Addendum
10
IPv4 Routing
This chapter describes the IPv4 routing-related enhancements and contains the following sections:
• IPv4 Path MTU Discovery Overview
• Configuring the Duration to Establish a TCP Connection
• Using Loopback Address in ICMP Unreachable Messages
IPv4 Path MTU Discovery Overview
In common network topologies, hosts send large large volumes of data to other neighboring devices
using IP packets. For effective utilization of network resources, enhanced performance, and easy
reassembly of packets that are transmitted, devices attempt to forward packets from the origin to the
endpoint of the network without the need of fragmentation as much as possible. The size of the packet
that can be sent across each hop in the network path without being fragmented is called the path
maximum transmission unit (PMTU). This value might vary for the same route between two devices,
mainly over a public network, depending on the network load and speed, and it is not a symmetric,
consistent value. This MTU size can also be different for various types of traffic sent from one host to the
same endpoint.
Path MTU discovery (PMTD) is a mechanism that identifies the path MTU value between the sender and
the receiver, and uses the determined value to transmit packets across the network. PMTD, as described
in RFC 1191, denotes that the default byte size of an IP packet is 576. The IP and TCP portions of the
frame constitute 40 bytes and the remaining 536 bytes form the data paylod. This packet size is called the
maximum transmission unit (MTU) for IP4 frames. PMTD operates by containing the do not fragment (DF)
bit set in the IP headers of outgoing packets. When any device along the network path contains an MTU
that is smaller than the size of the packet that it receives, the device drops the packet and sends an
Internet Control Message Protocol (ICMP) Fragmentation Needed (Type 3, Code 4) message with its MTU
value contained in the message to the source or the sending device. This message enables the source to
identify that the transmitted packet size must be reduced. The packet is retransmitted with a lower size
than the previous value. This process is repeated in an interative way until the MTU of the transmitted
packet is lower or equal to the MTU of the receiving device for it to obtain the packet without
fragmentation. If the ICMP message from the receiving device that is sent to the originating device
contains the next-hop MTU, then the sending device lowers the packet size accordingly and resends the
packet. Otherwise, the iterative method is followed until the packet can traverse without being
fragmented.
PMTD is enabled by default on the switches that support this capability. To enable PMTD to function
correctly, you must enter the ip unreachables command on a VLAN interface to enable the
generation of ICMP unreachable messages. PMTD is supported on all the layer 3 VLAN interfaces.
Because all of the Layer 3 interfaces are mapped to the VLAN ID of 4095 when VLAN subinterfaces are
configured on it, it is not possible to configure unique layer 3 MTU values for each of the layer 3
interfaces. If a VLAN interface contains both IPv4 and IPv6 addresses configured on it, both the IPv4 and
IPv6 traffic are applied the same MTU size; you cannot specify different MTU values for IPv4 and IPv6
packets. This functionality is supported on the S4810, S4820T, Z9000, and MXL platforms.
IPv4 Routing
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