3 Replies Latest reply: Jan 28, 2019 1:32 AM by Juan RSS

    ECMP resiliency and fast convergence


      Hi all,


      I have something I would like to clarify about ECMP.

      1. Is enabling ECMP considered a resiliency feature?

      2. Is using ECMP considered a fast convergence feature if used with a fast failure detection mechanism like LoS or BDF?




        • 1. Re: ECMP resiliency and fast convergence

          Hi Alex,


          ECMP is operating mode of the routing protocol. ECMP requires two routing entries in the RIB database.

          Resiliency is providing additional pathes to the destination.

          Fast convergence is enabled by precalculation of the additional path and upload the information in the FIB/RIB.

          Fast failure detection is used to speed up a routing calculation in the device. LoS or BFD helps you with this. The path in this case is not preloaded in the RIB/FIB.


          My recommendation is this session for your further studies: BRKRST-3363 from "Fish" <Routed Fast Convergence> in Cisco Live libary


          Please mark my feedback as "helpful" or just reply if you have any further questions.




          • 2. Re: ECMP resiliency and fast convergence

            I mean, ECMP is not a resiliency function in itself, but if you engineer your network to have ECMP like in a DC CLOS architecture, should not ECMP be considerd a "make before break" function as it installs addidtional routes in the FIB? It even uses all ECMP routes. You don't need a "classic" FRR function in your CLOS core because of ECMP.


            With regular FRR functionality the router still has to somehow switch to the pre-calculated backup path, but with ECMP all the paths are already installed and in use.


            My point being: ECMP makes your network converge faster than any other FRR functionality (as long as you have multiple equal paths to all your destinations), so should not having ECMP be considered a fast convergence/resiliency feature?

            • 3. Re: ECMP resiliency and fast convergence

              Hi UDA


              You described pretty well the ECMP and FRR mechanisms.


              ECMP and FRR have some significant differences that need to be spotted to select the technology that better suits your application requirements. FRR is a reroute mechanism, fast is the key. The alternative route is calculated in advance and ready to be used 'almost' immediately. You have several options depending on your environment (SP vs Enterprise):

              • EIGRP FS
              • OSPF LFA
              • MPLS TE


              With ECMP you just can avoid the convergence event. You have both paths available, if one path fails, you simply use the remaining ones. ECMP should not be considered a fast convergence feature, but increases resiliency of your network.


              Have a nice day.