External design and hardware


Firmware upgrade


Command line interface




This time our lab hosts ASUS RT-AC51U wireless router for review, which is a classic all-in-one device that was made to meet the needs even of the most exigent user. ASUS RT-AC51U supports a wide variety of modern and popular technologies. Among them are media server, the latest generation wireless network, cloud storage server, 4G modems and MFPs, and a couple of other small features that we will be talking about in detail in the next chapters.

External design and hardware

The package box contains the comprehensive information about  what the device is really capable of. It has a USB port as well as supports cloud technologies, current modems, and high-speed wireless network. Among other things, the vendor promises the device to support IPv6 and Windows 8 (although at the moment one really should implement support of Windows 10, which has been released on July 29, 2015). The box includes the device itself, power supply unit, eight-contact patch-cord, user's manual, warranty leaflet, and a CD with the electronic version of the instruction and the utilities. Unfortunately, the PSU is not meant for vertical placement unlike the one that comes with certain other ASUS routers. Apart from this, the user's manual claims that the PSU included in the box is vertical.

The external design of the router is very typical for all devices in this series. The only difference is that the surface is completely opaque and the antennae cannot be dismounted.

A user settings reset button, ON/OFF button, power socket, USB 2.0 port, adjustable WPS/Wi-Fi OFF button, WAN port, and four LAN ports are located on the device rear panel.

On the upper panel of the device there are LEDs that indicate the correct operation of all interfaces. Side panels and the bottom panel have ventilation grates located on them. Also, there are two X-shaped mounting holes used for wall mounting of the device located on the bottom panel.

The electronic stuffing of ASUS RT-AC51U wireless router is one green textolite card which has all essential elements located on both of its sides. On the lower surface there is a Spansion FL128SAIF00 flash memory module with the size of 16 Mbytes.

The system is powered by MediaTek MT7620A SoC CPU and MediaTek MT7610EN wireless chip. Winbond W9751G6KB-25 with the overall memory size of 64 Mbytes acts as the device RAM. We already encountered the same components in, for example, ASUS RP-AC52 repeater.

Now let's pass on to reviewing of the software capabilities of the device.


Upon the first booting of the device one can detect two wireless networks: ASUS and ASUS_5G. If the administrator decides to get connected to one of them, Windows will automatically offer him/her to adjust the device using WPS technology in a several simple steps.


If one decides to ignore this Windows notification upon connection to one of the networks, s/he will need to enter website in the browser for the initial setup of the router where s/he—following the same two-step process—can prepare the device for the first use by specifying web-interface and wireless network passwords.

Now let's pass on to reviewing of the firmware upgrade process.

Firmware upgrade

Firmware upgrade is carried out in Firmware Upgrade tab, Administration menu item. Firmware update may be carried out both in a manual and semi-automatic mode. In order to perform the latter one needs to be connected to the Internet. The whole firmware upgrade process takes about three minutes and does not require any technical proficiency from the administrator.

As of when this article was being written, there was an alternative firmware version by Padavan for ASUS RT-AC51U wireless router. The user can change to the above-mentioned firmware through the common upgrade process. However, it's worth noticing that when the upgrade process has been finished, one will need to reset all the device settings to factory defaults and adjust it manually once again.

Changing to the original firmware is done in Firmware Upgrade tab, Administration menu.

The same way, the user will need to reset the settings to factory defaults and adjust the device manually after changing to the original firmware. We have seen various complaints from users on the web that the router doesn't work after changing to the latest original firmware versions. If this is the case, one will need to boot the device upon holding WPS button, which will lead to complete reset of the user settings by the boot loader. We don't consider such behaviour of the device as problematic since the original firmware doesn't have to support configuration modes from other firmwares.

In case there was a failure during the firmware upgrade process, the firmware can be restored using Firmware Restoration utility.

Now let's pass on to reviewing the web-interface capabilities of the device.


Once the ASUS RT-AC51U has been rebooted, the administrator will be able to finetune it. Let's list all menu items of the web-interface: Network Map, Guest Network, Traffic Manager, Parental Controls, USB Application, AiCloud 2.0, Wireless, LAN, WAN, IPv6, VPN, Firewall, Administration, System Log, and System Tools.

