Allied Telesis X900-48FE-N, AT-9924T/4SP-A-20, AT-8948, AT-9924T-40 manual Ports and Recovery Times

Ports and Recovery Times

Ports and Recovery Times

In practice, recovery time in an EPSR ring is generally between 50 and 100ms. However, it depends on the port type, because this determines how long it takes for the port to report that it is down and send a Link-Down message.

The following ports report that they are down immediately or within a few milliseconds, which leads to an EPSR recovery time of 50 to 100ms:

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10/100M copper RJ-45 ports

tri-speed copper RJ-45 ports operating at 10 or 100M

fiber 1000M ports

10G ports

However, for tri-speed copper RJ-45 ports operating at 1000M, there is a short delay— either 350ms or 750ms—before the port reports that it is down. This is because the IEEE standard for 1000BASE-T specifies that a port must wait for a certain length of time after a link goes down before it decides that the link is actually down (see Section 40.4.5.2 of IEEE Std 802.3-2002). The length of the wait depends on whether the 1000BASE-T port is “master” or “slave” end of the link (“master” and “slave” are determined when the port autonegotiates and are not related to the master node of EPSR). If a 1000BASE-T port is the master the wait is 750ms; if it is the slave, the wait is 350ms.

This means that if a 1000M copper link goes down between two transit nodes, EPSR recovers after approximately 350ms. The EPSR nodes at both ends of the broken link send a Link-Down message when they detect that the link has gone down. As the diagram shows, the node at the slave end of the link sends a Link-Down message in 350ms. The node at the master end does not send a Link-Down message until 750ms have passed, but by then the EPSR master node has already handled the first Link- Down message. You can see the messages in the debugging output in "Link Down Between Two Transit Nodes" on page 47.

		Master	1	Link-Down
		Node
				after 350ms
		Transit
	Transit	Node
	Node		slave end
			of link
2	Link-Down	Transit
		Node
	after 750ms	master end
		of link
				epsr-copper

For almost all networks, this slight delay in recovery has no practical effect. For networks with extremely stringent failover requirements, we recommend using fiber 1000M ports instead of copper.

Page 30 AlliedWare™ OS How To Note: EPSR