Blog Posts in November 2007
We all know that BGP is meant to converge slowly… well, the MPLS/VPN service providers tend to disagree, as their users are not used to minute-long convergence times. One of the major components of slow BGP convergence is the time it takes a router to discover that a neighbor has disappeared. Traditionally, the BGP keepalive packets were sent every minute and it took up to three minutes to discover that a neighbor is down.
Of course you could fine-tune those times with the neighbor timers configuration command, but the reduced timers resulted in increased TCP traffic and consequently increased CPU load, which could reach tens of percents if the timers were set to a few seconds and the router had lots of BGP neighbors.
After I've fixed the default routing in my home office, I've stumbled across another problem: the two ISPs I'm using for my primary and backup link have DNS servers that reply solely to the DNS requests sent from their own IP address range:
When the traffic is switched from the primary to the backup ISP, I therefore also need to switch the DNS servers. Fortunately, this is quite easy to do on a router; you just need to configure ppp ipcp dns request on the dialer interface and the router starts asking for the DNS server address as part of the IPCP negotiation.
It's amazing how many options (most of them still undocumented) the show interfaces command accepts in IOS release 12.4T (I won't even start guessing when each one was introduced, if you're running old IOS releases, please feel free to comment):
- show interfaces description displays interface names, L1 and L2 status (line and line-protocol status) and interface description. Extremely handy if you want to check which interfaces are up/down.
- show interfaces counters protocol status displays the L3 protocols active on each interface.
- show interfaces summary displays the state of various interface queues and related drop counters in a nice tabular format.
- show interfaces accounting displays per-protocol in/out counters.
Here are a few sample printouts:
When I’ve been describing the limitations of kron, someone quickly asked an interesting question:
As I cannot insert extra input keystrokes with EEM applet, can I run a Tcl script from it with the action sequence cli command “tclsh script” command and use the typeahead function call to get around the limitation?”
The only answer I could give at that time was “maybe” … and obviously it was time for a more thorough test. The short result is: YES, you can do it (at least in IOS release 12.4(15)T1).
When two groups within Cisco needed time-based command execution in Cisco IOS, they (in a typical big-corporation fashion) decided to implement the same wheel from two different sets of spokes and rims. One group built the Embedded Event Manager with its event timer cron command (introduced in 12.2(25)S and 12.3(14)T), the other group created the more limited kron command set (introduced in 12.3(1)).
In my home office, I'm using DSL access to the Internet with ISDN backup to another ISP, as shown on the next figure:
Obviously, I would like the ISDN backup to kick in whenever the primary connection goes down; two static default routes and reliable static routing on the primary default seem like a perfect solution.
- Type-7 encryption used in enable password has been broken. Source code for the decrypt program and cracker programs are available online, or you could use a router to do it for you.
- The type-7 encryption is reversible (and easily breakable due to a weak algorithm), whereas type-5 encryption is a one-way encryption that probably requires a dictionary attack to break.
- Based on the previous two facts, you should never use enable password. Use enable secret.
- The service password-encryption encodes passwords attached to local usernames with type-7 encryption. The usage of type-7 encryption is necessary as you might need the cleartext passwords in some authentication mechanisms (for example, CHAP). However, it's still better to have scrambled passwords than cleartext ones; at least a casual observer will not be able to read them. Conclusion: use service password-encryption.
- If your authentication methods don't need cleartext passwords (examples: local username/password authentication, local AAA authentication or PAP authentication), use username secret configuration command (available from IOS releases 12.2T, 12.3 and 12.0S).
interface Serial1/0… and this is the “server”-side configuration:
ip address negotiated
ppp authentication pap optional
ppp pap sent-username client password 0 client
interface Serial1/0To trigger PPP negotiations, shut down and re-enable the serial interface on either side.
ip address 10.0.0.33 255.255.255.252
peer default ip address 10.0.0.34
ppp authentication pap callin
username client password client
Note: As I'm using PAP authentication, I could use the more secure username secret configuration command, which would not work with CHAP.
A while ago I was asked to write an article about IPv6 training. I could just cover the training aspect, like what’s offered (answer: not much) and whether someone can train the whole operations team like you could in the IPv4 or MPLS/VPN world (answer: no), but I wanted to understand whether anyone is really using IPv6 in a production network.
I found a few academic networks (after all, there are about 2000 IPv6 prefixes assigned and someone should be doing something with them), but not much of what I would call a real production environment, which is a bad thing, as it looks like the IPv4 address space will get saturated in a few years.
Martin Kluge sent me an interesting BGP question: he has two upstream links and runs BGP on both. Since his router is low on RAM, he cannot accept full routing, so he’s just announcing his IP prefix and using static default routing toward upstream ISPs.