As always, Network Map menu item is a user-friendly tool for access to all the frequently used network parameters and connected devices. Internet Status group shows the connection characteristics and lets one enable a capability of connection backup with the Internet through a connected 3G/4G dongle. One can specify the safety parameters and review the router utilization in the wireless network status.

Apart from all the common information, one can find out the vendor of a certain device by clicking on a MAC address in the client list. This information will be exact irrespective of the device firmware version since the request will be sent to the external MAC address database of the network equipment vendors. Also, one can assign a unique icon to every connected device and bind an address for it in a static manner.

Guest Network menu item is used to enable up to three guest networks for every frequency range (overall, there are six networks) with unique security settings, capability to adjust limitations of access to the intranet, and MAC address filter. It's nice that one can specify the lifespan of the guest network in order to not to worry about forgetting to disable it.

Traffic Manager menu item answers for management of QoS or the transferred data and monitoring of the traffic usage. QoS can be adjusted either in a manual or semi-automatic mode upon specifying the maximum speed according to a certain service plan.

Traffic monitoring features let one review the utilization on the wired and wireless networks in real time as well as review the traffic usage history during the last 24 hours or daily.

Parental controls let one set limitations on certain devices really easily (for example, on smartphones or tablets that children use) for entering the web according to a daily schedule. The access is limited completely without a possibility to create either black or white lists. On the page that contains parental controls settings there is a video that explains how to use this feature.

We believe that USB Application menu item is the most interesting one. It offers the administrator to use the USB port in six different modes. One can use all these modes at the same time upon connecting necessary devices through a USB hub. These devices may be printers, MFPs, modems, and external USB storage devices.

The device supports 3G/4G modems not just for the sake of appearance, ASUS RT-AC51U wireless router lets one use a modem as the primary or back-up channel that's automatically connected. The list of supported modems keeps on growing and the most widely-used models are supported. We had several modems like Huawei E5830 at hand, but we couldn't make them work even though they were detected successfully. It's worth noticing that these modems are not on the list of the supported models. Naturally, we notified the vendor about this and are expecting to see this modem on the list of supported devices in the next firmware versions. The latest firmware versions support operation of Android phones as a 3G/4G modem.

There are two printing protocols supported for printers: LPR and ASUS EZ. There's nothing difficult about the former, but the latter calls for some explaining. It's an easy-to-adjust software for Windows that lets one use the scanner in the MFPs, too. Contrary to what had happened with the modem support, our experience with the printer support turned out much better. And though Epson L800 we used was not on the list of supported device, however there were many other Epson L series printers, we could make the printer work after setting it up using the instructions located on the ASUS customer support website.

And, finally, support of USB sticks came to be the most elaborate. ASUS RT-AC51U wireless router can be used in order to provide public access to the user data through FTP and Samba as well as manage access to them over the web or mobile app.

Among other available features are installation of Download Master and Media Server applications. The latest versions of the software will be downloaded from the web and installed in the selected USB drive. The first application can be used to download files from HTTP and FTP resources, BitTorrent networks, eDonkey, and even Usenet. Web-interface of Download Master is located at

Yet another advantage of Download Master is a utility for Windows that is registered in the system as the client for BitTorrent and Usenet files by default. The same utility can be used in order to install plug-ins for Chrome/IE browsers for direct download of file through Download Master without the need to enter its web-interface.

Upon usage of Download Master the user may receive an error that hides downloads from the list right after they were added. When the downloads are added once again, the user receives a notification that these tasks are already on the list. After getting connected through Samba or FTP, one will see these files in /Download/Complete folder with zero size. This error may appear after upgrading the firmware and that is why the vendor recommends the users to reinstall Download Master after every firmware upgrade.

Media Server lets common devices obtain access to media content using DLNA and iTunes standards. Web-interface of this service is available at: It has literally no settings, doesn't divide content by categories, and makes all disks connected to the router available for use by default.

