Posted: March 8th, 2010 | Author: charlie | Filed under: Networking | Tags: cisco, networks, performance, VoIP | No Comments »
WANs often need Quality of Service (QoS) configured to ensure that certain traffic is classified as “more important” than other traffic. Until now, it took a serious Cisco guru to configure a network properly for VoIP if the network was at all bandwidth constrained. AutoQoS, a new IOS feature for Cisco routers, makes deploying VoIP easy, even on busy WAN links. In this article we’ll cover the basics, what AutoQoS does, and some of its limitations.
The first whack at AutoQoS was Cisco recognizing the need to simplify VoIP traffic prioritization. VoIP is especially sensitive to any latency, jitter, or loss, and users will notice problems. To ensure the best possible VoIP call, the network must ensure that lower priority 
traffic does not interfere with time-sensitive VoIP. AutoQoS can be enabled on both WAN links and Ethernet switches to automatically provide a nice best-practices based template for VoIP prioritization.
How it Works
QoS allows a router to classify which types of traffic are most important, and ensure that that traffic passed as quickly as possible. If necessary, other traffic will be queued until the higher priority traffic has had a chance to pass. Before a router can know when to queue versus when to attempt to pass all traffic, it must be configured with bandwidth settings for each link.
Configuring QoS on a Cisco router normally involves a complex series of interactions, which require understanding not only the protocols, but a router’s strange way of associating policies. The basic steps are:
- Use an ACL to define which traffic gets matched
- A class-map classifies matched traffic into classes
- A policy-map assigns priorities to the classes
- The policy-map is applied to the interface, which enables the processing of all packets through the ACL, class-map, and policy-map
Each of these “maps” are quite complicated and prone to error. Most sites are going to be duplicating effort because of common problems, like VoIP, needing QoS help.
Why AutoQoS
QoS configuration is not simple. It requires understanding the protocols your network interfaces are using, as well as the type of data you’re passing. To configure QoS for VoIP, for example, you must understand how VoIP works. In short, it requires a guru. If you’re like me, you literally giggled out loud the first time you encountered the word, “AutoQoS.”
AutoQoS enables any network administrator to just “turn on” a solid solution for ensuring VoIP is happy. VoIP is the pain point for most organizations, so that’s what Cisco focused on first, and that’s what we’re focusing on here. Given the limited scope of AutoQoS, it’s believable that it works well enough. In reality, QoS configurations generally classify many types of traffic, and then place a priority on each one.
The main benefit of AutoQoS is that administrator training is much quicker. It also means that VoIP deployments often go much smoother, and upgrading WAN links isn’t usually required. Finally, AutoQoS creates templates that can be modified as needed and copied elsewhere for deployment.
Limitations
Before talking about how to enable AutoQoS, which is literally three commands, let’s talk about where this works best, and what’s required to use AutoQoS.
First and foremost, you can only configure AutoQoS on a few types of router interfaces. These interfaces include:
- PPP or HDLC serial interfaces
- ATM PVCs
- Frame Relay (point-to-point links only)
Cisco catalyst switches also support an AutoQoS command to prioritize Cisco VoIP phones, but you cannot prioritize (using AutoQoS) generic VoIP protocols.
Next, there are some limitations with ATM sub-interfaces. If you have a low-speed ATM link (less than 768Kbps), then AutoQoS will only work on point-to-point sub-interfaces. Higher speed ATM PVCs are fully supported though. For standard serial links, AutoQoS is not supported at all on sub-interfaces. A quick litmus test to see if AutoQoS will work on your desired interfaces or not is to verify that the service-policy configuration is supported. If not, you’ll probably have to reconfigure some links.
AutoQoS will not work if an existing QoS configuration exists on an interface. Likewise, when you disable the AutoQoS configuration, any changes you may have made to the template after the initial configuration will be lost.
Bandwidth statements are used by AutoQoS to determine what settings it should use, so remember that after updating bandwidth statements in the future, you have to re-run the AutoQoS commands.
