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ACE Boot Camp – Day 2

September 24, 2010

Day 2 of ACE boot camp did not disappoint! Another full day of lecture and labs. We covered the following topics:

Modular Policy CLI
Managing the ACE Appliance and Service Module
Security Features
Layer 4 Load Balancing
Health Monitoring

I’ll cover some general things about each topic and go into additional details on the points I thought were interesting.

Modular Policy CLI – ACE classifies which traffic it will load-balance based on policy maps, which are comprised of class maps. If this sounds a lot like how you build QoS policies on IOS based routers, it is. The big difference is that ACE is far more restrictive in what those policies contain.

Managing the ACE Appliance and Service Module – Like most Cisco devices, ACE can be managed in a number of different ways. Telnet, SSH, HTTPS, and SNMP. You can even use the XML API if you want. With SNMP, versions 1 and 2 cannot understand contexts. SNMP version 3 can. In order for SNMP version 1 and 2 to work with contexts, you have to use the community string format of “community@context” where “community” is the community name and “context” is the name of the virtual context. When the GET, SET, or whatever SNMP action you choose hits the ACE, the “@context” portion is understood and passed along to the appropriate context.

Security Features – There are a ton of different ways to restrict traffic entering and leaving the ACE. Most of the time you will be focused on traffic entering the ACE. As with applying ACL’s to interfaces on switches and routers, very rarely will you see access lists applied in the outbound direction. That feature is there in case you have some special need to use it.

An interesting capability that the access lists have in ACE is the ability to use object groups to identify which traffic to permit or deny. If you have ever worked on the PIX, ASA, or FWSM, you will be familiar with object groups. They make traffic identification much easier not to mention the simplification of the ACE configuration itself.

The much more granular security options were of great interest to me. Take something like IP fragmentation and reassembly. You can specify the max number of fragments allowed from one packet. If it exceeds the number you specify, you can just drop the traffic. Many other options exist with regards to the packet stream itself. You can enforce certain flags from being set. If violations occur, not only can you drop the traffic, but you could actually reset the flag itself and then send the traffic through the ACE. While most options are configurable, there are some rules that are always enforced. For example, the source IP of a traffic flow can never equal the destination IP.

Layer 4 Load Balancing – This is exactly what it sounds like. Load balancing based on TCP/UDP flows. I think the neatest part about this particular topic was the fact that you can actually load balance traffic across multiple firewalls and have the return traffic come back through the same firewall. This of course requires an ACE on both sides of the firewall, but withe ability for the ACE module to have up to 250 virtual contexts, it doesn’t have to be 2 separate physical ACE modules. The same module can host both contexts that live on either side of the firewall. It is fairly clever how they make this work. Essentially, when traffic comes from one firewall into the ACE, it remembers the MAC address of the sending firewall and places that connection in a state table. When traffic comes back through the ACE, it already knows which firewall to send the traffic to based on that state table. I’m not sure I would want to use an ACE module for load balancing through firewalls, but there are plenty of customers out there that are already doing it or could see the benefit in doing something like that.

Health Monitoring – If there’s one thing the ACE seems to have a fairly large amount of options on, it’s the health monitoring or probes. All the major protocols have specific probes on the ACE that are used to check the health of the back end or “real” servers. This is way beyond the load balancer simply pinging the server to make sure it is up and running. Let’s say you used the HTTP probe. Instead of just trying a simple ping to check a back end servers’ status, the HTTP probe can actually go out and make an HTTP connection to the server or serverfarm. That’s a far more intelligent way to query server status. Based on the probe results, any number of things can be done to the various serverfarms and servers ACE may be providing services for. They may be taken out of active status, have their priority reduced, etc.

There’s a LOT more to this stuff. This was only day 2 of 4! More to come.

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