Technical Tip: Timers used for speedup Convergence, Failover and Recovery in ADVPN/SD-WAN setup

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Description	This article describes that in a complex ADVPN environment with SD-WAN enabled, multiple links are used to make sure the network is up and running at all times. During a failover and in a recovery process from one link to another, the time taken to normalize the traffic is significant, especially in a critical production environment.
Scope	In an ADVPN/SD-WAN deployment, the main 2 components that govern the setup are BGP and IPSEC. IPSEC corresponds to the overlay network and connectivity to its branches (spoke) and BGP is responsible for routing traffic from one location to another. BGP and IPSEC have their timers which can be adjusted to make failover and recovery of links quicker.
Solution	Below are the timers and their function which can be adjusted. On BGP: Spoke1 (bgp) # sh full config router bgp    set keepalive-timer 60    <----- Default values.    set holdtime-timer 180    <----- Default values. config neighbor    edit "172.17.201.1"       set advertisement-interval 30  <-----  Default values.      set connect-timer 4294967295    <-----  Default values      set link-down-failover disable  <-----  Default values. Keepalive and hold timers K77441326: Overview of BGP timers Advertisement interval K77441326: Overview of BGP timers  Connect timer How long in seconds the FortiGate will try to reach this neighbor before declaring it offline. link-down-failover enable By default, if an overlay link goes down, it will wait until BGP timers to show the neighbor ship is down. with this enabled it can make the BGP peer down immediately by enabling this under the neighbor. On IPSEC: Spoke1 (phase1-interface) # sh full config vpn ipsec phase1-interface     edit "Spoke1-MPLS"        set dpd-retrycount 3 <----- Default values.        set dpd-retryinterval 20 <----- Default values. dpd-retrycount How often will the DPD be attempted. dpd-retryinterval How long is the interval in seconds after which a DPD will be attempted again. By reducing these timers it is possible to put down a tunnel and use a redundant tunnel. Note: When reducing these timers, be mindful not to make them too low, which might cause frequent flapping. Hence, make sure the reduced timers are not affecting the normal operation of IPsec tunnels and BGP peers.