AiDisk technology offers remote access to external disks connected to the router. In order to set it up, one needs to use a special wizard that will adjust the necessary rights and open the ports. Also, if necessary, one can register a name on ASUS DDNS service and obtain an address like

Settings of the local and remote synchronization are located in a separate menu item named AiCloud 2.0. It contains features that are meant to be used for synchronization of files between a USB stick and ASUS cloud as well as for synchronization of files between two ASUS devices with support of AiCloud 2.0 through the web. It's worth noticing that the main feature of this cloud service is about providing online access to files and folders that are not only located on the disk connected to the router, but also to files and folders located on network disks of users in the LAN. WoL (Wake on LAN) is an auxiliary feature that lets one turn the computer on remotely. It's also worth noticing that there are software clients for operation with AiCloud for devices powered by Android as well as for iPhones and iPads.

Wireless network settings are quite common for a dual frequency range device with support of 802.11AC standard.

LAN menu item is used to set only those parameters associated with the operation of IPv4. IPv6 settings are located in a separate menu item. There is also a capability of creating routes manually and receiving them from the service provider's DHCP server. And a capability to specify one or two ports for connection of IPTV Set Top Boxes.

Switch control tab includes a feature of NAT hardware acceleration that we can see more and more frequently in top router models by ASUS.

Among settings associated with the connection to the Internet there are the following types of connections: static settings, DHCP, PPPoE, L2TP, and PPTP. The most interesting thing is that the latter lets one get connected to VPN servers even with 128 bit MPPE encryption. Also, there is a helpful feature Extend the TTL value, which lets one get connected to service providers' networks that prohibit usage of routers on the client side. Technically speaking, this kind of prohibition is made possible by specifying TTL=1 field value in packets that are sent to the client. We already reviewed this technology earlier in the review of ASUS RT-N11P wireless router.

The same menu item also features port forwarding settings, DMZ, DDNS, and NAT Passthrough. And though certain ports on the external router interface are made active upon usage of AiDisk service, they cannot be seen in the port forwarding table. One can only see a notification about a possible conflict of ports upon using them manually. Manual forwarding will be given priority in this case.

As we already said earlier, all IPv6 settings are located in the separate same-named menu item. The following connection types are supported: static and dynamic addresses (Stateful and Stateless) as well as 6to4, 6in4, and 6rd tunnels.

Availability of the built-in VPN server that functions using PPTP with MSCHAPv2 authorization, and MPPE encryption with the key length of up to 128 bits came to be a nice capability. Also, one can use a well-protected OpenVPN server with a lot of security settings for clients.

Apart from the VPN server feature, there is also a VPN client capability. It may come in handy if one needs to get connected to a corporate network remotely. The following protocols are supported: OpenVPN, PPTP, and L2TP.

Firewall settings feature both the classic security means—protection against DoS attacks, denying sending echo-replies through ICMP, filtration of websites according to the URL addresses—and more elaborate technologies like filtration of web pages according to the key words (apart from compressed pages and HTTPS) and port filtering.

The same menu item is used to manage IPv6 traffic filtering. All outgoing and reply IPv6 traffic is permitted, but one will need to permit the incoming traffic separately.

Administration menu item lets the administrator choose the device operation mode: wireless router or access point; specify the administrator's password, manage time synchronization, enable or disable access through telnet.

The same menu item is used for checking the firmware updates, backing up settings, or resetting the user settings.

System Log is divided into several categories: general output of messages of all active services, wireless network, DHCP, IPv6, routing table, port forwarding table, and NAT translations table.

Among network utilities are the following: ping, traceroute, nslookup, netstat as well as Wake-On-Lan feature that lets one save devices' MAC addresses, which may come in handy since the administrator won't need to type in MAC addresses by hand.

That is where we bring review of the router web-interface to a conclusion and pass on to examining the capabilities of its command line.

Command line interface

Managing the access to the command line is performed using System tab, Administration menu item in the web-interface.

Firmware of the model under review is built on Linux 2.6.36 OS using Busy Box 1.17.4.

