Implementing sub 50msec convergence in NW

BANGALORE, INDIA: Industry default standard in building-wide LAN, campus wide network, MAN is Spanning tree. Spanning tree is enabled by default. When Ethernet switches are interconnected, even more than one link connected between two switches or multiple switches interconnected forming Layer 2 loop.

For Spanning tree the alternate path is in blocking mode. The failover time for Blocked port to go into forwarding mode takes 50 sec.

Hello time = 2 seconds, Max Age = 20 seconds, Listening = 15 seconds, Learning =

15 seconds = 52 seconds of convergence

Whereas, rapid spanning tree takes near 20 seconds of convergence

Further fine-turning the parameters can reduce the convergence time. Still this will face the downtime which may not be acceptable by mission critical business applications.

Fastest failover can be achieved on network using Ethernet Protection Switching Ring (EPSR)

EPSR operates on physical rings of switches (not on meshed networks). When all nodes and links in the rings are up, EPSR prevents a loop by blocking data transmission across one port.

When a node or link fails, EPSR detects the failure rapidly and responds by unblocking the blocked port so that the data can flow around the ring.

Following diagram shows a basic ring with all the switches in the ring architecture.

In EPSR, each ring of switches forms an EPSR domain. One of the domain's switches is the master node and the others are transit nodes. Each node connects to the ring via two ports.

One or more data VLANs sends data around the ring and a control VLAN sends EPSR messages.

A physical ring can have more than one EPSR domain, but each domain operates as a separate logical group of VLANs and has its own control VLAN and master node.

On the master node, one port is the primary port and the other is the secondary port. When all the nodes in the ring are up, EPSR prevents loops by blocking the data VLAN on the secondary port.

The master node does not need to block any port on the control VLAN because loops never form on the control VLAN. This is because the master node never forwards any EPSR messages that it receives.

EPSR Components are as follow:

- EPSR domain: A protection scheme for an Ethernet ring that consists of one or more data VLANs and a control VLAN.

- Master node: The controlling node for a domain, responsible for polling the ring state, collecting error messages, and controlling the flow of traffic in the domain.

- Transit node: Other nodes in the domain.

- Ring port: A port that connects the node to the ring. On the master node, each ring port is either the primary port or the secondary port. On transit nodes, ring ports do not have roles.

- Primary port: A ring port on the master node. This port determines the direction of the traffic flow, and is always operational.

- Secondary port: A second ring port on the master node. This port remains active, but blocks all protected VLANs from operating unless the ring fails. Similar to the blocking port in an STP/RSTP instance.

- Control VLAN: The VLAN over which all control messages are sent and received. EPSR never blocks this VLAN.

- Data VLAN: A VLAN that needs to be protected from loops. Each EPSR domain has one or more data VLANs.

The author is country manager, India & SAARC, Allied Telesis.