Ever thought you might be having some Layer 4 connectivity issues? Pings as you should know are ICMP transmissions and ICMP is a Layer 3 protocol (commonly used to send error messages). Luckily Cisco has a hidden ( as in it wont show up when using IOS help ) utility for testing TCP connectivity.
The command, ttcp is actually a very powerful tool for testing TCP connections, it requires IOS 11.2 or higher with IP Plus or Service Provider images. Here is an example of it being used to test connectivity between two locations.
SiteA#ttcp transmit or receive [receive]: buflen : bufalign : bufoffset : port : sinkmode [y]: rcvwndsize : 4096 show tcp information at end [n]: ttcp-r: buflen=8192, nbuf=2048, align=16384/0, port=5001, rcvwndsize=4096 tcp SiteA# SiteB#ttcp transmit or receive [receive]: trans Target IP address: 188.8.131.52 buflen : nbuf : 50 bufalign : bufoffset : port : sinkmode [y]: buffering on writes [y]: show tcp information at end [n]: ttcp-t: buflen=8192, nbuf=50, align=16384/0, port=5001 tcp -> 184.108.40.206 SiteB# SiteA# ttcp-r: accept from 220.127.116.11 (mss 1460, sndwnd 2144, rcvwnd 4096) ttcp-r: 409600 bytes in 61064 ms (61.064 real seconds) (~5 kB/sec) +++ ttcp-r: 301 I/O calls ttcp-r: 0 sleeps (0 ms total) (0 ms average) SiteA# SiteB# ttcp-t: connect (mss 1460, sndwnd 4096, rcvwnd 2144) ttcp-t: 409600 bytes in 60504 ms (60.504 real seconds) (~5 kB/sec) +++ ttcp-t: 50 I/O calls ttcp-t: 0 sleeps (0 ms total) (0 ms average) SiteB#
The downside of ttcp is that it really makes me miss tcpspray for Linux / on ImageStream routers. As you see in this example ttcp requires that you start the receiver process on the remote end, while tcpspray just uses the echo and discard inetd services. I’ll cover some details about tcpspray in a future post.
Obviously another advantage of this utility is speed testing. Obviously, this is not a reliable means to test high bandwidth links as the router needs enough CPU to generate the packets. If your bandwidth tests are not pegging the router and you’re still not happy with the results, latency could be the issue. Check our my previous post on the relationship between latency and throughput.
Network engineer turned management currently servicing the enterprise data center market. I started working on networks in the ’90s and still feel like that was just a few years ago. Jack of all trades, master of none; I love to learn about everything. Feel free to ask me about photography, woodworking, nhra, watches, or even networking! — For feedback, please leave a comment on the article in question, and I’ll respond as soon as I can. For everything else including fan mail or death threats, contact me via twitter.