Making it Work
In the most standard situation, where VoIP isn’t performing as it was promised, the network admin can quickly save the day by running the following on the WAN interface:
interface Serial0
bandwidth 256
autoqos voip
If it’s the local network that needs tuning, the following can be run on Catalyst switches (if running Enhanced Images):
auto qos voip cisco-phone
auto qos voip trust
It really couldn’t be easier than that. For the WAN example, we told the router that interface Serial0 has 256 Kbps, and to enable VoIP QoS. The switch example is similar, for Cisco phones.
The neat part about this is that AutoQoS is actually doing more than just generating a configuration for you and forgetting about it. If you run the command show autoqos interface s0, you will see much more than just your standard old interface configuration. It will show that a Virtual Template “interface” has been created, and that a class is applied to the interface. The same output will also show you the configuration of the template and class-map, with an asterisk next to each entry that was generated by AutoQoS. It’s actually keeping track of what was done automatically so that you can learn what AutoQoS is doing. As mentioned previously, however, don’t forget that removing the AutoQoS configuration will destroy all QoS settings on an interface, not just the ones that AutoQoS configured.
Finally, remember to enable QoS on both sides of a WAN link to truly prioritize VoIP packets. Don’t forget to read through the Cisco documentation before deploying it, even though AutoQoS is simple, in comparison. It is simple, but the more prepared you are the easier it is to deploy.
Cisco will hopefully continue this trend of providing Auto features for complicated, but common tasks. AutoQoS for VoIP sure does enable a much larger audience to correctly deploy VoIP over a wide variety of networks.
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Posted: February 27th, 2010 | Author: charlie | Filed under: Networking, Networking 101 | Tags: ccna, cisco, education, networks | 3 Comments »
What’s the point of creating subnets anyways? How do I remember those strange looking subnet masks? How the heck does this work with those crazy looking IPv6 addresses? This edition of Networking 101 will expand on the previous Subnets and CIDR article, in the interest of promoting a thorough understanding of subnetting.
An oft-asked question in networking classes is “why can’t we just put everyone on the same subnet and stop worrying about routing?” The reason is very simple. Every time someone needs to talk, be it to a router or another host, they have to send an ARP request. Also, there’s broadcast packets that aren’t necessarily limited to ARP, which everyone hears. When there are only 255 devices on a /24 subnet, the amount of broadcast packets are fairly limited. It is important to keep this number low, because every time a packet destined for a specific host or a broadcast address is seen, the host must handle the packet. A hardware interrupt is created, and the kernel of the operating system must read enough of the packet to determine whether or not it cares about it.
Broadcast storms happen at times, mainly because of layer 2 topology loops. We’ll explain layer 2 topology issues in excruciating (actually, enlightening) detail in a future issue. When thousands of packets hit a computer at a time, slow and fast computers alike can become very slow. The kernel spends so much time handling interrupts that it doesn’t have much left for dealing with “trivial” things like making sure your web browser process gets a chance to run. So that, my friends, is why subnets are very important. This is also known as a broadcast domain, because it limits the amount of broadcasts that you will hear.
The natural follow-up question normally involves a host’s notion of a broadcast address and netmask. We hopefully understand that a host needs to understand what computers are on the same subnet. Those IP addresses can be spoken to directly, making a router unnecessary. When the netmask or broadcast address is incorrectly configured, you’ll quickly find that some hosts are unreachable.
The most common erroneous configuration happens when someone configure an IP address without specifying the netmask and broadcast address. For some reason, most operating systems don’t take the liberty of updating these things, even though one can be determined from the other. If you run ‘ifconfig eth0 130.211.0.1 netmask 255.255.255.0′ you might expect that everything is ready to go. Unfortunately, it’s very likely that your broadcast address was set to 255.255.0.0. It largely depends on the router’s configuration, but normally this results in all broadcast packets being dropped. Conversely, if the netmask is configured incorrectly, the computer wouldn’t know where the subnet starts and begins. If a computer thinks a host is on the same subnet when it actually isn’t, it will attempt to ARP for it instead of the router. Routers can be configured to handle this and pretend they are the host (called Proxy Arp), but normally the result is unreachable hosts.
