++blog / ra / without default router

content/u/blog/ipv6-router-advertisement-without-default-router/contents.lr

@@ -0,0 +1,68 @@
+title: Announce IPv6 networks without becoming the defaulto router
+---
+pub_date: 2020-01-31
+---
+author: ungleich network team
+---
+twitter_handle: ungleich
+---
+_hidden: no
+---
+_discoverable: yes
+---
+abstract:
+You can also announce prefixes without becoming the default router
+---
+body:
+
+## TL;DR
+
+Use **AdvDefaultLifetime 0** in your radvd.conf
+
+## Background
+
+Sometimes you have multiple routers in a network and you don't want a
+router to become the default router for clients, but it should still
+announce an IPv6 prefix.
+
+## radvd
+
+Luckily radvd supports an option to notify the clients of this. From the manpage of radvd.conf(5):
+
+
+```
+       AdvDefaultLifetime seconds
+
+              The lifetime associated with the default router in units of seconds.   The
+              maximum  value  corresponds to 18.2 hours.  A lifetime of 0 indicates that
+              the router is not a default router and should not appear  on  the  default
+              router  list.  The router lifetime applies only to the router's usefulness
+              as a default router; it does not apply to information contained  in  other
+              message fields or options.  Options that need time limits for their infor‐
+              mation include their own lifetime fields.
+
+              Must be either zero or between MaxRtrAdvInterval and 9000 seconds.
+
+              Default: 3 * MaxRtrAdvInterval (Minimum 1 second).
+
+```
+
+## Sample configuration
+
+A configuration from one of our test sites looks as follows:
+
+```
+interface eth0
+{
+  AdvSendAdvert on;
+  MinRtrAdvInterval 3;
+  MaxRtrAdvInterval 5;
+
+  AdvDefaultLifetime 0;
+
+  prefix 2a0a:e5c1:111:10c::/64 { };
+};
+```
+
+Client in this test network assign themselves an additional IPv6
+address from this prefix.