RT-AC51U login: admin
ASUSWRT RT-AC51U_3.0.0.4 Sat Jan 10 18:55:04 UTC 2015
admin@RT-AC51U:/tmp/home/root# cd /
admin@RT-AC51U:/# busybox
BusyBox v1.17.4 (2015-01-11 02:55:02 CST) multi-call binary.
Copyright (C) 1998-2009 Erik Andersen, Rob Landley, Denys Vlasenko
and others. Licensed under GPLv2.
See source distribution for full notice.
Usage: busybox [function] [arguments]...
 or: function [arguments]...
 BusyBox is a multi-call binary that combines many common Unix
 utilities into a single executable.  Most people will create a
 link to busybox for each function they wish to use and BusyBox
 will act like whatever it was invoked as.
Currently defined functions:
 [, [[, arp, ash, awk, basename, blkid, cat, chmod, chown, chpasswd, clear, cmp, cp, crond,
 cut, date, dd, df, dirname, dmesg, du, e2fsck, echo, egrep, env, ether-wake, expr, fdisk,
 fgrep, find, flock, free, fsck.ext2, fsck.ext3, fsck.minix, fsync, grep, gunzip, gzip,
 head, ifconfig, insmod, ionice, kill, killall, klogd, less, ln, logger, login, ls, lsmod,
 lsusb, md5sum, mdev, mkdir, mke2fs, mkfs.ext2, mkfs.ext3, mknod, mkswap, modprobe, more,
 mount, mv, netstat, nice, nohup, nslookup, pidof, ping, ping6, printf, ps, pwd, readlink,
 renice, rm, rmdir, rmmod, route, sed, setconsole, sh, sleep, sort, strings, swapoff,
 swapon, sync, syslogd, tail, tar, telnetd, test, top, touch, tr, traceroute, traceroute6,
 true, tune2fs, udhcpc, umount, uname, unzip, uptime, usleep, vconfig, vi, watch, wc, wget,
 which, zcat, zcip
admin@RT-AC51U:/# uname -a
Linux RT-AC51U 2.6.36 #1 Sun Jan 11 02:58:33 CST 2015 mips GNU/Linux

Let's see what processes are currently running using ps command. By using top utility one can obtain information on the current activity of the launched processes.

