Aaron’s Worthless Words

Did you catch the article on setting up fault tolerance on the CSM?  In that article, I mentioned that Cisco recommends a dedicated trunk for the FT VLAN if you have two HA CSMs in two chassis.  Discuss amongst yourselves while I drone on.

Why should you set up a dedicated trunk for this stuff?  The most obvious reason is to be sure that normal traffic doesn’t step on the syncing traffic.  Since we’re syncing state information as well as configuration, the frames need to arrive in a timely manner.  Any errors could potentially disrupt the FT process, which is bad.  You surely don’t want the primary to fail only to find out that the standby doesn’t have the complete or current config.

Another reason is to keep the syncing traffic from stepping on normal traffic.  The CSM is a pretty robust box and can handle a pretty good chunk of data.  If you had a 100Mbps trunk between your chassis, there is the potential for the link to get flooded if the CSM ever starts sending some real data.  All things being equal, though, your trunks are probably sized properly for your network, and the addition of the syncing traffic probably won’t affect much.

Let’s review our configuration from the other article.

vlan 83
 name CSM-Sync
!
module csm 3
 ft group 1 vlan 83
  priority 100 alt 90
  preempt

This snippet creates VLAN 83 and tells the CSM to use it for syncing, but how do we dedicate a trunk for that VLAN?  We use the switchport trunk allowed vlan directive.  We’ll assume that G1/1 on your primary switch is connected to G1/1 on your standby.

interface GigabitEthernet1/1
 description CSM Syncing
 switchport
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 83
 switchport mode trunk

This sets G1/1 up to only allow VLAN 83 across it.  If you do a show int G1/1 trunk, you’ll see that this VLAN is the only one allowed, the only one active, and the only one one forwarding on that link.  Of course, you’ll need to do the same on the other side to keep traffic flow sane, but it’s fairly easy.

What if G1/1 goes down, though?  You’d lose sync, so you probably want to look at a solution for that little problem.  You could put in multiple links and let Spanning Tree do the work.  You could even turn those links into an EtherChannel for redundancy and throughput.  If you have more than two chassis, you could full mesh them with trunks dedicated to VLAN 83.  There are a number of ways around the problem.  Be creative.

Be sure to send turkey questions my way.

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There are three different ways that a CSM checks for the health of the servers — active probes, inband health checking, and inband HTTP monitoring.  Let’s talk about active probes.

Active probes (or just probes) typically send traffic to one of the RIPs of a serverfarm, do some stuff, and give a pass or fail grade.  If the probe fails a certain number of times in a row, that server is considered sick and taken out of the pool for use.  The CSM keeps checking the unhealthy until it passes a number of times in a row, at which point it is placed back in the pool for use.  Almost everything is configurable, of course, so let’s look at some of those settings.

These all have their defaults, so you don’t need to actually configure them, but it’s important to know they’re there to tweak later.  There are other parameters to the more specific types of probes as well.  You’ll have to venture forth on your own to figure those out, though I’ll be glad to help.

• interval:  The time between probes when the server is healthy.
• retries:  The number of consecutive failures before a healthy server is considered failed.
• failed:  The time between probes when the server is failed.
• recover:  The number of consecutive successes before a failed server is considered health.

I always said that the CSM only speaks HTTP, but it knows how to order a ham and cheese sandwich in a few other protocols, including doing some decent stuff like watching for SMTP banners, looking for ICMP reachability, or getting a response from a Tcl script.  Here’s a list of the probes with some boring commentary.  Depending on IOS versions, you may have more or less available to you.

• tcp: Establishes a connection to a TCP port.  If the port is open, we pass.
• udp: Same as TCP, but in UDP.  Duh.
• icmp: Ping-a-ling.  Do I need to explain this one?
• http: Requests a URL from a webserver and looks for HTTP return codes.
• dns, smtp, telnet: These guys just attach to the service and look for a proper response header.  It doesn’t do any transactions or anything but simply makes sure that DNS, SMTP, or telnet is running on the port.
• script:  These probes run a Tcl script (that you write) and look for the return code of the script.  Very powerful!
• kal-ap-tcp, kal-ap-udp:  Admittedly, I have no earthly idea what those are.  Most references I’ve seen to it involve the Cisco ACE, but I have no clue.  Can someone fill in for me here?

