unreachable messages through the ip unreachable command in Interface mode. When a ping ortraceroute packet from an endpoint or a device arrives at the null 0 interface configured with a staticroute, it is discarded. In such cases, you can configure Internet Control Message Protocol (ICMP)unreachable messages to be sent to the transmitting device.Configuring the ICMP Source InterfaceYou can enable the ICMP error and unreachable messages to contain the configured IP address of thesource device instead of the previous hop's IP address. This configuration helps identify the devices alongthe path because the DNS server maps the loopback IP address to the host name, and does not translatethe IP address of every interface of the switch to the host name.Configure the source to send the configured source interface IP address instead of using its front-end IPaddress in the ICMP unreachable messages and in the traceroute command output. Use the ip icmpsource-interface interface or the ipv6 icmp source-interface interface commands inConfiguration mode to enable the ICMP error messages to be sent with the source interface IP address.This functionality is supported on loopback, VLAN, port channel, and physical interfaces for IPv4 and IPv6messages. feature is not supported on tunnel interfaces. ICMP error relay, PATH MTU transmission, andfragmented packets are not supported for tunnel interfaces. The traceroute utilities for IPv4 and IPv6 listthe IP addresses of the devices in the hops of the path for which ICMP source interface is configured.Configuring the Duration to Establish a TCP ConnectionYou can configure the duration for which the device must wait before it attempts to establish a TCPconnection. Using this capability, you can limit the wait times for TCP connection requests. Uponresponding to the initial SYN packet that requests a connection to the router for a specific service (suchas SSH or BGP) with a SYN ACK, the router waits for a period of time for the ACK packet to be sent fromthe requesting host that will establish the TCP connection.You can set this duration or interval for which the TCP connection waits to be established to asignificantly high value to prevent the device from moving into an out-of-service condition or becomingunresponsive during a SYN flood attack that occurs on the device. You can set the wait time to be 10seconds or lower. If the device does not contain any BGP connections with the BGP neighbors acrossWAN links, you must set this interval to a higher value, depending on the complexity of your network andthe configuration attributes.To configure the duration for which the device waits for the ACK packet to be sent from the requestinghost to establish the TCP connection, perform the following steps:1. Define the wait duration in seconds for the TCP connection to be established.CONFIGURATION modeDell(conf)#ip tcp reduced-syn-ack-wait <9-75>You can use the no ip tcp reduced-syn-ack-wait command to restore the default behavior,which causes the wait period to be set as 8 seconds.2. View the interval that you configured for the device to wait before the TCP connection is attemptedto be established.EXEC modeDell>show ip tcp reduced-syn-ack-wait424 IPv4 Routing
