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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- What the Heck is a TCAM?
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.
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Posted: February 14th, 2010 | Author: charlie | Filed under: Networking, Security | Tags: ccna, cisco, networks, Security | No Comments »
Another new feature available in IOS (12.3) is Cisco’s Intrusion Prevention System. An IDS has been part of IOS for a long time, but they recently took it a step further. As part of its Self-Defending Network campaign, Cisco realized that an IPS should be integrated into the network fabric. We’ll explain what this means, and show you how to implement it.
Actively preventing the attack makes it an IPS. The standard old IDS solution means that it can detect and alert, but blocking attacks is not normally part of an IDS’s feature set. Thus, if you want to prevent attacks rather than just receive alerts, you need an IPS. Cisco’s IPS works like any other: you get a signature file, called the Signature Definition File (SDF) by Cisco, and if the IPS finds that a packet matches a signature, it’s blocked.
There are appliances, Catalyst switch modules, and router modules, but IPS is also built-in to certain IOS images now. Since Cisco claims IPS features won’t impact router performance (since the latest release), it may be possible to skip the purchase of a dedicated module for IPS.
The catch, of course, is that an IPS is not robust without constant signature updates. Attacks are constantly evolving, and without an update you aren’t protected against the latest and greatest attacks. Something completely new could sneak in, but the idea is that after the first few attacks Cisco will update the SDF and you’ll be notified that it’s time to download a new version. That’s right, you have to manually download and install a new signature file. This requires a subscription service above and beyond what you pay for SMARTnet. Services for IPS, as it’s called, provides SDF updates and the other features (support, warranty) that SMARTnet does as well. Accordingly, your SMARTnet contract is discounted when you purchase a Cisco Services for IPS contract, according to Cisco’s Q&A documentation.
Configuring IPS for Sensor Modules
There are many different cases for configuring IPS depending on your device. First, we’ll show you how to enable it on any IPS sensor module that uses the IPS 5.1 or later, then we’ll show you how to take advantage of the IOS built-in default IPS features.
The IDS Device Manager (IDM) is a graphical interface for configuring all IDS (and IPS) functionality. If you prefer that, then refer to the Cisco documentation after reading about how it’s done via the CLI here.
The general idea we’re working with here is called the VLAN pair method. This means that we’ll configure two VLANs in a pair group, and all traffic received by a sensor will be inspected and either forwarded on to the other VLAN, or dropped. Up to 255 VLAN pairs can be configured on most sensors.
First we enter configuration mode, then the service interface, and finally select the physical interface that we wish to configure:
sensor#configure terminal
sensor(config)#service interface
sensor(config-int)#physical-interfaces GigabitEthernet0/1
Next, we must configure the VLAN pair (and give it a meaningful description):
sensor(config-int-phy)#subinterface-type inline-vlan-pair
sensor(config-int-phy-inl)#subinterface 1
sensor(config-int-phy-inl-sub)#vlan1 10
sensor(config-int-phy-inl-sub)#vlan2 11
sensor(config-int-phy-inl-sub)#description vlans 10 and 11
Conceptually, the interface will now be added to a virtual sensor, and once it’s enabled it will monitor traffic. We now need to enable a virtual sensor:
sensor(config)#service analysis-engine
sensor(config-ana)#virtual-sensor vs0
Once that’s completed, we simply add the previously-defined subinterface to the sensor, and we’re done:
sensor(config-ana-vir)#physical-interface GigabitEthernet0/2 subinterface-number 1
Configuring IPS for IOS
You can enable IPS features in IOS using the default SDF. Signatures may be added manually to the SDF, or you can pay Cisco for the latest signatures.
First we need to enable what’s called Security Device Event Exchange notifications:
router(config)#ip ips notify sdee
Then we must configure an IPS rule name that will be used for associating with interfaces.
router(config)# ip ips name MYIPSRULES
The next step is to specify where the SDF file will come from. The following command specifies that the file 256MB.sdf can be found in flash memory. You can also specify tftp or any other protocol your Cisco knows how to handle, but it’s best to use flash memory to ensure no dependencies on other servers.
router(config)# ip ips sdf location flash:256MB.sdf
Finally, we simply enable IPS on the interface (in both directions). It is also a good idea to enable IP reassembly on the interface, so that the IPS rule can evaluate entire IP packets at once.
router(config)#interface fastEthernet 0
router(config-if)#ip ips MYIPSRULES in
router(config-if)#ip ips MYIPSRULES out
router(config-if)#ip virtual-reassembly
Now you have a working IPS, based on the file in your flash called 256MB.sdf. That file must be downloaded from Cisco using your CCO login linked to a valid support contract.
The Power of Community
If you don’t feel like paying Cisco for signature updates, you can update the SDF yourself. When a new attack surfaces, you’ll often find Cisco IPS XML signatures posted to various online forums. You can and should use them.
To view your current SDF version, you can run: sh ip ips signatures
To merge the IPS SDF configuration with new information, you can copy in an XML file. Just like copying in any configuration snippet, the updates will be merged, not replaced. Say we got sigs.xml from a helpful network operator. To enable these signatures, we simply run:
router#copy tftp://serer.fqdn/sigs.xml ips-sdf
That’s it! You’ll see that 256MB.sdf on the flash memory is now a bit larger. It’s a good idea (and is recommended by Cisco) to rename 256MB.sdf to avoid confusion, now that you are no longer running a Cisco-sanctioned version.
