Blog Posts in September 2007
Stop Inter-VRF static route leaking
The MPLS VPN implementation on Cisco IOS has always allowed you to create VRF static routes that pointed to interfaces belonging to other VRFs. The feature can be used to implement interesting overlapping VPN (or common services VPN) designs, some of which are explained in the MPLS and VPN Architectures books.
However, quite often the ability to create inter-VRF static routes is considered a major security problem, as an operator configuration error could establish undesired inter-VPN connectivity. In these cases, use the no ip route static inter-vrf configuration command to prevent such routes from being installed in the VRF routing table.
Router as a TFTP server
Shaun needed an extra TFTP server in CCNP labs and asked whether you could use a router to act as one. The read-only (download only) TFTP functionality has been available in Cisco IOS for a long time, but the common wisdom was that you could only use the TFTP server function to serve current IOS image.
Fortunately, as of IOS 11.0, the function is more generic; you can serve any file residing on the router (you still cannot upload files), but you have to declare each file to be served with the tftp-server path global configuration command. You could even specify an alias to have the file available under a different name and attach an access list to each configured file to restrict its availability.
EEM syslog messages look like debugging messages
If you use EEM applets with action syslog commands, the messages generated by those commands will be displayed in the same format as the debugging messages, regardless of their severity. This might be a problem if you decide to use different timestamps for debugging and regular syslog messages (or disable debugging timestamps completely).
Five routers on your laptop
In case you've missed yesterday's post … the weather was just way too good to stay in the office :) However, even if I would decide to work on my routers, I could take them with me (well, the laptop would be a bit heavy and the sun was too bright) thanks to Christophe Fillot (Dynamips) and Greg Anuzelli (Dynagen).
In case you haven't heard about Dynamips/Dynagen yet: Dynamips emulates a variety of IOS platforms (from 2600 to 7200) on Intel platform and Dynagen provides friendlier user interface (more than friendly enough for me, probably too cryptic for GUI addicts). I've seen Dynamips a year or two ago, checked what it can do and decided to stay with the real routers in a remote lab environment. In the meantime, the software has improved drastically, allowing you to test all sorts of IOS features and topologies, as long as you don't expect QoS to work or real-time features to act in real-time (simulation is, after all, a bit slower than the real life).
Frame Relay congestion management
In the “good old days”, we've been teaching our students that although a router can act as a Frame Relay switch, it supports only the rudimentary functionality of switching the packets, but not the policing/marking features available in Frame Relay switches. That hasn't been true for a while - in IOS release 12.1T, Cisco has introduced the congestion management features. You can specify the congestion management per-interface (with the frame-relay congestion-management interface configuration command) and set the DE drop/ECN mark percentages for all PVCs on the interface, or you can set the parameters within a map-class.
Assigning Server IP addresses with DHCP
Using DHCP to assign server IP addresses is usually not a wise decision. To start with, you have to define static DHCP mappings, which rely on client-id attribute in the DHCP request (usually the MAC address of the client). For me, the easiest way to find the correct client ID is as follows:
- Use DHCP to assign the IP address to the server
- Note the newly assigned IP address
- Use the show ip dhcp bindings | include ip-address command to display the client-id to IP address binding.
- Create a static DHCP mapping (for example, by configuring a host DHCP pool on the router) and release/renew IP address on the server
CEF accounting
The "How could we figure out if any traffic uses the default route" challenge was obviously too easy; a number of readers quickly realized that the CEF accounting can do what we need (and I have to admit I've completely missed it).
However, when I started to explore the various CEF accounting features, it turned out the whole thing is not as simple as it looks. To start with, the ip cef accounting global configuration command configures three completely unrelated accounting features: per-prefix accounting (that we need), traffic matrix accounting (configured with the non-recursive keyword) and prefix-length accounting.
Increased Number of OSPF processes in MPLS VPN Environments
When we were writing the MPLS and VPN architectures books, there was a limit on the number of OSPF processes you could configure per PE-router. The limit was based on the fact that IOS supports up to 32 routing information sources. Two of them are static and connected; you also need an IGP and BGP in the MPLS VPN backbone, resulting in 28 OSPF processes that could be configured on a single PE router. This “feature” severely limited OSPF-based MPLS VPN deployments until IOS release 12.3(4)T when the limitation was removed, resulting in the availability of up to 30 routing processes per VRF.
