Deployment Guide

Configure Static Routes for the Management
Interface
When an IP address that a protocol uses and a static management route exists for the same prefix, the protocol route takes
precedence over the static management route.
To configure a static route for the management port, use the following command.
● Assign a static route to point to the management interface or forwarding router.
CONFIGURATION mode
management route ip-address mask {forwarding-router-address | ManagementEthernet slot/
port}
To view the configured static routes for the management port, use the show ip management-route command in EXEC
privilege mode.
DellEMC#show ip management-route
Destination Gateway State Route Source
----------- ------- ----- ------------
10.16.0.0/16 ManagementEthernet 1/1 Connected Connected
172.16.1.0/24 10.16.151.4 Active Static
IPv4 Path MTU Discovery Overview
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 consistent value. The MTU size can also be different for
various types of traffic sent from one host to the same endpoint.
Path MTU discovery (PMTD) 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. This packet size is called the maximum transmission unit (MTU) for IPv4 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 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 interactive 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, which 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.
To use the PMTD in the physical interface, you must allocate and activate the fedgov CAM ACL space using the cam-acl
command. The fedgov CAM ACL space is defined as a value (0-8) and you can select the required value to define the space.
When the space is activated, you can send the ICMP type 3 messages through the physical port.
To use the PMTD functionality, you must enter the ip unreachables command on a VLAN interface to enable the
generation of ICMP unreachable messages in the intermediate nodes. The PMTD functionality is based on the ICMPv4
destination unreachable message, which is generated by the intermediate device only when the ip unreachables command
is configured. PMTD is supported on all the layer 3 VLAN interfaces. Since all of the Layer 3 interfaces are mapped to the VLAN
ID of 4095, you cannot configure unique layer 3 MTU values for each of the layer 3 interfaces. If a VLAN interface contains both
IPv4 and IPv6 addresses configured in the system, both the IPv4 and IPv6 traffic are applied to the same MTU size; you cannot
specify different MTU values for IPv4 and IPv6 packets.
Packet handling during MTU mismatch
When you configure the MTU size on an interface, ensure that the MTU size of both ingress and egress interfaces are set to
the same value for IPv4 traffic to work correctly. If there is an MTU mismatch between the ingress and egress interface, there
may be a high CPU usage. If egress interface MTU size is smaller than the ingress interface, packets may get fragmented.
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IPv4 Routing