174 BigIron RX Series Configuration Guide53-1001810-01Changing the router ID7 Globally changing the IP MTUTo globally enable jumbo support on all ports, enter commands such as the following.BigIron RX(config)# ip mtu 5000BigIron RX(config)# write memorySyntax: [no] ip mtu The parameter specifies the maximum number of bytes an Ethernet frame can have inorder to be forwarded on a port. Enter 64 – 9212, but this value must be 18 bytes less than thevalue of the global maximum frame size.NOTEThe BigIron RX will always use 22 Bytes less than the configured MTU in order to compensate for the4Bytes required for VLAN tags. This is so if a packet is forwarded on both a tagged and untaggedlink within a VLAN, it will get through.Changing the maximum transmission unit on an individual interfaceBy default, the maximum IP MTU sizes are as follows:• 1500 bytes – The maximum for Ethernet II encapsulation• 1492 bytes – The maximum for SNAP encapsulationNOTEThe IP MTU configured at the physical interface level takes precedence over the IP MTU configuredat the global level for that physical interface.To change the IP MTU for interface 1/5 to 1000, enter the following commands.BigIron RX(config)# int e 1/5BigIron RX(config-if-e10000-5)# ip mtu 1000Syntax: [no] ip mtu The parameter specifies the IP MTU. Ethernet II packets can hold IP packets from 572 –1500 bytes long. Ethernet SNAP packets can hold IP packets from 572 – 1492 bytes long.However, the value of IP MTU on an interface cannot exceed the configured value of IP MTU for aninterface, minus 18 bytes. The default IP MTU for Ethernet II packets is 1500. The default IP MTUfor SNAP packets is 1492.Changing the router IDIn most configurations, a device has multiple IP addresses, usually configured on differentinterfaces. As a result, a device’s identity to other devices varies depending on the interface towhich the other device is attached. Some routing protocols, including OSPF and BGP4, identify adevice by just one of the IP addresses configured on the device, regardless of the interfaces thatconnect the device devices. This IP address is the router ID.NOTERIP does not use the router ID.