Shall we try one or two?  How about a TCP probe that makes sure your custom app is still running on TCP/8839?  Since it’s a custom app, we can use a TCP probe on that port to make sure something is listening (If you need something more, you’ll need to check out script probes.).

probe MYAPP tcp
description My app on TCP/8839
port 8839

Now we apply the probe to the serverfarm.

serverfarm MYFARM
probe MYAPP
real 1.1.1.1
inservice
real 1.1.1.2
inservice

Easy enough.  How about another?  I want to configure an HTTP probe that gets the URL /test.php and looks for the status code 200 and apply it to a serverfarm.

probe LOOKFOR200 http
request url /test.php
expect status 200

serverfarm YOURFARM
probe LOOKFOR200
real 2.2.2.1
inservice
real 2.2.2.2
inservice

By the way, you don’t want to use this in production at all.  You’ll probably want to elect to look for ranges of status codes instead of a single value.  Google up HTTP return codes and you’ll see why.

I will mention again, though, that the script probe is very interesting.  If you know Tcl or have done any development, check these things out.  You can do whatever you want with them instead of using the canned probes that come with your CSM.  I suggest taking a look at Ivan Pepelnjak’s page for some insight into Tcl on Cisco devices.

I think you can take it from here.  As always, comments are welcome.

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Like (nearly) everything in the Cisco world, you can set up your CSM to fail over to another module when the primary dies a horrible death.  You can have two in the same chassis or even have them in separate chassis — the process is the same no matter how you have it set up.  Either way, you have a primary and a secondary module in fault tolerance (FT) mode.

First, we’ll establish a VLAN that the CSM will use to do its configuration and state syncing over.  This is just an ordinary VLAN; there’s nothing special about it, really, but it should be dedicated for the CSM to use for syncing.  Let’s randomly choose VLAN 83.

vlan 83
name CSM-Sync

You will, of course, have to do this on every switch that holds a CSM, so, if you’re using them in two different chassis, you’ll put the same VLAN on each making sure they can see each other through a trunk.  Cisco recommends that you dedicate a trunk between the two switches for the sync VLAN in order to remove the chance of other traffic stepping on the sync packets, but I’m not convinced that’s necessary.  Use your judgement on that one.

Back to it.  Next, you need to decide on a FT group ID.  This is similar to a HSRP group and lets you run multiple FT groups on the same VLAN.  The group ID needs to be in the range of 1 to 256, so, since this is the first one, let’s just use 1.  Get into config mode for the CSM that you want to be the primary and do this.

ft group 1 vlan 83

This takes you to the config-slb-ft prompt.  Just like HSRP, we need to set priorities for each device and whether or not it should preempt, so let’s configure.  Yes, we want to preempt, right?  Let’s set the priorities to 100 and 90, too.

priority 100 alt 90
preempt

This sets the primary CSM to priority 100 and the secondary to 90; both will preempt.

What about configuring the secondary for FT? That’s easy.  Go into CSM config mode on the secondary and enter the ft group 1 vlan 83 command.  That’s it. The two CSMs will do a little arguing and come back as the primary and secondary.  After that, all configuration is done on the primary, which is synced over to the secondary just like an ASA.  Pretty cool, eh?

When configuring things like IP addresses, though, you’ll need to make provisions for the secondary with the alt directive (remember that one from the priority).  I won’t go into much, but you’ll need it mostly when settings IPs to VLANs.  Here’s an example of setting an IP address on client VLAN 100 for both the primary and secondary.

vlan 100 client
ip address 192.168.0.11 255.255.255.0 alt 192.168.0.12 255.255.255.0

Alright…one more thing.  The configurations don’t sync automagically (at least not on my old version of code).  If you make a change to the primary CSM, you’ll see an out-of-sync message when you look at the FT status.