Understand how the netmask is configured, to avoid this problem. Figuring out the network and broadcast address isn’t very difficult when you remember that the netmask simply means “cover some bits,” but deciphering netmask representation can induce a double-take. The netmask for a /24 network is 255.255.255.0, that’s easy. But what does 255.255.240.0 mean? The best way to decipher it is to begin with the masked off part. Comparing it to the /24, which had three octets masked, we see that 255.255.240.0 has two octets masked, and part of another. We know it’s between a /16 and a /24. We have to understand binary, and realize how many bits are masked. The last 16 bits are clearly part of the network portion. The third octet, 240, allows 16 IP addresses beyond the mask, so it must mean that four bits are left (2^4=16). The four remaining bits, plus the 16 bits used for the first two octets means that we’re dealing with a /20!
What about 1.0.0.0/255.255.255.248? We’re definitely in a land smaller than the /24 subnet. If we look at the remaining bits in the last octet, we can see that there are eight IP addresses available. Remember that only 2^3 can make eight, so we’re using all but three bits in the network portion. This is a /29 network. Of course, the easy ones are pretty clear: 255.255.255.128 allows half as many host addresses in the last octet compared to the /24 network, so it’s a /25.
On the topic of confusing netmasks, IPv6 addresses certainly have a place. The netmask isn’t really an issue–the same concept applies, just with larger numbers to remember. The real problem lies within the address representation itself; the IETF seemed to take pride in creating confusion. Typically an IPv6 address is represented in hex, or base-16. Our old friend IPv4 could represent an IP address in hex too, which would look like B.B.B.B for the address 11.11.11.11. Unfortunately, IPv6 isn’t quite that nice looking. To represent 128 bits, IPv6 normally breaks up the address into eight 16-bit segments.
An IPv6 address looks like: 2013:4567:0000:CDEF:0000:0000:00AD:0000. It does get a bit easier. For example, leading zeros are not written, and contiguous quads of zeros get collapsed to ::. Trailing zeros ,however, must be shown. This is a bit confusing, but the rules always allow for a non-ambiguous IP address. Leading zeros in each quad can always be removed, but the collapsing of contiguous blocks of zeros can only happen once per address. The above address with collapsed zeros will look like: 2013:4567:0000:CDEF::AD:0000. IPv6 provides 2^128 addresses, more than enough to allocate roughly 1000+ IP addresses per square meter of the earth.
If you remember the rules of binary, the address representation rules with IPv6, and a few simple subnets for reference, you’ll be Master of Subnets – the one who everyone asks for help.
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Posted: February 18th, 2010 | Author: charlie | Filed under: Networking | Tags: ccie, ccna, cisco, networks | 2 Comments »
Cisco Flex Links gives network operators a simple, reliable, and more scalable method of layer 2 redundancy. The Spanning Tree Protocol (STP) is not destined for the scrap bin, but it will certainly fall out of favor with many enterprise networks.
Flex Links are a pair of layer 2 interfaces configured to act as a backup of each other. Configuring Flex Links is very simple, but it’s a manual process. Spanning tree can configure itself if you just enable it, albeit likely a sub-optimal configuration, but a working one nonetheless. Flex Links, on the other hand, require manual setup and layout of your layer 2 network. If you don’t want to leave anything to chance, then Flex Links are preferred over STP.
The benefits of FlexLinks include:
- simplicity, which equals stability.
- instant failover.
- rudimentary load balancing capabilities, so one link isn’t wastefully idle.
- load balancing works across switches in a stack, including port channels.
Flex Links’ primary operating mode is just like spanning tree: one on, one off. With per-VLAN spanning tree, a trunk port can have some VLANs enabled and some blocked at the same time, so on the surface it seems that STP is superior. In reality, you can configure Flex Links to load balance VLANs, and we’ll show you how shortly.
Configuration
Conceptually, you configure Flex Links by telling one link it’s the active link, and another that it’s the backup of that
Flex Links Design Map
primary (active) one. Without configuring VLAN load balancing, it will completely disable the backup, and if the active link goes down the backup will take over.
For example, to configure port gi1/0/1 as a active link, and gi1/0/2 as the backup, you’d run:
Switch# configure terminal
Switch(conf)# interface gigabitethernet1/0/1
Switch(conf-if)# switchport backup interface gigabitethernet1/0/2
That’s all there is to configuring the basic mode, which gets you failover but no load balancing. Before talking about load balancing, let’s take a look at preemption and “mac address-table move update.”
