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Ask the Expert: FSPF Concepts and Troubleshooting in Cisco SAN Environments

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Feb 28th, 2014
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            Read the bioWith Upinder Sujlana

Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to learn and ask questions about FSPF, VSAN interaction, load balancing, and troubleshooting with Upinder Sujlana.

According to the FC-SW-2 standard, Fabric Shortest Path First (FSPF) is a link state path selection protocol. FSPF keeps track of the links on all switches in the fabric and associates a cost with each link. FSPF tracks the state of links on all switches in the fabric, associates a cost with each link in its database, and then chooses the path with a minimal cost. The cost associated with an interface can be administratively changed to implement the FSPF route selection. Upinder will discuss Cisco's implementation of FSPF.

Upinder Sujlana is a customer support engineer for Cisco's SAN TAC team based in San Jose, CA. He has worked in the TAC for the past five years with a focus on WAN technologies (L2TP, T1, T3, SCE 2K, 8K) and data center technologies such as MDS; Cisco Nexus 7000, 5000, and 2000; FCoE; and FC. Prior to joining the TAC, Upinder was a Java client-side programmer for an NMS startup company and then transitioned to network testing for a cloud company. He holds a master's degree in electrical engineering from Santa Clara University and has CCIE certification (no. 37318) in routing and switching. These days he is enthusiastic about Python programming. 

Remember to use the rating system to let Upinder know if you have received an adequate response. 

Upinder might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation in Data Center community,  sub-community, Storage Networking   discussion forum shortly after the event. This event lasts through March 14, 2014. Visit this forum often to view responses to your questions and the questions of other community members.

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egordon310 Mon, 03/03/2014 - 16:59
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Hi Upinder,

Really appreciate you covering this topic.  So my question is what's the load-balacing mechanism used in FSPF ?  Thanks and I look forward to your answer.


usujlana Tue, 03/04/2014 - 10:26
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Hi Evan,

Please note that in MDS for FSPF uses load-balancing algorithms based on source FC ID, destination FC ID, and exchange ID which are enforced in hardware. Further please note that :

> FSPF supports load balancing of traffic between equal cost paths

> Lowest 16 equal cost next hop interfaces  programmed for a domain

> Another level of load balancing within members of port channel

Below document is good :


Upinder Sujlana

egordon310 Wed, 03/05/2014 - 15:58
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Thanks for your answer Upinder.  Really appreciate it.  Can you also tell me how I can find out why my traffic is using one link over another? 


usujlana Wed, 03/05/2014 - 16:08
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Hi Evan,

You can use my favorite command as below to find out the cost and check what path traffic will take. Here is a example :

switch1# show fspf internal route vsan 2

FSPF Unicast Routes 


 VSAN     Number          Dest Domain          Route Cost          Next hops


1                   0x01(1)                    1000                  fc1/2

1                   0xEF(239)                  1000                  fc1/1

1                   0xED(238)                  2000                  fc1/1


This shows the total cost of all links.

The next hop (238) has two interfaces. This indicates that both paths will be used during load sharing. Up to sixteen paths can be used by FSPF with a Cisco MDS 9000 Family switch.



tenaro.gusatu.novici Fri, 03/07/2014 - 04:57
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Hi Upinder,

let me mention first that I don't know anything about FSPF. Actually, this is the very first time I've seen this abbreviation. Even more, I was pretty sure that there are no routing protocols in the Fiber Channel world. Now, when you have some idea about my level of knowledge, can you please try to explain in few simple words what is the benefit of using FSPF, why (and when) it was created and is it by any chance important in UCS server world.

BTW, I also tried to dig up some info on my own but only useful link (that one you already posted) doesn't display any pictures:



usujlana Fri, 03/07/2014 - 13:44
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Hi Tenaro,

The link you mentioned is a good one.

Wikipedia has a great and simplest definition around :

Fabric Shortest Path First (FSPF) is a routing protocol used in Fibre Channel networks.

It calculates the best path between switches, establishes routes across the fabric and calculates

alternate routes in event of a failure or topology change. FSPF can guarantee in-sequence delivery

of frames, even if the routing topology has changed during a failure, by enforcing a 'hold down' time before a new path is activated.

Highlights of the Cisco FSPF implementation are :

•Fiber channel standard defined in FC-SW2

•Supports multipath routing.

•Bases path status on a link state protocol.

.Computes routes based on domain ID – a domain ID identifies a switch

•Routes hop by hop, based only on the domain ID.

•Runs only on E ports or TE ports and provides a loop free topology.

•Runs on a per VSAN basis. Connectivity in a given VSAN in a fabric is guaranteed only for the switches configured in that VSAN.

•Uses a topology database to keep track of the state of the links on all switches in the fabric and associates a cost with each link.

•Guarantees a fast reconvergence time in case of a topology change. Uses the standard Dijkstra's algorithm, but there is a static

dynamic option for a more robust, efficient, and incremental Dijkstra's algorithm. The reconvergence time is fast and efficient

as the route computation is done on a per VSAN basis.




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