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Does FCoE really need TRILL?

Cisco says no - so?

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Comment Does Fibre Channel over Ethernet (FCoE) require TRILL (Transparent Interconnection of Lots of Links)? Brocade and NetApp say or imply it does. A distinguished engineer from Cisco says it doesn't.

The background to the issue is that FCoE is going to run over Ethernet networks and these run at layer 2 in the OSI 7-layer network reference model, lacking routing and multi-pathing; having more than one link between switches. Both of these issues are separate from the carrying of Fibre Channel frames inside Ethernet frames and having them cross the Ethernet network without frames being dropped, a showstopper for Fibre Channel fabric-using applications.

The routing is needed for multi-hop FCoE.

A Brocade FCoE handbook (pdf) states: "TRILL provides a Layer 2 (L2) multi-path alternative to the single path and network bandwidth limiting Spanning Tree Protocol (STP), currently deployed in data centre networks.

TRILL will also deliver L2 multi-hop routing capabilities, which are essential for expanding the deployment of DCB/FCoE solutions beyond access layer server I/O consolidation and into larger data centre networks."

A NetApp document (pdf) states: "TRILL is a new draft standard being created by IETF to accommodate FCoE converged networks and is scheduled to be completed later this year.

"The goal of TRILL is to create an L2 shortest path robust multipath and mult-ihop routing protocol that eventually will replace the limited and restricted L3 STP. The more resilient L2 will fulfill the needs of 10Gb Ethernet networks with virtualised ecosystem and data migration. TRILL will also enable multi-hop capabilities for FCoE environments."

The implied conclusion here is that TRILL is needed for multi-hop FCoE networks beyond access layer concentration.

Lincoln Dale, a Cisco distinguished engineer, says: “TRILL is not needed for FCoE; the reason is simple: Spanning Tree (STP) is NOT used for FCoE, at all, ever."

He adds: "From a NetApp perspective, TRILL is neither here nor there,  because TRILL is something that is done on a 'network' port (switch to switch), you still have 'classical Ethernet' behaviour on 'edge' ports. I think the paper basically says the same thing.

"The paper does talk about TRILL as being the 'replacement' for STP - I agree with that, but it implies that its necessary for 'multi-hop FCoE'.  That is not correct."

"There is nothing preventing multi-hop FCoE today.  it is part of the FC-BB-5 (FCoE) standard. What it requires are the 'unified I/O' switches to support it. There is no pre-requisite for STP - or TRILL - because FCoE is based on FC-BB-5 which uses all the FC [Fibre Channel] aspects to provide a switch and its services."

Interestingly Cisco is going to support TRILL with a software upgrade for its Nexus 7000 F-Series I/O module, saying: "It supports the Data Centre Bridging and TRILL standards with Fibre Channel over Ethernet (FCoE) to be enabled in the near future through a software upgrade." ®

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