admin@RT-AC51U:/# ps
 1 admin     4076 S    /sbin/init
 2 admin        0 SW   [kthreadd]
 3 admin        0 SW   [ksoftirqd/0]
 4 admin        0 SW   [kworker/0:0]
 5 admin        0 SW   [kworker/u:0]
 6 admin        0 SW<  [khelper]
 7 admin        0 SW   [sync_supers]
 8 admin        0 SW   [bdi-default]
 9 admin        0 SW<  [kintegrityd]
 10 admin        0 SW<  [kblockd]
 11 admin        0 SW   [kswapd0]
 12 admin        0 SW   [fsnotify_mark]
 13 admin        0 SW<  [crypto]
 17 admin        0 SW   [mtdblock0]
 18 admin        0 SW   [mtdblock1]
 19 admin        0 SW   [mtdblock2]
 20 admin        0 SW   [mtdblock3]
 21 admin        0 SW   [mtdblock4]
 22 admin        0 SW   [mtdblock5]
 23 admin        0 SW   [mtdblock6]
 24 admin        0 SW   [kworker/u:1]
 31 admin        0 SW   [kworker/0:1]
 42 admin      660 S    hotplug2 --persistent --no-coldplug
 89 admin     4060 S    console
 90 admin     1596 S    /bin/sh
 97 admin        0 SW   [khubd]
 177 admin     4068 S    usbled
 270 admin     4068 S    wpsaide
 304 admin     1592 S    crond
 305 admin     1092 S    /usr/sbin/infosvr br0
 308 admin     4068 S    watchdog
 310 admin     1300 S    rstats
 324 admin     1184 S    lld2d br0
 326 admin     4068 S    ots
 328 admin      748 S    miniupnpd -f /etc/upnp/config
 344 admin     1188 S    lpd
 528 admin     4068 S    ntp
 609 admin     1580 S    syslogd -m 0 -S -O /tmp/syslog.log -s 256 -l 6
 611 admin     1580 S    /sbin/klogd
 620 admin     1584 S    telnetd
 731 admin        0 SW   [RtmpCmdQTask]
 732 admin        0 SW   [RtmpWscTask]
 749 admin        0 SW   [RtmpCmdQTask]
 750 admin        0 SW   [RtmpWscTask]
 768 admin     4068 S    /sbin/wanduck
 771 nobody     988 S    dnsmasq --log-async
 774 admin     1588 S    udhcpc -i vlan2 -p /var/run/ -s /tmp/udhcpc -O33 -O249
 826 admin     3988 S    httpd
 827 admin     1196 S    networkmap
 829 admin     2108 S    u2ec
 831 admin     2108 S    u2ec
 833 admin     2108 S    u2ec
 844 admin     3260 S <  /usr/sbin/smbd -D -s /etc/smb.conf
 845 admin     2392 S    nmbd -D -s /etc/smb.conf
 848 admin     1604 S    -sh
 864 admin     1584 R    ps
Mem: 46376K used, 15128K free, 0K shrd, 5188K buff, 15792K cached
CPU:   0% usr   0% sys   0% nic  98% idle   0% io   0% irq   1% sirq
Load average: 0.00 0.06 0.04 1/56 866
 865   848 admin    R     1592   3%   1% top
 1     0 admin    S     4076   7%   0% /sbin/init
 308     1 admin    S     4068   7%   0% watchdog
 270     1 admin    S     4068   7%   0% wpsaide
 326   308 admin    S     4068   7%   0% ots
 768     1 admin    S     4068   7%   0% /sbin/wanduck
 528     1 admin    S     4068   7%   0% ntp
 177     1 admin    S     4068   7%   0% usbled
 89     1 admin    S     4060   7%   0% console
 826     1 admin    S     3988   6%   0% httpd
 844     1 admin    S <   3260   5%   0% /usr/sbin/smbd -D -s /etc/smb.conf
 845     1 admin    S     2392   4%   0% nmbd -D -s /etc/smb.conf
 833   831 admin    S     2108   3%   0% u2ec
 829     1 admin    S     2108   3%   0% u2ec
 831   829 admin    S     2108   3%   0% u2ec
 848   620 admin    S     1604   3%   0% -sh
 90    89 admin    S     1596   3%   0% /bin/sh
 304     1 admin    S     1592   3%   0% crond
 774     1 admin    S     1588   3%   0% udhcpc -i vlan2 -p /var/run/ -s /tmp/udhcpc -O
 620     1 admin    S     1584   3%   0% telnetd
 611     1 admin    S     1580   3%   0% /sbin/klogd
 609     1 admin    S     1580   3%   0% syslogd -m 0 -S -O /tmp/syslog.log -s 256 -l 6
 310     1 admin    S     1300   2%   0% rstats
 827     1 admin    S     1196   2%   0% networkmap
 344     1 admin    S     1188   2%   0% lpd
 324     1 admin    S     1184   2%   0% lld2d br0
 305     1 admin    S     1092   2%   0% /usr/sbin/infosvr br0
 771     1 nobody   S      988   2%   0% dnsmasq --log-async
 328     1 admin    S      748   1%   0% miniupnpd -f /etc/upnp/config
 42     1 admin    S      660   1%   0% hotplug2 --persistent --no-coldplug
 21     2 admin    SW       0   0%   0% [mtdblock4]
 3     2 admin    SW       0   0%   0% [ksoftirqd/0]
 4     2 admin    SW       0   0%   0% [kworker/0:0]
 20     2 admin    SW       0   0%   0% [mtdblock3]
 31     2 admin    SW       0   0%   0% [kworker/0:1]
 2     0 admin    SW       0   0%   0% [kthreadd]
 23     2 admin    SW       0   0%   0% [mtdblock6]
 24     2 admin    SW       0   0%   0% [kworker/u:1]
 97     2 admin    SW       0   0%   0% [khubd]
 22     2 admin    SW       0   0%   0% [mtdblock5]
 731     2 admin    SW       0   0%   0% [RtmpCmdQTask]

Contents of /bin, /sbin, /usr/bin, and /usr/sbin catalogs, as well as the output of sysinfo utility, are located in a separate file.

