Quick Reference Guide

584 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
Configuring IP parameters – Layer 3 Switches
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NOTE
The new command ip-port-mtu replace the command ip mtu. On the PowerConnect the IP MTU
check on egress is validated based on the physical port instead of the ip interface. Therefore, the
command ip-port-mtu can be set only on a physical port. In the case of a VE, we can set the
ip-port-mtu on a port member of a VE. In contrast with the ip mtu command, the multiple physical
ports in a VE can have a different IP MTU. However, on PowerConnect, all VLANs of a port would have
the same IP MTU size.
The <num> parameter specifies the MTU. Ethernet II packets can hold IP packets from 576 –
1500 bytes long. If jumbo mode is enabled, Ethernet II packets can hold IP packets up to 9198
bytes long. Ethernet SNAP packets can hold IP packets from 576 – 1492 bytes long. If jumbo
mode is enabled, SNAP packets can hold IP packets up to 9190 bytes long. The default MTU for
Ethernet II packets is 1500. The default MTU for SNAP packets is 1492.
Path MTU discovery (RFC 1191) support
Most devices support the path MTU discovery method described in RFC 1191. When the device
receives an IP packet that has its Do not Fragment (DF) bit set, and the packet size is greater than
the MTU value of the outbound interface, then the device returns an ICMP Destination Unreachable
message to the source of the packet, with the Code indicating "fragmentation needed and DF set".
The ICMP Destination Unreachable message includes the MTU of the outbound interface. The
source host can use this information to help determine the maximum MTU of a path to a
destination.
RFC 1191 is supported on all interfaces.
Changing the router ID
In most configurations, a Layer 3 Switch has multiple IP addresses, usually configured on different
interfaces. As a result, a Layer 3 Switch identity to other devices varies depending on the interface
to which the other device is attached. Some routing protocols, including Open Shortest Path First
(OSPF) and Border Gateway Protocol version 4 (BGP4), identify a Layer 3 Switch by just one of the
IP addresses configured on the Layer 3 Switch, regardless of the interfaces that connect the Layer
3 Switches. This IP address is the router ID.
NOTE
Routing Information Protocol (RIP) does not use the router ID.
NOTE
If you change the router ID, all current BGP4 sessions are cleared.
By default, the router ID on a Layer 3 Switch is one of the following:
• If the router has loopback interfaces, the default router ID is the IP address configured on the
lowest numbered loopback interface configured on the Layer 3 Switch. For example, if you
configure loopback interfaces 1, 2, and 3 as follows, the default router ID is 9.9.9.9/24:
• Loopback interface 1, 9.9.9.9/24
• Loopback interface 2, 4.4.4.4/24
• Loopback interface 3, 1.1.1.1/24
• If the device does not have any loopback interfaces, the default router ID is the lowest
numbered IP interface configured on the device.