Enabling IPS on supported routers is quite easy, but can lead to some interesting troubleshooting sessions. Be sure you have a syslog server that your routers all log to: it will save hours of work. Also, search around; you may find a source for XML updates that you wish to trust, and then it’s pretty easy to automate daily merges into your local SDF.
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Posted: February 13th, 2010 | Author: charlie | Filed under: Networking 101 | Tags: ccna, cisco, networks | No Comments »
Networks don’t work without addresses: whenever you are sending something, you need to specify where it should go, and where it came from. To be an effective network engineer or administrator, you need to understand IP addresses backwards and forwards: you need to think be able to think on your feet. If something breaks, likely as not it’s some address assignment has been screwed up. And spotting the problem quickly is likely to be the difference between being the hero, or the guy who “takes a long time to fix the problem.” Before covering subnetting in the next Networking 101 installment, we’d like to thoroughly explore IP addresses in their primal form. This is crucial to understanding subnets.
IPv4 Addresses and 32-bit Numbers
IP addresses are just 32-bit binary numbers, but they’re important binary numbers: you need to how to work with them. When working with subnet masks, new network administrators generally get confused with the ones they haven’t memorized. All the subnet mask amounts to is moving the boundary between the part of the address that represents a “network” and the part that represents a “host.” Once you’re comfortable with this method of thinking about IP addresses and masks, you’re mastered IP addressing.
Binary is quite simple. In binary the only numbers are zeros and ones, and a 32-bit number holds 32 zeros and ones. We’re all used to base-10 numbers, where each place in a number can hold any number from 0-9. In binary each place holds either a zero or a one. Here’s the address 255.255.255.0 in binary:
11111111.11111111.11111111.00000000
For convenience, network engineers typically break IP addresses into four 8-bit blocks, or octets. In an 8-bit number, if all of the bits are set to 1, then the number is equal to 255. In the previous address, 11111111 represents 255 and 00000000 represents zero.
The way binary really works is based on powers of two. Each bit represents a different power-of-two. Starting at the left-hand side, the most significant bit, numbers form in the following manner:
| Power-of-two | 232 | ... | 27 | 26 | 25 | 24 | 23 | 22 | 21 | 20 |
| Decimal | 4,294,967,296 | ... | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
The result is additive, meaning that if all bits are set, you simply add the power-of-two value up for each place. For example, if we have an 8-bit number, 11111111 , we simply add: 27 + 2 6 + 2 5 + 2 4 + 2 3 + 2 2 + 2 1 + 2 0 = 255
Try a non-trivial example now: 11110000
We can see that four bits are “set” in the above 8-bit number. Summing the power-of-two values in those places yields: 27 + 26 + 2 5 + 2 4 = 240
It is just that simple. If you can convert a binary number to decimal form, you can easily figure out subnet masks and network addresses, and we’ll show you how in the next issue of Networking 101.
Focusing on 32-bit IPv4 addresses themselves now, there are a few different types that need to be understood. All IP addresses can be in the range 0.0.0.0 to 255.255.255.255, but some have special uses.
Loopback:
Packets that will not leave the host–i.e. they will not traverse an external network interface. Example: 127.0.0.1
Unicast:
Packets that are destined for a single IP address. Example: 2.2.2.2
Multicast:
Packets that will be duplicated by the router, and eventually routed by multicast routing mechanisms. Example: 226.0.0.2
Limited Broadcast:
A broadcast packet, sent to every host, limited to the local subnet. Example: 255.255.255.255
Directed Broadcast:
Packets that are routed to a specific subnet, and then broadcast. Example, assuming we are not on this subnet: 1.1.1.255
There are also some special cases of IP addresses, including private and multicast addresses. Addresses in the range 224.0.0.0 – 239.255.255.255 are reserved for multicast. Everything below that range is fair game on the Internet, excluding addresses reserved by RFC 1918 and a few other special-purpose assignments. These “1918 addresses” are private addresses, meaning Internet routers will not route them. The ranges include:
- 10.0.0.0 -10.255.255.255
- 172.16.0.0 – 172.31.255.255
- 192.168.0.0 – 192.168.255.255
These IP addresses can be assigned locally to as many computers as you want, but before those computers access the Internet, the addresses must be translated to a globally routable address. This is commonly done via Network Address Translation ( NAT ). The 1918 addresses aren’t the only reserved spaces, but they are defined to be “site local.” Multicast also has a reserved range of addresses that aren’t designed to escape onto the Internet: 224.0.0.0 – 224.0.0.255 are multicast “link-local” addresses.To give the necessary background for the next issue of Networking 101, we need to make sure everyone understands the concept of a local subnet. Once we have assigned a valid IP address to a computer, it will be able to speak to the local network, assuming the subnet mask is configured properly. The subnet mask tells the operating system which IP addresses are on the local subnet and which are not. If an IP we wish to talk to is located on the local subnet, then the operating system can speak directly to it without using the router. In other words, it can ARP for the machine, and just start talking. IP address and subnet mask configuration is fairly straightforward for general /24 networks. The standard 255.255.255.0 mask means that the first three octets are the network address, and the last part is reserved for hosts. For example, a computer assigned the IP of 10.0.0.1 and a mask of 255.255.255.0 (a /24, or 24-bits if you write it out in binary) can talk to anyone inside the range 10.0.0.1-10.0.0.255.Be sure to digest everything here, because in part two we’ll get to the meat of subnetting with CIDR.
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