Now let's turn to /proc catalogue to view its contents and find out the system uptime, its average utilisation, information on the CPU installed, and the amount of RAM. Actually, system uptime and average system utilisation can also be learnt using uptime command.

admin@RT-AC51U:/# cd /proc
admin@RT-AC51U:/proc# ls
1              305            732            960            kcore          softirqs
10             308            749            97             kmsg           stat
11             31             750            buddyinfo      kpagecount     swaps
12             310            768            bus            kpageflags     sys
13             324            771            cmdline        loadavg        sysrq-trigger
17             326            774            cpuinfo        locks          sysvipc
177            328            8              crypto         meminfo        timer_list
18             344            826            devices        misc           tty
19             4              827            diskstats      modules        uptime
2              42             829            driver         mounts         version
20             5              831            execdomains    mt7620         vmallocinfo
21             528            833            filesystems    mtd            vmstat
22             6              844            fs             net            zoneinfo
23             609            845            interrupts     nvram
24             611            848            iomem          pagetypeinfo
270            620            89             ioports        partitions
3              7              9              irq            scsi
304            731            90             kallsyms       self
admin@RT-AC51U:/proc# cat uptime
1190.00 1130.26
admin@RT-AC51U:/proc# cat loadavg
0.01 0.02 0.01 1/56 962
admin@RT-AC51U:/proc# cat cpuinfo
system type             : Ralink SoC
processor               : 0
cpu model               : MIPS 24Kc V5.0
BogoMIPS                : 386.04
wait instruction        : yes
microsecond timers      : yes
tlb_entries             : 32
extra interrupt vector  : yes
hardware watchpoint     : yes, count: 4, address/irw mask: [0x0ffc, 0x0ffc, 0x0ffb, 0x0ffb]
ASEs implemented        : mips16 dsp
shadow register sets    : 1
core                    : 0
VCED exceptions         : not available
VCEI exceptions         : not available
admin@RT-AC51U:/proc# cat meminfo
MemTotal:          61504 kB
MemFree:           14972 kB
Buffers:            5220 kB
Cached:            15844 kB
SwapCached:            0 kB
Active:            16632 kB
Inactive:           9368 kB
Active(anon):       5124 kB
Inactive(anon):      224 kB
Active(file):      11508 kB
Inactive(file):     9144 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:          4940 kB
Mapped:             3472 kB
Shmem:               412 kB
Slab:              12112 kB
SReclaimable:       2188 kB
SUnreclaim:         9924 kB
KernelStack:         448 kB
PageTables:          404 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:       30752 kB
Committed_AS:      11644 kB
VmallocTotal:    1048372 kB
VmallocUsed:        7892 kB
VmallocChunk:    1008456 kB
admin@RT-AC51U:/proc# uptime
 03:20:10 up 20 min, load average: 0.00, 0.02, 0.00

We can't help but mention nvram utility that allows changing certain important device operation parameters.

admin@RT-AC51U:/proc# nvram
usage: nvram [get name] [set name=value] [unset name] [show] [save file] [restore file]
admin@RT-AC51U:/proc# nvram show | grep admin
size: 31379 bytes (30061 left)

That's where we proceed to completion of the brief review of the command line interface capabilities and pass directly on to testing the device.


The first testing procedure we always begin our testing section with is estimating the booting time of the device, which is a time interval starting with the moment when the power is on until the first echo reply is received through ICMP protocol. ASUS RT-AC51U wireless router boots in 41 seconds. We believe that the result is decent.

The second traditional test was a security scanning procedure, which has been carried out using Positive Technologies XSpider 7.7 (Demo build 3100) utility. On the whole, there were 15 open ports discovered. The most interesting data are presented below.

Before getting straight down to performance tests we would like to mention the key specifications of the test stand we used.

