Administrator Guide

• When the interface comes up, the system re-installs the route.
• When the recursive resolution is “broken,” the system withdraws the route.
• When the recursive resolution is satised, the system re-installs the route.
Congure Static Routes for the Management Interface
When an IP address that a protocol uses and a static management route exists for the same prex, the protocol route takes
precedence over the static management route.
To congure 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}
Example of the show ip route static Command
To view the congured static routes for the management port, use the show ip management-route command in EXEC
privilege mode.
Dell#show ip management-route all
Destination Gateway State
----------- ------- -----
1.1.1.0/24 172.31.1.250 Active
172.16.1.0/24 172.31.1.250 Active
172.31.1.0/24 ManagementEthernet 1/0 Connected
Dell#
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 dierent for various types of trac sent
from one host to the same endpoint.
Path MTU discovery (PMTD) identies 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.
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 sub-
interfaces are congured on it, it is not possible to congure unique layer 3 MTU values for each of the layer 3 interfaces. If a VLAN
interface contains both IPv4 and IPv6 addresses congured on it, both the IPv4 and IPv6 trac are applied the same MTU size; you
cannot specify dierent MTU values for IPv4 and IPv6 packets.
IPv4 Routing
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