Description
What is the minimum recommended length of twisted pair copper cable that I should use with my SonicWall firewalls’ HA ports for high availability?
Resolution
Question:
What is the minimum recommended length of twisted pair copper cable that I should use with my SonicWall firewalls’ HA ports for high availability?
Answer:
At present, there is no officially published minimum length for a twisted pair cable from IEEE or ANSI, though there is information about maximum lengths for twisted pair cable.
However, sometimes customers want to use a cable that is only several inches in length for this. The reason why is because it looks neat and tidy, and it’s one less cable that has to be strung through a rackmount cable channel. However, using such a small cable often causes problems. Customers have reported issues where the firewalls appear to lock up, and they can neither be managed, nor can they pass traffic.
The reason why this occurs, is because the extremely short HA cable causes problems with the transmissions of the HA heartbeats. In an HA pair, when the idle unit does not receive heartbeats for the configured interval and time threshold, it will go active. However, if the other unit in the HA pair is still active, both units’ interfaces will be competing for the addressing, which means there is an IP conflict between all of each firewall’s interfaces. This will prevent the devices from being managed, and it will also prevent them from passing traffic.
With regards to cable length, the following needs to be considered:
—
1.) Crosstalk
Crosstalk is when a signal sent on one circuit interferes with another signal sent on a separate but adjacent circuit. This is usually caused by circuits being close together. With ethernet cabling, this effect is reduced by twisting the circuit pairs. This reduces the circuits’ ability to interfere with one another while traveling the length of the copper media. With an extremely short cable, there is usually not enough twisted pair to prevent crosstalk interference.
2.) Return Loss
Return loss is essentially the loss of a signal’s power which is returned or reflected by a discontinuity in the cabling (ie: a point in the transmission line where the signal cannot conduct fully to the next leg of the pathway). It is desirable to have a high level of return loss (ie: the loss of reflectivity). Low return loss can be caused by problems at the termination point of the cable, or by a device which is in line with the transmission pathway. A shorter cable presents a potential for lower return loss, because there is less wire to degrade the reflection of signals.
3.) Cable Quality
The quality of cabling will vary from vendor to vendor, depending upon how accurate the equipment is which is used in the manufacturing process. Some vendors do not twist their cabling as effeciently as others do, and some have lower-quality crimps than others. Cables which are crimped by individuals often suffer greatly by comparison to manufactured cables, as one can only be so precise with a hand-crimping tool. The most common problem with custom cables is a loss of twisting near the termination point of the cable. Most vendors who make cables less than half of 1 meter in length do not have those cables certified by any standards body.
For high availability, SonicWall support recommends using a patch or crossover (NSA units have MDIx autosensing capabilities on their interfaces) cable which is no shorter than 1 meter in length (about three feet). There are many posted discussions on this topic available to read online, however, this post from a Fluke Networks employee at forucms.bicsi.org sums up these discussions very well.
http://forums.bicsi.org/Topic2210-4-1.aspx#bm2215
—-
“If you are talking specifically about patch cords, then 0.5 m is the implied minimum length in ANSI/TIA/EIA-568-B.2-1 for a certified patch cord. That’s because the math for the limit lines really does not work below this. Infact, getting a certified patch cord of 0.5 is going to be tricky. Many vendors only offer a certified patch cord of 1.0 m or longer.”
Related Articles
- Why is my SonicWall’s power, test, or alarm LED blinking?
- Sonicos 7.1.1 FAQ
- Microsoft Teams randomly dropping (Video conferencing applications)