What is the design purpose of FabricPath?

[Steve Zhou]

Hi all,

     Is FabricPath a Cisco implemention of TRILL? What's the difference between FabricPath and OTV? Looks like they are both technologies enabling L2 over L3.

     thanks for any inputs!

[Peter Paluch]

Hello Steve,

FabricPath is a Cisco's take on TRILL but the frame format used inside the FabricPath cloud and some of its internal workings is different from TRILL. However, Cisco claims interoperability of FabricPath and the TRILL, and actually tries to get its own FabricPath improvements into the TRILL standard. Both TRILL and FabricPath do not encapsulate Ethernet frames into IP packets, though - they are still working purely on Layer2 and do not involve IP for their own workings. Therefore, I would not call TRILL nor FabricPath to be L2 over L3. It is true that TRILL and FabricPath use the IS-IS routing protocol, however, they are using it to compute shortest paths between any pair of switches. However, both these technologies still constitute a single broadcast domain and there is no concept of hierarchical addressing typical for L3 routed technologies. I admit, though, that the boundary between L2 and L3 in TRILL and FabricPath is blurred.

OTV is different - OTV is basically MAC-in-IP. Once the Ethernet frame is encapsulated into IP packet, the packet is subject to normal routing etc. OTV would be more reminiscent of "L2 over L3".

Best regards,

Peter

[Steve Zhou]

Hello Peter,

Wonderful! Thanks a lot for your help. I would like to know more from an architecture or design purpose point of view. Could you help me on that? I will be very appreciated!

1. Both TRILL and FabricPath build larger L2 domain, why do we need a larger L2 domain? Why don't just leverage L3 fucntion, like IP? What's the challenge in today's data center networking that result in this requirement?

2. There are standards like VXLAN, NVGRE, STT that provide overlay networking to decouple IP addresses of virtual machines from the underlying hardware. Is OTV also designed to meet the same VM mobility requirement?

thank you!