In AIX, I’ve found that networking is probably one of the most complicated subjects around. Some components of networking are addressed by making changes to flat files, while others are handled through the Object Data Manager (ODM). There are typically at least three commands or ways of doing something, from adding a route to turning off an interface. And what seems like a subtle or innocuous action can wind up causing headaches on the server in due time—for example, a reboot that brings out duplicate default routes.
I have extensive experience in both setting up and messing up networking on AIX servers worldwide. Along the way, I’ve taken copious notes on how to manage networking, so that others will hopefully avoid the same plights I encountered. The following is a condensed version of the knowledge I've gleaned about AIX networking through hard-won experience.
I have extensive experience in both setting up and messing up networking on AIX servers worldwide. Along the way, I’ve taken copious notes on how to manage networking, so that others will hopefully avoid the same plights I encountered. The following is a condensed version of the knowledge I've gleaned about AIX networking through hard-won experience.
Understand Physical and Logical Relationships
Although most forms of UNIX and Linux will have only one device under the /dev directory to describe each networking interface, AIX is different in that it will create both a device for the physical interface (e.g., ent0) and a device for the logical interface (e.g., en0). Each device serves a unique purpose; for example, media speeds are set on the physical interface, whereas IP addresses are defined on the logical interface.
Knowing this about the physical and logical devices, your one key take-away is that if you need to do something to the physical interface, you’ll need to shut down, disable, or remove the logical interface before AIX will accept any commands to manipulate the physical interface.
Knowing this about the physical and logical devices, your one key take-away is that if you need to do something to the physical interface, you’ll need to shut down, disable, or remove the logical interface before AIX will accept any commands to manipulate the physical interface.
Establish EtherChannel Devices First
Before you define any IPs on your servers, to create redundancy and get higher performance, I recommend setting up devices that use Cisco EtherChannel technology wherever possible. Much like bonding in Linux or IP network multipathing (IPMP) in Solaris, EtherChannel lets you gather multiple physical interfaces into one connection. The interfaces can be aggregated so that traffic goes down both pipes simultaneously, or they can work in an active-passive mode conditional upon failover. The "smitty etherchannel" fastpath is the easiest way I’ve found to define EtherChannel devices.
One thing worth noting is that the EtherChannel device, although logical in nature, still has a physical and a logical component. For example, if you choose to use ent0 and ent1 to create an EtherChannel adapter, the physical device will be created as ent2 with a logical device of en2. The EtherChannel adapter behaves almost identically to any other standard network adapter as far as setting network tunable parameters or establishing IP aliases, but the physical interface will show some additional information when viewed using the lsdev or lsattr command.
One thing worth noting is that the EtherChannel device, although logical in nature, still has a physical and a logical component. For example, if you choose to use ent0 and ent1 to create an EtherChannel adapter, the physical device will be created as ent2 with a logical device of en2. The EtherChannel adapter behaves almost identically to any other standard network adapter as far as setting network tunable parameters or establishing IP aliases, but the physical interface will show some additional information when viewed using the lsdev or lsattr command.
Use SMIT for Your Initial IPs
I’ve always subscribed to the philosophy that the proficiency of an AIX administrator can generally be determined by how often or seldom the admin relies upon the System Management Interface Tool (SMIT) to do his or her work. But there are a few tasks where, rather than memorize long commands with dozens of flags, I will turn to SMIT to accomplish my work. Setting up initial IP addresses is one of those tasks.
The "smitty tcpip" fastpath is the quickest route to navigating to the menus for setting up everything network related. From the SMIT Minimum Configuration & Startup screen, define your first IP address and default gateway through the menu. Then, as you need to add IPs to other interfaces, use the "smitty chinet" fastpath for those other devices. Using the Minimum Configuration & Startup screen to define additional interfaces could adversely affect the routing on the server.
The "smitty tcpip" fastpath is the quickest route to navigating to the menus for setting up everything network related. From the SMIT Minimum Configuration & Startup screen, define your first IP address and default gateway through the menu. Then, as you need to add IPs to other interfaces, use the "smitty chinet" fastpath for those other devices. Using the Minimum Configuration & Startup screen to define additional interfaces could adversely affect the routing on the server.
Use the ODM Wisely
Most networking information in AIX is stored within the ODM. Only a few flat files that pertain to networking can be modified manually, such as /etc/hosts or the /etc/rc.tcpip and /etc/rc.nfs files. This can make it difficult to manage, change, or remove items like network routing from the server quickly and easily.
Aside from the initial plumbing-up of IPs on interfaces, the one other thing I’ll do through the ODM is to assign any IP aliases onto devices. I do so using the following command:
Aside from the initial plumbing-up of IPs on interfaces, the one other thing I’ll do through the ODM is to assign any IP aliases onto devices. I do so using the following command:
chdev -l enX -a alias4=$IP,$NETMASK
This way, following a reboot, all IPs on that specific adapter will come up automatically. However, beyond this point, I stick with modifying other flat files.
Case in point: I’ve found the task of adding additional routes to a server by means of the ODM to be a beast. I've had to hunt through obscure corners of the CuAt database when trying to remove or adjust a single route. Instead of trying to jump through hoops in adding or removing routes through the ODM, I typically modify the /etc/rc.net file under "Part II - Traditional Configuration" and add all my routes to this area using the route command. This way, I can delete active routes off the server using a route delete command and strip them from the file without having to do a reboot or worrying about the ODM.
Case in point: I’ve found the task of adding additional routes to a server by means of the ODM to be a beast. I've had to hunt through obscure corners of the CuAt database when trying to remove or adjust a single route. Instead of trying to jump through hoops in adding or removing routes through the ODM, I typically modify the /etc/rc.net file under "Part II - Traditional Configuration" and add all my routes to this area using the route command. This way, I can delete active routes off the server using a route delete command and strip them from the file without having to do a reboot or worrying about the ODM.
Smoother Networking
The tips I've provided here should help make your AIX networking routine go a little more smoothly. In my next article, I'll pull together the information I've provided in this article and the other "Secrets of an AIX Administrator" articles (see the Learning Path for an articles list and links), and I’ll conclude this series by giving you some tips on what to do following your first reboot after building a new server.
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