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ECMP fast reroute: What problem does it solve?

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  • 1.  ECMP fast reroute: What problem does it solve?

    Posted 08-22-2017 07:44

    Hi everybody,


    I am trying to find  links  that helps me undersantd what problem ECMP fast reroute feature resolves before delving into ECMP fast reroute.


    I goodled it but found some Juniper techical links that do not describe the problem in detail.



    Thanks and have a nice day!!

  • 2.  RE: ECMP fast reroute: What problem does it solve?
    Best Answer

    Posted 08-22-2017 19:51

    Most of the information is available from multiple links, however sometimes Juniper does not complete dots with a summary statement. So here goes some base information for someone who is just reading the question and not researched any docs:
    An equal-cost multipath (ECMP) set is formed when the routing table contains multiple next-hop addresses for the same destination with equal cost. (Routes of equal cost have the same preference and metric values.) If there is an ECMP set for the active route, Junos OS uses a hash algorithm to choose one of the next-hop addresses in the ECMP set to install in the forwarding table.
    You can configure Junos OS so that multiple next-hop entries in an ECMP set are installed in the forwarding table. On Juniper Networks devices, per-flow load balancing can be performed to spread traffic across multiple paths between routing devices.
    Per-packet load balancing allows you to spread traffic across multiple equal-cost paths. By default, when a failure occurs in one or more paths, the hashing algorithm recalculates the next hop for all paths, typically resulting in the reordering of all flows. The reordering of all flows when a link fails potentially results in significant traffic loss or a loss of service to servers whose links remain active.
    "If a link goes down, ECMP uses fast reroute protection to shift packet forwarding to use operational links, thereby decreasing packet loss. Fast reroute protection updates ECMP sets for the interface without having to wait for the route table update process. When the next route table update occurs, a new ECMP set can be added with fewer links or the route might point to a single next hop. " set forwarding-table ecmp-fast-reroute

    Without fast, the ECMP sets will have to wait until the route is removed from the routing table, which then produces a new ECMP set which allows the hashing algorythm to recalculate the paths again. With the set forwarding-table ecmp-fast-reroute option enabled, this latency waiting for the routing table to update the ECMP set is eliminated.
    Consistent load balancing maintains all active links and instead remaps only those flows affected by one or more link failures. This feature ensures that flows connected to links that remain active continue uninterrupted.
    Starting in Junos OS 13.3R3, for MX Series 3D Universal Edge routers with modular port concentrators (MPCs) only, you can configure consistent load balancing, which prevents the reordering of all flows to active paths in an equal-cost multipath (ECMP) group when one or more next-hop paths fail. Only flows for paths that are inactive are redirected to another active next-hop path. Flows mapped to servers that remain active are maintained. This feature applies only to external BGP peers.

  • 3.  RE: ECMP fast reroute: What problem does it solve?

    Posted 08-24-2017 12:24

    Wow, not a single link in JUNOS  DOC that can explain this concept in just a few paragraphs the way you did!!


    It makes all the sense now.


    Thanks and have a nice day!!



  • 4.  RE: ECMP fast reroute: What problem does it solve?

    Posted 02-08-2021 09:02
    It  is not clear to me what the scope of this command is. 

    Does it apply to  multiple BGP PATHs  (i.e. when the BGP Multipath knob is on)  only ? Or it applies to IGP/LDP multipaths too ?   

    Andrea Di Donato

    Andrea Di Donato