Preemption
Preemption, that is, the preferred port for forwarding traffic, is also configurable. This is most often used in combination with multiple links that have differing bandwidth capacities. If you wish to ensure that port 1, a primary port that has more bandwidth, will return to the active link when it comes back up, you would set: interface preemption mode bandwidth andswitchport backup interface preemption delay. The delay is used to set the amount of time (in seconds) to wait before allowing port 1 to preempt port 2 and begin taking over traffic again.
MAC Address-Table Move Update
Enabling the MAC address-table move update feature allows for rapid convergence when a primary link goes down and the backup takes over traffic forwarding duties. Without this feature enabled, neighboring switches may continue to forward traffic for a short time to a dead port, since they have learned MAC addresses associated with that link.
When move update is enabled, the switch containing Flex Links will broadcast an update packet to let other switches know what happened, and they will in turn un-learn that false MAC address mapping.
On the switch with Flex Links, simply configure:
Switch(conf)# mac address-table move update transmit
All switches, including ones with Flex Links, need to receive these updates. This is not enabled by default, so you’ll need to run the following command on all of your devices:
Switch(conf)# mac address-table move update receive
To see the status and verify that “move update” is enabled, run: show mac address-table move update. Checking the status of your Flex Links is much the same: show interfaces [interface-id] switchport backup.
Load Balancing
Flex Links should be configured such that both ports are forwarding traffic at the same time. This way, you get load balancing in addition to redundancy. The limitation is that only one port can be forwarding a single VLAN at a time. If we have VLANs 1-200, we need to choose which VLANs are forwarded primarily through which port. The most simple configuration, ignoring traffic requirements, would be that VLANs 1-100 use port 1, and VLANs 101-200 use port 2.
Before we get into configuring preferred VLANs, let’s talk about multicast. Multicast, of course, becomes an issue with this type of setup. If a port passed an IGMP join, and the switch is part of a multicast group, when the port goes down the switch will no longer be able to receive multicast traffic for that group. The quick fix is to make both Flex Links always be part of learned groups, with the command: switchport backup interface gigabitEthernet 1/0/12 multicast fast-convergence.
Now, on to VLAN load balancing. It is quite easy; just specify which VLANs you prefer on which links:
Switch(config-if)#switchport backup interface gigabitEthernet1/0/2 prefer vlan 101-200.
If you have VLANs 1-200 on the switch, show interfaces switchport backup will show you:
Vlans Preferred on Active Interface: 1-100
Vlans Preferred on Backup Interface: 101-200
If a link goes down, VLANs that are preferred on that interface will be moved to the other link in the pair. Likewise, when a link returns to service, its preferred VLANs are blocked on the backup and returned to the preferred link.
Be sure to run show interfaces switchport backup detail to see the full status, including link speeds, preemption modes, the MAC address-table move update status.
In summary, the simplicity of Flex Links make it a better choice for carrier and core enterprise networks over the ubiquitous spanning tree protocol. Link-level redundancy is had via STP, but with Flex Links you have more control and better load balancing capabilities. This certainly means that it takes longer to configure since you are planning the layer 2 network manually, but when you need a stable no-surprises link-layer network, Flex Links are definitely the way to go.
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Posted: February 16th, 2010 | Author: charlie | Filed under: Networking | Tags: ccie, ccna, cisco, networks | No Comments »
Let’s talk about TCAM hardware, Cisco SDM templates, and try to answer that elusive question: “why do I have to reboot my router to enable certain features, which in turn disables others?”
First, CAM stands for Content Addressable Memory. A CAM is a special type of memory; some would say the opposite of RAM. With normal computer memory (RAM) the operating system provides an address, and receives the data stored at the supplied address. With a CAM, the operating system supplies the data, and the CAM returns a list of addresses where the data is stored, if it finds any. Furthermore, a CAM searches the entire memory in one operation, so it is considerably faster than RAM.
