Ring Protection Protocol: ERPS vs. EAPS

Updated on Jul 25, 2019 by

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Used for ring topology, the ring protection protocol adopts the pervasion mechanism to transmit the change of network topology, which is critical to reduce interference between nodes and extend lifetime of the network. Ethernet ring protection switching (ERPS) and Ethernet automatic protection switching (EAPS) are two types of ring protection protocol. Here makes a detailed illustration of ERPS vs. EAPS.

What Is ERPS and How Does It Work?

ERPS (Ethernet ring protection switching), is a protocol defined by ITU-T under G.8032 Recommendation, because of which it is also named G.8032. This protocol can provide sub-50ms protection and recovery switching for Ethernet traffic in a ring topology and at the same time eliminating loops at the Ethernet layer 2.

Terms introduction before the ERPS working principle illustration.

RPL: it refers to ring protection link and normally, is blocked by the RPL owner node.

RPL owner: it checks the state of the ports of local Ethernet ring and sends out the notifications in case of link breakdown.

Ring node: each switch on an Ethernet ring is a ring node.

ERPS uses RPL to protect the whole ring. When the link fails, protection switching of the RPL traffic is triggered to block traffic on the failed link and unblocks the traffic on the RPL. Once the failure eliminates, revertive protection switching blocks traffic over the RPL and permits the traffic through the repaired link. The link of the ring topology gets back to its normal. In this way, links in the ring will never form loops that fatally affect the network operation and services availability. ERPS gets prevalent as there are various types of network switches that support ERPS. For example, FS S5850-48T4Q 10GBase-T switch supports ERPS to ensure the error-free Ethernet networking.

Figure1. ERPS Ring

What Is EAPS and How Does It Work?

EAPS (Ethernet automatic protection switching), invented by Extreme Networks and submitted to IETF as RFC3619, is used to create a fault tolerant topology by configuring a primary and secondary path for each VLAN.

Terms introduction before the EAPS working principle illustration.

Control VLAN: it transmits protocol packets.

Data VLAN: it transmits the packets of normal services or the management packets.

Master node: it checks the state of the ring’s topology, removes loopback, and controls other switches to update topology information.

Transit node: it checks the state of the local port of the ring, and notifies the master node of the invalid link.

Transit port: it can only be configured on the transit node. Both ports through which the transit node connects the ring network are all transit ports.

Under the Ethernet automatic protection switching, the master node is configured with a primary port and a secondary port. The master node transmits the ring detection packets via the control VLAN through the primary port. If the secondary port receives the packets, it verifies that the topology of the ring network is complete. In complete topology, all the transit ports in the ring are in a forwarding state transmitting tagged data traffic via data VLAN and EAPS control message via control VLAN. However, the secondary port on the master node is logically blocked to this tagged data traffic in order to prevent loops. When a link or node fault occurs between two nodes, they will immediately feedback to the master node via the control VLAN. Then the master node removes the blocking state of the secondary port to make the data traffic flow on the ring without interruption.

Figure2. EAPS Ring

ERPS vs. EAPS: Similarities and Differences

Both ERPS and EAPS are used in ring topology to protect the network. How do they differ from each other and what do they have in common?

Similarities of ERPS and EAPS

Both EAPS and ERPS can be revertive and let the ring converge back to its normal state after a broken link gets recovered. Additionally, they allow a physical or aggregation port to be set as a ring port. However, the physical port where link aggregation, 802.1X or port security has been configured cannot be set to be a ring port any more. Whether EAPS or ERPS, to configure a switch to belong to multiple rings, the switch must connect different rings through different physical ports.

Differences of ERPS and EAPS

First, with the automatic discovery mechanism, ERPS does not need to configure the node type manually, whereas the role of each node in EAPS should be specified by user through configuration.

Second, different from the master node of EAPS, the RPL owner will not check the completeness of Ethernet ring positively and continuously, nor control the MAC address ageing of other nodes.

Third, ERPS has no need to distinguish control VLAN and data VLAN as the EAPS does.

Fourth, ERPS allows for the user to set the ring in a broken state for maintenance and then recover the ring manually, which overshadows the EAPS.

Conclusion

Both ERPS and EAPS are used for loop avoidance by means of controlling the MAC table aging. They share something in common while preserve their own distinctions. FS S58/80 series support ERPS and multiple Metro Ethernet network features, ideally suited for high density and advanced data center. For more detailed information, contact sales@fs.com.

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