Switch#sh mod csm X ft
FT group 1, vlan 83
This box is active
Configuration is out-of-sync
priority 100, heartbeat 1, failover 3, preemption is on
alternate priority 90

If the primary goes down now and the secondary takes over, the changes you just made won’t be reflected on the secondary.  You fix this with the hw-module contentSwitchingModule X standby config-sync command (where X is the module slot in the chassis).  Alternatively, you can just type hw c X s c as a shortcut.  It’ll take a few minutes depending on your configuration, so check your logs for when it’s finished.  Note that the secondary does not save the new configuration to its startup-config; you’ll have to log in and save that manually (or automatically through CiscoWorks or something) to save changes there.

Let me know if you have any questions and check out my page on getting output from Cisco’s fine mid-tier load balancer. 

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I like logging on an IOS device.  I like to look at the buffer and tell you that your interface went down 30 seconds ago.  I like to look on the box and see that BGP with my Internet provider has been flapping since 02:13ET.  I like to look and see that one of the other guys has been making changes to the gear all morning.  I could go on and on.

There are lots of ways to monitor a Cisco box — SNMP polling, SNMP traps, show commands, etc. — but there’s nothing so handy as the log buffer.   A show logging can provide you all sorts of information on things you do and don’t care about, so it’s important to know the destinations and levels when setting up logging.

There are four logging destinations.

• Console — logs to the console ports of the device
• Monitor — logs to any device or pseudo-device that’s in monitoring mode.  The most common application is when you do a terminal monitor to see output of debugs.
• Buffer — logs to a memory device that let’s you see the log messages on demand.  It has a finite size and scrolls old messages out after X bytes have been written.
• Host or Trap — logs to an external syslog server.

What’s the most important destination?  There’s not one.  I personally thing the syslog host is the most important since it allows you to log messages to disk on a server somewhere.  The buffer is also important since it lets anyone with access see what’s going on with the device.  Your mileage will vary depending on what you have set up.

There are eight logging levels as well.

• Debug - level 7
• Informational - level 6
• Notifications - level 5
• Warnings - level 4
• Errors - level 3
• Critical - level 2
• Alerts - level 1
• Emergencies - level 0

Wow.  There’s some numbers.  What does it mean?  Every logging message comes with a level.  If my CSM loses a RIP, it generates a level 6 message (%CSM_SLB-6-RSERVERSTATE) telling me.  If I configure the box, I get a level 5 message (%SYS-5-CONFIG_I) saying that I’ve done so.  If my router is on fire in the rack, I’ll get a level 0 message telling me.  Now, when you configure a destination, you have to give a logging level, and every message at that level or below will be logged.  If I set my logging buffer to 5, I’ll see the configuration message and the “Oh, the humanity!” message, but not the RIP failures.  If I set it to 0, I only see the emergencies.  If I set it to 7, I see everything.

Let’s do the configuration, then.  After hours or research, you’ve decided to use a remote syslog server at 1.2.3.4 for warnings and the buffer for informational.  Here’s what you’d do.

logging host 1.2.3.4
logging trap warnings
logging buffer information

It’s not that hard.  You can even use the number instead of the words for the logging level if you would like.  The same procedure holds true for the console and monitor mechanisms — logging <mechanism> <level>.  Easy.

If you care, here’s what I usually run.

• Console — off.  I’ve seen a console rendered unusable because the console was being obliterated with syslog messages.  Not only is it an issue with being able to see what you’re typing when stuff is scrolling, but some older devices wind up using 100% CPU because they’re sending messages to the console.
• Monitor — debug.  It doesn’t really log anything unless you do a term mon or something, and, in that case, I want to see my debugs.
• Buffer — informational, but it depends on the device.  It lets me see all the messages except for debugs, which is probably just right for most routers and switches.  If you’re switch is in a closet somewhere with users plugged directly into it, you may be flooded with up/down messages, so keep an eye out for stuff like that.
• Host or Trap — informational.  Debug’s a little too much for the corporate environment, but, depending on how much disk space you have, you may be able to handle it.

There’s a lot more to syslog and log messages, so see this nifty Cisco page.

