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Cisco Employee

“Datacenter troubleshooting guide” – a blog by Gilles Dufour.

Day 1 – Start with the basic

Very often I see questions regarding troubleshooting steps to follow when working with a loadbalancer like ACE (Application Control Engine).

Some people believe that because this is a new platform, it works differently than all previous equipments they ever worked with.

Suddenly, because this is a loadbalancer, we see CCIE not knowing what to do when the device does not respond to ping requests.

It is true that a new platform will have new features, new functions, new GUI and maybe a new CLI.  But at the end, a loadbalancer remains a TCP/IP device. 

If your loadbalancer does not loadbalance, I would suggest to perform standard routing/switching troubleshooting first.  This includes checking the path from the source to the destination, making sure routing tables and arp tables are correct.  You will also check the reverse path making sure the server will respond to the client THROUGH the ACE (asymetric routing is often not accepted by loadbalancers).

And I strongly recommend to have a sniffer tool ready to capture traces to verify that traffic gets to your loadbalancer and leaves it in the right direction.

Next time, I will focus on loadbalancing at layer 3 and 4.

Gilles Dufour.

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Cisco has a new solution called ITD:


ITD (Intelligent Traffic Director) is a hardware based multi-Tbps Layer 4 load-balancing, traffic steering and clustering solution on Nexus 5K/6K/7K series of switches. It supports IP-stickiness, resiliency, NAT (EFT), VIP, health monitoring, sophisticated failure handling policies, N+M redundancy, IPv4, IPv6, VRF, weighted load-balancing, bi-directional flow-coherency, and IPSLA probes including DNS. There is no service module or external appliance needed.


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