active’) is allowed to send and receive traffic in ES in its EVI. The ‘active’ redundant device is selected using a deterministic Designated Forwarder (DF) election algorithm that may be performed in the EVI (e.g., cooperatively by the redundant network devices). The DF election algorithm may use as, an input, all received Ethernet ES routes (e.g., type-4 EVPN routes) for a particular ES. The DF election algorithm designates a single redundant network device as the ‘active’ device, and all other redundant network device as ‘standby’ devices.

When several redundant network devices in a ‘single-active’ configuration provide multihoming to a host, they will advertise EVPN auto discovery (AD) routes for the ES to remote network devices (e.g., Provider Edges) that are not connected to the ES of the redundant network devices. Using the data of the EVPN AD routes, the remote network device will learn that only one of the several redundant network devices is ‘active.’ When the ‘active’ redundant network device advertises an access to a host on its ES (e.g., using type-2 MAC/IP Advertisement Route) to the remote network device, that remote network device will configure its forwarding table to forward traffic to the host via the ‘active’ redundant network device (e.g., by setting the address of the ‘active’ redundant network device as ‘active’ in a nexthop group associated with the host).

If the currently ‘active’ redundant network device loses its connection to ES, it may withdraw the EVPN AD route advertisement from the remote network device. In some embodiments, if there were only two redundant network devices connected to the ES, the remote network device may immediately switch to using the other redundant network devices, when the ‘active’ redundant network device fails. For example, the remote network device may set the address of the second redundant network device as ‘active’ in a nexthop group associated with the host. This is possible because if one of the two redundant network device failed, there is only one option for which device will become the next Designated Forwarder.

However, if there were more than two redundant network devices connected to the ES, the remote device is unable to perform the quick switch because it has not information about which of the surviving devices will be the next Designated Forwarder after the failure of the ‘active’ redundant network device. Because of this, the remote device will have to purge its forwarding table of all the addresses learned on the ES, which will result in traffic disruption until the addresses are re-learnt. For example, after the DF election algorithm is re-run again by the surviving redundant network devices to identify the new DF, the new active redundant network device will re-advertise all addresses of the multihomed hosts to the remote host. Such an event will significantly slow down the network traffic.

To overcome this problem, techniques are presented for allowing the use of ‘single-active’ active configuration for large groups (e.g., groups with more than two devices) of redundant network devices for an ES without the loss of fast-switch over capability by a remote network device (e.g., a device not directly connected to the ES).

To solve the fast-switch problem, a remote network device may be configured to no longer reject ES routes (e.g., EVPN type-4 route) that are received for an ES that the remote network device is not connected to (as is the conventional practice). Instead, the remote network device is configured to store such ES routes for the remote ES it its local memory. If the remote network device becomes aware of a loss of a multihoming link by a currently ‘active’ device of a set of redundant networking devices that provide multihoming to host via that remote ES (e.g., when the remote device receive a withdrawal of EVPN AD route advertisement), the remote network device may use the data from the stored ES routes (e.g., EVPN type-4 route) to identify which of the surviving redundant networking devices, that are connected to a remote ES, will become designated as the next Designated Forwarder for that ES.

In particular, the remote network device may run its own instance of the DF election algorithm while the same DF election algorithm is performed by the surviving redundant networking devices. Because the DF election algorithm is deterministic, the remote devices and the surviving redundant networking devices will identify the same single redundant networking device as the next Designated Forwarder for that ES. Accordingly, when the loss of the ES link occurs at the currently ‘active’ redundant networking device, the remote network device is able to accurately predict the next ‘active’ redundant networking device of the ES it's not connected to.

Since the next ‘active’ redundant networking device is predicted, the remote network device may quickly adjust its forwarding table to indicate that the host multihomed on the ES is now reachable via the newly ‘active’ redundant networking device. For example, the remote network device may update the next hop group of each ES-connected host to indicate the address of the precited Designated Forwarder as ‘active.’

As a result, the remote network device is able to, in an event of an interruption, of an ES link of the Designated Forwarder device, to quickly switch traffic forwarding to the correct one of the surviving redundant networking devices even when the redundant networking devices serving the same ES include more than two devices in a ‘single-active’ active configuration.