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A lot of IOS commands give you a lot of information. Most of the time, though, it’s way too much information, and it sure would be nice to do some grep-like stuff on the output, right? Well, just like on Linux, you can use the pipe (|) to do such. That’s not a butt cheek, by the way.

The most useful function is the include directive. This is the equivalent of just plain grep on Linux, and will show you only lines that match a string that you give it. Say that you want to find what ports on your switch are down, but don’t want to grind through all the lines of a show ip interface brief. If you just pipe it to the include command followed by the word “down”, you’ll see something like this.

Switch#show ip interface brief | include down
GigabitEthernet0/4 unassigned YES unset down down
GigabitEthernet0/7 unassigned YES unset down down
GigabitEthernet0/17 unassigned YES unset down down
GigabitEthernet0/18 unassigned YES unset down down
GigabitEthernet0/19 unassigned YES unset down down
GigabitEthernet0/20 unassigned YES unset down down

You can also use the exclude directive, which is the same as a grep -v on Linux. I hope you figured out that this gives you all lines that don’t match the word, so, let’s use the exclude directive to found out what ports are down. How about we just ignore the lines that are up.

Switch#show ip interface brief | exclude up
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/4 unassigned YES unset down down
GigabitEthernet0/7 unassigned YES unset down down
GigabitEthernet0/17 unassigned YES unset down down
GigabitEthernet0/18 unassigned YES unset down down
GigabitEthernet0/19 unassigned YES unset down down
GigabitEthernet0/20 unassigned YES unset down down

Well, this won’t exactly give you all the ports that are down. What if the port is up/down?

What else can you use with the pipe? What if you want to look at the configurations of all the ports or interfaces on a box but don’t want to go through the config hitting the spacebar over and over. If you use the begin command, you’ll see the output beginning from the first match, so, using the string interface will show you the config starting at the first interface.

Switch#show running-config | begin interface
interface GigabitEthernet0/1
description Server
switchport access vlan 4
switchport mode access
spanning-tree portfast

…

Another good one is the section command. It’s usually used on the output of show running-config and shows you sections of the config that match your string. Huh? If you want to see the BGP section of the configuration, you can do something like thing just to see that part of the configuration.

Router#show running-config | section bgp
router bgp 1
neighbor 1.1.1.1 remote-as 65000
neighbor 1.1.1.1 version 4

There are a few other commands for use with the pipe, so explore on your own. You might also want to check out regular expressions on the Cisco IOS if you want to match more than just simple text.

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We covered QoS tagging the other day, but that just marks packets. I think you’re old enough now that we should actually do some policing. Policing is where you restrict the amount of bandwidth that a flow or set of flows can use. For example, say you have a site that serves webpages to the rest of the network. HTTP is the primary function, but the SysAdmins also have to maintain the boxes via SSH, right? To make sure that their SSH sessions don’t squash the bandwidth that your HTTP servers need, you can police the SSH sessions by giving the a bandwidth ceiling that they can’t cross.

Let’s set up a scenario. You have a router with two Ethernet ports. E0/0 goes to your webservers on 192.168.1.0/24, while E0/1 goes to SysAdmins’ PCs on 192.168.2.0/24. We want to restrict all SSH access from 192.168.2.0/24 to 192.168.1.0/24 to 8k of bandwidth. That’s right, you don’t like your admins and want them to struggle with their daily tasks. Let’s set it up.

Remember the steps? ACL, class-map, policy-map, interface. First, create an ACL that matches the traffic you want to police.

ip access-list extended SQUASHSSH
permit tcp 192.168.2.0 0.0.0.255 192.168.1.0 0.0.0.255 eq 22

Next, create a class-map that matches that traffic.

class-map SQUASHSSH-CMAP
match access-group name SQUASHSSH

Create your policy-map. This is where you actually set the bandwidth restrictions.

policy-map SQUASHSSH-PMAP
class SQUASHSSH-CMAP
police 8000 conform-action transmit exceed-action drop

Last time, we just did a set dscp af43, but this time we actually take action on a packet instead of just marking it. The long lines does three things — it sets the ceiling to 8000 bits per second, transmits if the rate is below that number, and drops if its exceeded. So, if one of the admins tries to SCP a file over, he’s gonna be in for a surprise with only an 8k window to use. If he goes over 8k, the packets are just dropped, and it’s up to TCP to retransmit. It’ll take a while. Remember, too, that this policy applies to anything matching the ACL, so, if two admins are SCPing files at the same time, they have to share that 8k. Wow, you’re mean!

