diff --git a/docs/manpages/startup-config.5 b/docs/manpages/startup-config.5
index e1babbd..62002d1 100644
--- a/docs/manpages/startup-config.5
+++ b/docs/manpages/startup-config.5
@@ -418,6 +418,14 @@ Each throttled session requires two buckets (in and out).
 Determines whether or not to enable kernel acceleration.
 Note that only one l2tpns instance can use it per network namespace,
 otherwise they will step on each other.
+Also, if you have a lot of clients (e.g.\ at least a hundred), listening
+for DHCPv6 and RS requires a lot of igmp6 subscriptions, tuning sysctl
+may be needed, such as
+.PP
+sysctl net.core.optmem_max=1048576
+.PP
+otherwise the logs will mention failures to subscribe due to lack of
+memory.
 .SS DHCPv6 And IPv6 SETTINGS
 .PP
 \f[B]dhcp6_preferred_lifetime\f[R] (int)
diff --git a/docs/src/man/startup-config.5.md b/docs/src/man/startup-config.5.md
index 60176e3..1ca67a2 100644
--- a/docs/src/man/startup-config.5.md
+++ b/docs/src/man/startup-config.5.md
@@ -253,7 +253,11 @@ The following `variables` may be set:
 
 **kernel\_accel** (boolean)
 
-  Determines whether or not to enable kernel acceleration. Note that only one l2tpns instance can use it per network namespace, otherwise they will step on each other.
+  Determines whether or not to enable kernel acceleration. Note that only one l2tpns instance can use it per network namespace, otherwise they will step on each other. Also, if you have a lot of clients (e.g. at least a hundred), listening for DHCPv6 and RS requires a lot of igmp6 subscriptions, tuning sysctl may be needed, such as
+
+sysctl net.core.optmem\_max=1048576
+
+otherwise the logs will mention failures to subscribe due to lack of memory.
 
 ## DHCPv6 And IPv6 SETTINGS