Path MTU Discovery for IPv6 over GRE

IPv6 GRE tunnel PMTU uses the PMTU mechanism of the underlying IPv6 layer of the Junos stack. The underlying IPv6 PMTU discovery timeout has a default value of 10 minutes and is configurable; using the ipv6-path-mtu-discovery-timeout command option. The GRE tunnel layer PMTU discovery timeout has a fixed value of 13 minutes. When an IPv6 GRE interface receives an ICMPv6 Packet Too Big message, the MTU of the interface is reduced to either the MTU value in the message or to 1336 (IPv6 minimum link MTU and tunnel overheads), whichever is higher. A 13-minute timer is started at the tunnel layer. Null-route filtering can occur if the MTU of a link between tunnel endpoints falls to a value less than 1336. After tunnel timeout expiry, a check is made whether the IPv6 PMTU timeout has already occurred. If it has, the tunnel MTU reverts to the default value. If not, a new timerfor13 minutes is started for a subsequent check.