CAMs are very expensive, so they aren’t normally found in PCs. Even router vendors will sometimes skimp, opting to instead implement advanced software-based searching algorithms to plod through RAM. Most commonly, CAMs and TCAMs are found in network processing devices, including Intel IXP cards and various routers or switches. The most commonly implemented CAMs are called binary CAMs. They search only for ones and zeros; a simple operation. MAC address tables in switches commonly get stored inside binary CAMs. You can bet that any
A Renesas TCAM
switch capable of forwarding Ethernet frames at line-speed gigabit is using CAMs for lookups. If they were using RAM, the operating system would have to remember the address where everything is stored. With CAMs, the operating system can find what it needs in a single operation. In this case desired data is the switchport that data should be sent out, based on the given MAC address, i.e. the essence of a MAC table. Some older Cisco switches running CatOS even opted to call this table the cam table, thereby causing great confusion across the land. Bridge table, forwarding table, mac-address table, cam table; it’s all the same.
Finally, a TCAM is a Ternary CAM. This allows the operating system to match a third state, “X.” The X state is a mask, which means you don’t care what it is. This naturally lends itself to networking, since netmasks operate this way. To calculate a subnet address we mask the bits we don’t care about, and then apply the logical AND operation to the rest. Being able to do this in hardware is a great benefit for routers. Additionally, routers can store their entire routing table in these TCAMs, allowing for very quick lookups. A router with routing tables in TCAMs can find the next-hop destination in a single operation every time instead of trying to search through a tree (or other data structure) in RAM.
Hardware can sometimes seem magic, but it isn’t always transparent. When configuring routers most people will run into a situation where enabling a new feature will require that the Cisco SDM (Switching Database Manager) template be changed. This template is actually a method Cisco uses to assign specific applications to specific TCAM resources.
Some routers will allow you to manually specify how much TCAM space you want to allocate to a specific feature. Others aren’t so nice. They make you choose from a few restrictive templates, which allocate the resources automatically based on a few predetermined settings. For example, on the Cisco 3750, we recently wanted to enable policy-based routing (PBR) to implement a layer 3 jail. The basic idea with template-only routers is that you have to choose where you want most of the optimizations, and compromise on the rest.
For this platform, there are four templates: default, routing, PBR, and VLAN. Each of these tries to allow for a bit more resources allocated to the specified task. For policy routing, we’d have to choose “routing” or “PBR,” which in turn limits the amount of unicast MAC addresses that can be held in TCAMs. Likewise, selecting a VLAN template will make PBR impossible, but allow for more VLAN database information to be held in TCAMs. There are always compromises when we need to use more advanced features. Keeping true with the spirit of router operating systems, there’s also some mysterious side-effects when a new template is chosen. On our specific router, if the PBR template is chosen, the router will become unable to support VPN routing/forwarding tables (VRF). The next unsightly gotcha is that with the IOS version that supports IPv6, you cannot even enable PBR. There is no template to allow both policy routing and IPv6.
Perhaps the main idea of TCAM allocation still isn’t clear. Just because, for example, 8K is allocated to routing tables, this doesn’t mean that you can only have a routing table of that size. There’s always the fallback of process switching. Process switching means that everything will be done by the processor instead of in hardware (TCAMs). Processor intervention is not desirable, mostly because it is much slower than hardware lookups. Also, the processor is supposed to be used for things like sending logs to a syslog server and controlling SSH sessions. If a router doing process switching gets really busy, it may be unable to service your console access attempts. Generally speaking, the more expensive the router, the less it will use the processor.
Hardware is finite, and we always need more. More expensive routers don’t always suffer from the constant struggle for TCAMs because they have enough to support most features that currently exist. Unfortunately, most companies won’t want to purchase the latest and greatest router with seemingly endless hardware resources unless they can justify the added cost by showing a current need for them. So, most of us are stuck having to adjust TCAM allocations.
Further reading: an interesting blog from Plixer.
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Posted: February 15th, 2010 | Author: charlie | Filed under: Networking, Security | Tags: cisco, configuration management, Security | No Comments »
Ever wanted to make something “just work” in a secure and reliable way? We, too, have often thought that common configurations should just be selectable. The Cisco Security Device Manager(SDM) is a Java-based Web application for managing Cisco devices. It implements many management features aside from just security-related tasks, and it’s quite interesting. In this article we’ll explain what it can do, and why you might want to take it for a test drive.
