Metro vendors question Spanning Tree standard
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A debate is brewing over whether a new standard will help boost the resiliency of Ethernet enough to enable metropolitan-area services to really take off.
While Ethernet-based metropolitan services have gained a following in pockets across the country, observers say the networks delivering these services need to be as reliable as circuit-switched SONET networks before large numbers of customers will move traffic to them.
The question is whether the recently ratified IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) is the answer to Ethernet's shortcomings. All parties seem to agree that RSTP offers a vast improvement on the 30 seconds it takes traditional Spanning Tree to help a failed Ethernet link recover. But they differ on the extent of that improvement - estimates run from 10 msec to a full second or more - and whether it's sufficient for guaranteeing service-level agreements (SLA).
Faster reconvergence
The consensus seems to be that RSTP is one component of several needed to make Ethernet as resilient as SONET, which recovers from link outages in 50 msec or less, in the metropolitan area.
"[Recovery] times all depend on how one implements the standard," says Tony Jeffree, chairman of the 802.1 working group and editor of the 802.1w document. "If one combines the technology with other Ethernet technologies - specifically the Link Aggregation Protocol - it's possible for service providers to guarantee SLAs where they were not traditionally able to get [tens of milliseconds] resiliency and recovery time."
Some metropolitan Ethernet equipment vendors disagree with Jeffree's assertion that RSTP can provide recovery in tens of milliseconds.
David Yates, director of marketing for Atrica, says RSTP's recovery time is closer to 1 second, too slow for carrier-grade service levels.
"Rapid Spanning Tree results in faster recovery times, but it's still not in the submillisecond category, which doesn't allow it to carry certain types of traffic or guarantee SLAs," he says.
Even in the LAN, users are not seeing recovery time in 1 second, let alone milliseconds.
"Fast Spanning Tree helps some, but it's still not perfect for some hosts," says Phil Kwan, associate director of network infrastructure for Incyte Genomics. "I'm also still not seeing failover times of 1 second yet. We're lucky if we see 10 seconds from boot up to traffic transport."
Ashwin Moranganti, a product manager at metropolitan network equipment maker Appian Communications, says RSTP lacks the fault-management capabilities that service providers get with SONET and is not designed for ring topologies, which continue to be widely deployed in metro networks.
"[Rapid] Spanning Tree is not the right algorithm to show that the problems associated with Ethernet have been solved," Moranganti says. "The technology still isn't viable in a ring environment and often, especially in the metro, it's hard to deploy a mesh network."
That's why the IEEE is defining another standard for increasing the resiliency of Ethernet networks - 802.17 Resilient Packet Ring (RPR). Just as its name implies, RPR is designed for ring topologies carrying packets, but with the same resiliency attributes of a typical SONET ring.
Still, some Ethernet service providers say RSTP is a better bet than RPR.
"Unlike RPR, which has little consensus on anything yet - let alone something approaching a stable draft [standard] - RSTP is [complete]," says Jay Gill, director of IP service products at Telseon, a metropolitan-area service provider. "With RSTP solving one of the perceived problems of switched Ethernet mesh architectures, RPR seems to have a somewhat more dubious case for adding any value. For example, if you don't need ring topologies for restoration, then the concept of [RPR] is irrelevant."
Telseon plans to deploy RSTP throughout its network by year-end. Ethernet service provider Yipes Communications is also implementing RSTP.
Some analysts say it may be premature for Ethernet service providers to rely on RSTP for restoration of their networks. RSTP may be unable to scale along with the network, says Marian Stasney, an analyst with The Yankee Group.
"It's just not there yet," Stasney says. "It's just one piece of the tool kit."
Security is another issue in Ethernet virtual LAN services that RSTP cannot solve, says Tracey Vinik, technical director at market research firm RHK.
"If you cross service provider boundaries with [virtual] LANs, there's no guarantee that your VLAN ID hasn't already been assigned to some other VLAN," Vinik says. "[Rapid] Spanning Tree can support multiple VLANs, fast reroutes and prevent loops, but the biggest issue is security. Service providers have to be careful of how they implement these VLANs, especially across service provider boundaries."
Companies cannot be assured that just because their Ethernet service provider supports RSTP, traffic will be safe. They must consider the totality of the service provisioning environment before deciding whether to subscribe to Ethernet services.
"No one technology . . . is going to make Ethernet viable in the [metropolitan-area network]," Telseon's Gill says. "It's a collection of things that must come together for us to build the kind of network with rapid fail-over times like those of [SONET]. It's also going to take a good implementation of these standards and a smart network design.
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