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IPv6 Minimum MTU
The minimum MTU for IPv6 is 1280 bytes. You cannot enable IPv6 on an interface
with an MTU less than 1280 bytes.
IPv6 Mandatory Path MTU Discovery
Path MTU Discovery is a protocol that allows a host to determine the maximum size
packet that will successfully traverse a path to a destination without fragmentation.
As packets are generated and sent from the host, the MTU of the particular output
interface that the packet will be transmitted to is available.
If the packet will fit on the output interface, either as a whole or in fragments, it is
transmitted. If a router in the path needs to forward that packet onto a net with a
smaller MTU than the packet size, the packet will be dropped and an ICMP
message will be sent to the originator of the packet indicating the packet size that is
necessary to fit onto the output net of the intermediate router. The host receiving
this message will adjust the size of subsequent packets forwarded on the path. This
process may occur multiple times before the packet reaches its final destination.
Once the packet reaches its destination, subsequent packets should not be dropped
because their packet size being too large.
Because the route can change dynamically, the path MTU may increase and will
need adjustment in the host node. Learned path MTUs are aged and the Path MTU
Discovery process re-occurs. This allows the transmitted packet size to react to the
dynamic nature of routes through the network.
Path MTU Discovery is mandatory because fragmentation is not allowed on transit
routers.
If the device is acting as a transit router, it will not forward packets that are larger
than the output net’s MTU. It will generate an ICMP Packet Too Big message back
to the source of the packet.
The enable path-mtu-discovery command at the IPv6 Config> prompt can be
used to enable or disable path MTU discovery. Path MTU discovery is enabled by
default.
Use the set path-mtu-aging-timer command at the IPv6 Config> prompt to specify
the aging time for path MTUs that have been determined.
IPv6 Mandatory Security
An IPv6 node must support IP security. IP security can be enabled or disabled. See
“Using IP Security” and “Configuring and Monitoring IP Security” in the
Using and
Configuring Features
for additional information about IP security.
1. Use the add packet command at the IPv6 Config> prompt to add a packet
filter.
2. Use the update packet command at the IPv6 Config> prompt to update the
packet filter.
3. Use the add access command at the Packet-filter 'filter_name' Config>
prompt to add access controls.
Using IPv6
Chapter 13. Using IP Version 6 (IPv6) 379
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