Network admins can use SDM to generate Cisco TAC approved configurations with the click of a few buttons. It’s not just limited to simple configurations either. Some tricky configuration tasks such as QoS and VPNs also become easier with the SDM because it ensure that configuration errors don’t exist. In short, you can deploy new devices and services much quicker by using the SDM.
As the name implies, SDM also intently focuses on security. A feature called “one-click lockdown” will set your router up as Cisco recommends—a good starting point for new routers. Also, the security audit function of the SDM will check your configuration and offer up a surprisingly large set of recommendations for hardening security. Many are things that most administrators don’t worry about, but with the SDM you can easily click “fix it” for each item after reading a description. There’s no reason to leave any possible vulnerability open when you have a quick, easy GUI manager pointing out what should change.
Cisco SDM user interface
The SDM is also a management console that gives you a real-time look at your device. It provides a nice interface for viewing system logs, firewall logs, and even real-time performance statistics. You probably already gather performance data via SNMP for historical charting, but being able to see the real-time information while you’re logged into the device manager, where you can also make changes to the configuration, is quite convenient.
SDM is available for most IOS-based routers running 12.2 and above. It is install by downloading a zip file from Cisco and copying it to the router’s flash memory. It’s then accessed from your Web browser (Firefox or IE required, as well as certain Java versions).
Making it Work
First, we must point out that using the SDM requires that you enable the HTTP server on your device. Yes, most Cisco security holes involve the Web server, and yes, a Web spider can easily DoS your router if it starts crawling Web pages and runs it out of RAM. Fortunately, both of these are negligible if you don’t allow access to the Web server from external networks. So first things first, enable: ip http secure-server, then configure ACLs to limit access properly.
After unzipping the file downloaded from Cisco, you can browse to: https://$server/flash/sdm.shtml
Then, login with a highly privileged account (level 15 is required). Up comes the Java applet, and you’re in! It couldn’t be easier than that.
Features
At the top, you’ll see things like Wizard, Advanced, and Monitor. The left had side lists things you can do in Wizard mode, and includes things such as VPN, Firewall, and LAN configuration options.
At the top you’ll also see a “deliver” button, which is another way of saying “commit.” All changes made within the SDM are committed to flash and merged into the running configuration when deliver is clicked.
Various configuration menus exist, most of which make the task at hand slightly easier. For the advanced administrator, it means you can just select options quickly without remembering the specific syntax. More junior admins can make previously confusing concepts work with little effort as well, and then look at the configuration that was generated.
The neatest feature is the security audit. When run, it will gather information about your device and then provide a list of problems. A nice “fix it” check box next to each item can be clicked, or you can elect to choose “fix all.” Beware that Cisco’s idea of security is basically very locked down. Selecting “fix all,” for example, will disable SNMP. It’s true that exposing SNMP to the external world is unwise, but you really do need it enabled for internal access.
You can also configure ACLs and interface parameters from within the GUI. Interfaces can be configured completely via the SDM, and the really nice part is that it lists all available setting for the particular interface. You’ll see check boxes for every option, along with a nice description of each option. ACLs can also be configured, and the GUI presents a nice view of which services will be allowed, and in which direction, on each interface.
In advanced mode, you can easily change many things, including OSPF and BGP settings. It’s just a matter of a few clicks to add another OSPF process ID or add another network to an existing one. Being able to see networks each OSPF process advertises and configure passive interfaces in a single well laid out window is very exciting.
In Monitor mode, you can see which interfaces are down, how much CPU is being utilized, and how much RAM is being taken up by which processes. Very useful information, sure to put a smile on your face the first time you see it.
The SDM does not support everything you’d want to do on a router, but the majority of common tasks are covered. It’s definitely a time-saver, learning tool, and convenience crutch all in one. Don’t feel bad using the SDM; convenience always outweighs prestige, assuming you can do it via the command line too. Enable the “show changes before delivering config” option to see what commands the SDM is about to run, and you’ll avoid surprises and possibly learn something at the same time.
No Comments yet... be the first » Related posts:
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