Setting it up on the interface is pretty straightforward. It’s just like we did before.

interface E0/0
service-policy output SQUASHSSH-PMAP

I set this up in a dynagen lab to test it, but, instead of SSH traffic, I policed ICMP. If I did a normal, 5-packet ping, it worked great. If I sent a 100-packet set, drop, drop, drop. Or if I increased the size of the packets. Just to be sure it was the service-policy doing the dropping, I sent a 100-packet set and removed the policy from the interface; it missed a few at first, but, when the policy was removed, it zoomed right along without a single miss or hesitation.

NOTE: I do not condone policing traffic from your coworkers machines to 8k, but it would be funny.

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I’ve been trying to get some experience on Cisco VOIP, and, as you probably know, Quality of Service (QoS) is quite important in that realm. Since VOIP is very time-sensitive, you have to be sure your gear delivers the voice packets first. A packet in an HTTP transaction can wait another 200ms without any problems. A voice packet with another 200ms on it means static and digital artifact on the line. Not good. There are lots of things you can do in the world of QoS, but I’ll talk about tagging this time (I may get to some of the other topics later, though).

What is tagging, though? There is a header in an IP packet that, paraphrased from the standard, set the importance and drop-sensitivity of the packet in regard to the flow/application/data stream. A ping packet is quite unimportant in the grand scheme of things, and dropping the packet will have very little impact on the stuff going on in the network. Voice, however, needs every packet to arrive in order, so each packet’s importance is high and a drop hurts. To differentiate between these two, we set the Differentiated Service Code Point (DSCP) value in the pakcet. This is just the packet header thing I was talking about earlier.

Let me pause for a second to discuss something, though. Just tagging the packet does nothing to prioritize the packet. I can tag a packet with whatever I want, but, if a router or switch isn’t configured to use the tag, my packet just goes into the FIFO queue like everything else. Actually doing something different to a tagged packet is one of the many other parts of QoS that I may get to later.

Time to configure? Of course. Like everything in the Cisco world, it always usually starts with an access-list to define interesting traffic. You then create a class-map that defines what traffic you want to tag (usually with the ACL you created), then you create a policy-map that defines what to do with that traffic. Finally, you apply the policy-map to the interface.

Let’s try one. Let’s say we just put in a new VOIP system and want to tag the packets appropriately. The IP of the system is 10.0.0.10, and you want the gateway on that network to tag the packets as AF41 as they come into it (Google up DSCP if you want to learn about the AF41 thing). What comes first? The ACL…duh!. I like named ACLs, so we’ll create one called VOIP.

ip access-list extended VOIP
permit ip host 10.0.0.10 any

Time for the class-map. All that a class-map is used for is to create a list of values that are interesting to you, but a list can have an or or an and associated it. Basically, do you want to match all the items or just any of them. In our case, we just want to match the ACL we created (a single item), so it’s no big deal, but keep in mind that you can use the match-all or match-any keywords if you want. Here’s the class-map to match our ACL.

class-map match-any MYCLASSMAP
match access-group name VOIP

Next is what we want to do to the traffic, which is tag it to AF41. In the policy-map, you actually reference the class-map you made, so keep that in mind when you’re configuring.

policy-map MYPOLMAP
class MYCLASSMAP
set dscp af41

That says that anything that matches the class-map MYCLASSMAP gets tagged to AF41. Now we have to apply it to the gateway. Assuming that’s F0/0 of your router, give this a shot.

interface F0/0
service-policy input MYPOLMAP

That’s pretty much it. Now, when a packet lands on F0/0 that matches the ACL, it will be tagged as AF41 and sent on. I say again that this just tags packets. If you want to actually prioritize those packets, you have to do more, but I’m too sleepy to get into that right now.

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