Varnish grace period for when server is down

multiple_backends.vcl

vcl 4.0;

import directors;

backend server_1 {
    .host = "172.30.0.125";
    .port = "81";
}

backend server_2 {
    .host = "172.30.1.178";
    .port = "81";
}

sub vcl_init {
    new balancer = directors.round_robin();
    balancer.add_backend(server_1);
    balancer.add_backend(server_2);
}

sub vcl_recv {
   set req.backend_hint = balancer.backend();
}

sub vcl_backend_response {
     if (beresp.status >= 500) {
     #if (beresp.status >= 500 && bereq.is_bgfetch) {
          return (abandon);
     }

     set beresp.grace = 24h;

     return (deliver);
}

sub vcl_deliver {
    if (obj.hits > 0) { # Add debug header to see if it's a HIT/MISS and the number of hits, disable when not needed
      set resp.http.X-Cache = "HIT";
    } else {
      set resp.http.X-Cache = "MISS";
    }
    return (deliver);
}

single_backend.vcl

vcl 4.0;

# Default backend definition. Set this to point to your content server.
backend default {
    .host = "<IP or DNS>";
    .port = "<PORT>";
}

sub vcl_backend_response {
   	 if (beresp.status >= 500) {
     #if (beresp.status >= 500 && bereq.is_bgfetch) {
          return (abandon);
     }

     set beresp.grace = 24h;

     return (deliver);
}

sub vcl_deliver {
    if (obj.hits > 0) { # Add debug header to see if it's a HIT/MISS and the number of hits, disable when not needed
      set resp.http.X-Cache = "HIT";
    } else {
      set resp.http.X-Cache = "MISS";
    }
    return (deliver);
}