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Cisco MDS 9000 FSPF Link Cost Multiplier: Enhancing Path Selection on High-Speed Storage Networks

The necessity for optimum path choice

When embarking on a journey from one location to a different one, we at all times attempt to decide the very best path to observe. This occurs in response to some choice standards. Having a single route is the best scenario however that will result in lengthy delays in case of any drawback occurring alongside the best way. Availability of a number of paths to vacation spot is sweet by way of reliability. After all, we would wish an optimum path choice instrument and clear avenue indicators, or a navigation system with GPS, to keep away from loops or getting misplaced.

In the same manner, information networks are designed with a number of paths to vacation spot for larger availability. Particular protocols allow optimum path choice and loop avoidance. Ethernet networks have used the Spanning Tree Protocol or newer requirements like TRILL. IP networks depend on routing protocols like BGP, OSPF and RIP to find out the very best end-to-end path. Fibre Channel materials have their very own commonplace routing protocol, referred to as Cloth Shortest Path First (FSPF), outlined by INCITS T11 FC-SW-8.


FSPF on Cisco MDS switches

The FSPF protocol is enabled by default on all Cisco Fibre Channel switches. Usually you do not want to configure any FSPF parameters. FSPF mechanically calculates the very best path between any two switches in a material. It could possibly additionally choose an alternate path within the occasion of the failure of a given hyperlink. FSPF regulates visitors routing regardless of how complicated the material is likely to be, together with twin datacenter core-edge designs.

FSPF helps multipath routing and bases path standing on a hyperlink state protocol referred to as Shortest Path First. It runs on E ports or TE ports, offering a loop free topology. Routing occurs hop by hop, based mostly solely on the vacation spot area ID. FSPF makes use of a topology database to maintain monitor of the state of the hyperlinks on all switches within the cloth and associates a value with every hyperlink. It makes use of Dijkstra algorithm and ensures a quick reconvergence time in case of a topology change. Each VSAN runs its personal FSPF occasion. By combining VSAN and FSPF applied sciences, visitors engineering might be achieved on a material. One use case could be to pressure visitors for a VSAN on a particular ISL. Additionally, the usage of PortChannels as a substitute of particular person ISLs makes the implementation very environment friendly as fewer FSPF calculations are required.


FSPF hyperlink price calculation

FSPF protocol makes use of hyperlink prices to find out the shortest path in a material between a supply change and a vacation spot change. The protocol tracks the state of hyperlinks on all switches within the cloth and associates a value with every hyperlink in its database. Additionally, FSPF determines path price by including the prices of all of the ISLs in every path. Lastly, FSPF compares the price of numerous paths and chooses the trail with minimal price. If a number of paths exist with the identical minimal price, FSPF distributes the load amongst them.

You’ll be able to administratively set the associated fee related to an ISL hyperlink as an integer worth from 1 to 30000. Nonetheless, this operation will not be obligatory and usually FSPF will use its personal default mechanism for associating a value to all hyperlinks. That is specified inside INCITS T11 FC-SW-8 commonplace. Primarily, the hyperlink price is calculated based mostly on the velocity of the hyperlink occasions an administrative multiplier issue. By default, the worth of this multiplier is S=1. Virtually the hyperlink price is inversely proportional to its bandwidth. Therefore the default price for 1 Gbps hyperlinks is 1000, for two Gbps is 500, for 4 Gbps is 250, for 32 Gbps is 31 and so forth.


FSPF hyperlink price calculation challenges

It’s straightforward to understand that  high-speed hyperlinks introduce some challenges as a result of the hyperlink price computes smaller and smaller. This turns into a major subject when the full hyperlink bandwidth is over 128 Gbps. For these high-speed hyperlinks, the default hyperlink prices grow to be too much like each other and so resulting in inefficiencies.

The scenario will get even worse for logical hyperlinks. FSPF treats PortChannels as a single logical hyperlink between two switches. On Cisco MDS 9000 collection, a PortChannel can have a most of 16 member hyperlinks. With a number of bodily hyperlinks mixed right into a PortChannel, the mixture bandwidth scales upward and the logical hyperlink price reduces accordingly. Consequently, completely different paths might seem to have the identical price though they’ve a distinct member depend and completely different bandwidths. Path inefficiencies might happen when PortChannels with as little as 9 x 16 Gbps members are current. This results in poor path choice by FSPF. For instance, think about two different paths to identical vacation spot, one traversing a 9x16G PortChannel and one traversing a 10x16G PortChannel. Regardless of the 2 PortChannels have a distinct combination bandwidth, their hyperlink price would compute to the identical worth.


FSPF hyperlink price multiplier characteristic

To deal with the problem, for now and the longer term, Cisco MDS NX-OS 9.3(1) launch launched the FSPF hyperlink price multiplier characteristic. This new characteristic needs to be configured when parallel paths above the 128 Gbps threshold exist in a material. By doing so, FSPF can correctly distinguish larger bandwidth hyperlinks from each other and is ready to choose the very best path.

All switches in a material should use the identical FSPF hyperlink price multiplier worth. This fashion all of them use the identical foundation for path price calculations. This characteristic mechanically distributes the configured FSPF hyperlink price multiplier to all Cisco MDS switches within the cloth with Cisco NX-OS variations that help the characteristic. If any switches are current within the cloth that don’t help the characteristic, then the configuration fails and isn’t utilized to any switches. In any case switches settle for the brand new FSPF hyperlink price multiplier worth, a delay of 20 seconds happens earlier than being utilized. This ensures that each one switches apply the replace concurrently.

The brand new FSPF hyperlink price multiplier worth is S=20, versus 1 within the conventional implementation. With a easy change to at least one parameter, Cisco implementation retains utilizing the identical commonplace based mostly components as earlier than. With the brand new worth for this parameter, the FSPF hyperlink price computation will keep optimum even for PortChannels with 16 members of as much as 128 Gbps velocity.



Cisco is main the best way in supporting the design and implementation of high-speed Fibre Channel materials. The continued improvement of 128 Gbps serial interfaces by commonplace our bodies, mixed with the Cisco-only functionality to create logical hyperlinks with as much as 16 members, demanded for a change to the normal FSPF hyperlink price calculation. That’s why the MDS NX-OS 9.3(1) launch launched help for the brand new FSPF hyperlink price multiplier characteristic. Due to this, your SAN constructed with MDS 9000 units can optimally function at the moment and tomorrow, whereas sustaining FC-SW-8 compliance.

To be taught extra, overview these sources:

Cisco MDS 9000 NX-OS 9.3(1) launch notes

Cisco MDS 9000 NX-OS 9.x cloth configuration information


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