We'll turn on type-7 encryption for local passwords and generate a test username
R1(config)#username test password t35t:pa55w0rd
Next we'll inspect the generated username with the show running command
R1(config)#do show run | include username
username test password 7 08351F1B1D431516475E1B54382F
Now we'll create a key chain and enter the type-7 encrypted password as the key string …
R1(config)#key chain decrypt
R1(config-keychain-key)#key-string 7 08351F1B1D431516475E1B54382F
… and the show command does the decryption for us.
R1(config-keychain-key)#do show key chain decrypt
key 1 -- text "t35t:pa55w0rd"
accept lifetime (always valid) - (always valid) [valid now]
send lifetime (always valid) - (always valid) [valid now]
- The [0-9.]+% pattern will match any non-zero percentage;
- The 0.00% pattern will obviously match the zero-percentage display;
- As the percentage figures are separated by various amounts of whitespace characters, we have to use the ' +' pattern to match those;
The real solution is based on the appl_setinfo and appl_reqinfo calls. They work, but like many other Tcl-related IOS features they are … well … weird.
Thinking about the implications of bit stuffing I wrote about in the SDLC post, I realized that long sequences of ones would be transmitted slower than long sequences of zeroes due to an extra bit being inserted after every fifth consecutive one. The theory would predict a 20% decrease in transmission speed.
Of course I wanted to test this phenomenon immediately. I connected two routers with a low-speed (64 kbps) link, and started a series of pings. Not surprisingly, the results confirmed the theory:
The link to this white paper has been published in Joe Harris' blog.
IBM obviously also had problems with bad modems and solved it with the NRZI encoding that was part of SDLC standard (and a major pain in the good old days when the appliques on the old Cisco routers did not support it and we've been trying hard to penetrate IBM accounts). You can still configure NRZI encoding on most routers' serial links (it might depend on the actual hardware platform) with the nrzi-encoding interface configuration command (you had to do it with jumpers in the AGS+). Incidentally, changing interface encoding to NRZI was really helpful when you had to break things in the preparation for the troubleshooting part of the original CCIE lab).
Enough theory, let's summarize the proposed solutions:
- The nrzi-encoding (if available) is the best one, as it reliably solves the problem, is transparent and does not incur additional overhead.
- Compression or encryption are OK, but they result in significant CPU overhead (unless you have hardware encryption/compression modules) and might (at least in theory) still produce a long sequence of zeroes, although with a very low probability. IPSec also introduces overhead due to additional IPSec headers.
- LFI (effectively multilink PPP over a single link) is also a good solution, as the PPP framing and MLPPP headers break the long sequences of zeroes (you might have to fine-tune the fragment size with ppp multilink fragment size configuration command), but it introduces overhead on the WAN link.
- IP fragmentation would work, but would be quite bandwidth-consuming. If the fragmentation would be performed by the router, the overhead would be 20 bytes per fragment (IP header), if the sending host performs the fragmentation, the overhead is 40 bytes per fragment for TCP sessions. For example, if we reduce the IP MTU size to 256 bytes, the TCP session overhead is over 18% (and we were scoffing at the ATM designers that made us live with 10% overhead).
- The invert data command would only help if the modem has problems with long strings of zeroes, not with long strings of the same value.
- The tunnel key command just sets a 4-byte field in the GRE header but does not affect the encapsulated data at all.
William Chu sent me a working configuration he uses to measure jitter with the IP SLA tool and react to excessive jitter on the primary link. First you have to create the jitter probe with the IP SLA commands:
ip sla monitor 3000
type jitter →
dest-ipaddr 220.127.116.11 dest-port 12333 →
source-ipaddr 18.104.22.168 codec g729a →
Note: The continuation character (→) indicates that the configuration command spans multiple lines
Next you have to define the IP SLA reaction to excessive jitter. William configured his router to react when the jitter exceeds 300 milliseconds and returns back to normal when the jitter falls below 290 milliseconds (some hysteresis is always a good thing).
ip sla monitor reaction-configuration 3000 →
react MOS threshold-value 300 290 →
threshold-type consecutive →
As the last step in the SLA configuration, you have to start the probe:
ip sla monitor schedule 3000 →
life forever start-time now
After the SLA probe and out-of-bounds reaction have been configured, the router will generate syslog messages whenever the jitter gets above the threshold as well as when it falls below the second threshold. You can then use the EEM applets to act on the syslog messages:
event manager applet MOS-Below
event syslog occurs 1 period 120 →
pattern "Threshold below for MOS"
... actions ...
event manager applet MOS-Above
event syslog occurs 1 period 120 →
pattern "Threshold exceeded for MOS"
... actions ...
And now two questions for you:
- What could you do on the router to fix this problem?
- Why was the synchronization retained when transmitting a long string of ones?
Did you believe MPLS TE was a quality-of-service feature? Did someone persuade you it’s mandatory to run OSPF or IS-IS if you want to deploy MPLS TE? I’ve collected a few more myths like these two and explained the actual facts behind them in 10 MPLS traffic engineering myths and half truths published by ComputerWeekly..
Warning: Due to total lack of any security features in TFTP protocol, use this functionality only in lab environment.