Setting The Don't Fragment Bit - HP 7102dl - ProCurve Secure Router Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring Policy-Based Routing

Setting the Don't Fragment Bit

Packets may travel over a path that includes routers with varying MTUs. When
a router prepares to forward a packet, it checks the packet's size against the
MTU of the link that connects to the next hop router. If the packet exceeds
this MTU, then the router fragments the packet. Anytime that a device frag-
ments a packet, the packet runs the risk of becoming garbled. This risk
increases the more times that a packet is fragmented.
You can set the "don't fragment" bit in a packet's IP header to prevent devices
between the local router and the packet's destination from fragmenting the
packet. For example, it is important the VoIP packets not be fragmented.
You select packets with a route map entry and then set the "don't fragment"
bit. Define traffic as described in "Selecting Traffic for a Route Map Entry" on
page 15-128. For example, if a VoIP application marks packets with an IP
precedence value of 5, you could select packets with this value for the route
map. You could also select packets destined for the application port used by
your VoIP equipment.
To set the "don't fragment" bit for selected packets, enter this command from
the route map configuration mode context:
Syntax: set ip df
Setting the "don't fragment" bit can cause problems. If a packet is larger than
the MTU of a link over it must pass, and the router attempting to forward the
packet cannot fragment it, then the router will drop the packet. Typically, the
router will also return an ICMP packet informing the host that sent the packet
that the packet was too large. However, some systems have firewalls that
prevent routers from sending the ICMP message.
If packets will be traveling through the Internet or other external network with
policies you cannot control, setting the "don't fragment" bit can cause packets
to be dropped. However, if the local router is forwarding packets into a remote
network under your organization's control, you can often set the "don't
fragment" bit without fear. You should be certain that all routers in your
organization have compatible MTU.
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