163To configure an MTU for an interface:Step Command Remarks1. Enter system view. system-view N/A2. Enter interface view. interface interface-typeinterface-number N/A3. Configure an MTU for theinterface. ip mtu mtu-size By default, no MTU is configured.Configuring TCP MSS for an interfaceThe maximum segment size (MSS) option informs the receiver of the largest segment that the sender canaccept. Each end announces its MSS during TCP connection establishment. If the size of a TCP segmentis smaller than the MSS of the receiver, TCP sends the TCP segment without fragmentation. If not, itfragments the segment according to the receiver's MSS.If you configure a TCP MSS on an interface, the size of each TCP segment received or sent on theinterface cannot exceed the MSS value.This configuration takes effect only for TCP connections established after the configuration rather than theTCP connections that already exist.This configuration is effective only for IP packets. If MPLS is enabled on the interface, do not configure theTCP MSS on the interface.To configure a TCP MSS of the interface:Step Command Remarks1. Enter system view. system-view N/A2. Enter interface view. interface interface-typeinterface-number N/A3. Configure a TCP MSS for theinterface. tcp mss value By default, no TCP MSS is configured.Configuring TCP path MTU discoveryIMPORTANT:All the devices on a TCP connection must be enabled to send ICMP error messages by using the ipunreachables enable command.TCP path MTU discovery (in RFC 1191) discovers the path MTU between the source and destination endsof a TCP connection. It works as follows:1. A TCP source device sends a packet with the Don't Fragment (DF) bit set.2. A router that fails to forward the packet because it exceeds the MTU on the outgoing interfacediscards the packet and returns an ICMP error message, which contains the MTU of the outgoinginterface.