RIP, BGP, and EIGRP never experienced the same limitations as you configure VRF-specific routing instances within address families of a single routing protocol
Logging to flash disk
Cisco IOS release 12.4(15)T brought (among a plethora of voice features) the logging to non-volatile storage, a nice-sounding name for the ability to write syslog messages into files on your flash memory (or an embedded disk, if you have one). To configure it, use the logging persistent [url directory] [size filesystem-size] [filesize logging-file-size] global configuration command:
- The directory argument specifies where you want the files to be stored (for example, flash:/logging).
- The filesystem-size specifies the maximum disk space the logging files can consume (once you exceed the limit, the oldest file is deleted)
- The logging-file-size parameter specifies the maximum size of each file (once the file grows too large, a new file is created).
Note: You can store the log files on the router's flash memory if it appears as a disk file system (check with the show file systems command). Wouldn't it be great if this feature would also work on USB drives ...
DNS resolver package for IOS Tcl
I've ported the dns package of the Tcl standard library to Cisco IOS. You can download it and install it on your router in just a few steps:
- Extract all the files from the ZIP archive and copy the Tcl files into a subdirectory on your router's flash (I would recommend you use flash:tcllib/dns).
- Configure the package initialization script with the scripting tcl init flash:tcllib/dns/pkgIndex.tcl global configuration command
To test the successful installation, start the Tcl shell from the command prompt and try to load the DNS package:
Static routing with Catalyst 3750: and the winner is …
The Static routing with Catalyst 3750 post has generated a lot of good, creative ideas. Some of the proposed solutions were better than the others and some were simply not implementable (but nonetheless, had great creative potential :). Here is my list of the favorites:
A routing protocol: as a few of you have rightly pointed out, this is the best choice.
Aggressive Unidirectional Link Detection (UDLD): this is my second favorite, as it's a reliable link-level mechanism that will detect a break in the fiber cable … exactly the right tool for the job.
Workaround: track the actual IP routing status of an interface
In a previous post, I've described how the track interface ip routing command reports incorrect interface state if you use IP Event Dampening feature. To track the actual IP routing readiness of an interface, you could use the following workaround:
- Create a static IP route pointing to the interface you want to test. Make sure this route is not redistributed into any routing protocols.
- Track the reachability of the static route
Get Creative: Static Routing with Catalyst 3750
Here's an interesting scenario:
We have two sites, each using a Catalyst 3750 switch, and routing between them using static routes. There's a primary fiber link between them and we're using twisted-pair-to-fiber converters due to port limitations on Cat3750. These converters do not report fiber link down status correctly (the carrier is still present on twisted pair even if fiber is down), so the primary Ethernet interfaces do not go down if the fiber link breaks and the primary static route is not removed, requiring manual action to switch over to the backup link.
The setup is summarized in this diagram:
Using Tcl packages on Cisco IOS
Although it's not exactly trivial, you can use standard Tcl packages with Tcl
shell on Cisco IOS by following this procedure:
- Install a Tcl interpreter on your workstation (use ActiveState's ActiveTcl in Windows environment).
- Collect all the source files needed for your set of packages into one directory on your workstation.
- Execute Tcl pkg_mkIndex command in that directory.
$ tclsh
% pkg_mkIndex . *.tcl
% ^Z
$
Don't miss the obvious
I've recently replaced my old home router (well, actually a combination of two low-end models, one could handle ISDN and the other one 3DES) with a 1812. After I've struggled past the “interesting” interface names (it has 8 switched ports, named FastEthernet2 to FastEthernet9) and brushed up my BVI/VLAN skills, configuring it was a breeze … only the DHCP server was causing me problems; every time my laptop would wake from the standby mode, it would take almost half a minute before it got the LAN IP address. The obvious suspect (as I've installed the 12.4(15)T on it) was the software, the next one DHCP ping timers.