Component PC Notebook
Motherboard ASUS Maximus VI Extreme ASUS M60J
CPU Intel Core i7 4790K 4 GHz Intel Core i7 720QM 1.6 GHz
RAM DDR3 PC3-10700 SEC 32 Gbytes DDR3 PC3-10700 SEC 16 Gbytes
NIC Intel PRO/1000 PT
Atheros AR8131
OS Windows 7 x64 SP1 Rus Windows 7 x64 SP1 Rus


NAT/PAT feature in RT-AC51U router can be disabled and that's why we measured the device performance both upon performing ordinary routing and routing with NAT/PAT. In case of the latter the measurements were carried out in two modes: with enabled and disabled hardware acceleration. In order to test it we used JPERF utility, 2.0.2 version. The tests were carried out with 1, 5, and 15 concurrent TCP connections.

At first glance, usage of hardware acceleration doesn't help increase the routing speed. However, enabling hardware acceleration decreases the load on the CPU and makes it available for performing other tasks.

We couldn't keep away from support of PPTP/L2TP/PPPoE tunnels by the router. Usually, the PPTP performance is the lowest and that's why we decided to perform the measurements using this connection type. Results of the measurements are presented on the diagram below. The measurements were made both using MPPE128 encryption and without it.

Apart from the support of the current version of IP protocol, IPv4, ASUS RT-AC51U wireless router also supports operation with the next generation protocol, IPv6.

Presence of a USB port lets the administrator get external drives connected to the device under review: flash cards and HDDs with a USB interface. Access speeds to the user data stored on such an external device are presented on the diagram below. In order to make the measurements we used our standard 750 Gbytes Transcend StoreJet 25M3 hard disk, which we successively formatted into the following file systems: FAT32, NTFS, and EXT2/3.

It's obvious that in this case Fast Ethernet interface is a bottleneck.

Also, we decided to measure the performance of Download Master application that is being executed in the router. We decided to use a popular torrent upload that had an approximate size of 2 GBytes for our measurements. At first we downloaded this file using a torrent client installed on a PC connected to RT-AC51U LAN port. The measured speed was about 11 Mbyte/s, which corresponds to the performance of Fast Ethernet. Then we connected an external hard disk to the router and started downloading the file to it. The speeds that we received depended mainly on the file system used. This way, the average download speed for NTFS partition was 1.3 Mbyte/s. EXT3 partition demonstrated the average speed of 2.1 Mbyte/s. Graphs located below show the utilization of the router external interface upon operation with Download Master for both of the file systems. It's worth noticing that ASUS RT-AC51U CPU utilization during the test was 100% all the time and that makes us believe that the received speeds are limited by the performance of the router itself.

Finally, we are about to test the performance of the wireless module of the router. At first we carried out the measurements for one pair of client devices: one wired and one wireless client. Once again, it looked like the received speeds were limited by Fast Ethernet interface too. That's why we decided to add the second pair of client devices and load the wireless network segment to the max. Results of the measurements for both frequency ranges are presented on the diagram below.

During our performance tests, the router case temperature didn't get higher than 33°С. The environment temperature was 25°С.

That's where we draw the testing chapter to a close and move on to summing it all up.


Generally, we are quite glad about ASUS RT-AC51U wireless router, which is the successor of RT-AC52U model. The device is fitted with 100 Mbps Fast Ethernet ports and two wireless adapters that provide theoretically possible speeds of up to 300 Mbps in 2.4 GHz frequency range and 433 Mbps in 5 GHz frequency range. We believe that the router under review is well balanced when it comes to the interfaces' speed. Obviously, RT-AC51U won’t become a favourite of geeks, but it will definitely meet all needs of a common user.

Among the strength areas of ASUS RT-AC51U wireless router are the following.

  • High performance of VPN
  • A built-in VPN client and server
  • Support of 802.11AC wireless network
  • A lot of new interesting features
  • Support of IPv6
  • Availability of alternative firmware
  • Competitive price

Unfortunately, we cannot help but mention certain drawbacks of the model.

  • The web-interface is not completely translated
  • Certain insignificant errors in the firmware
  • No Gigabit Ethernet ports

As of when this article was being written, the average price for ASUS RT-AC51U in Moscow online shops was 4600 roubles.

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