RecoverPoint: How to Check MTU Size in RecoverPoint

The MTU is the largest possible frame on an OSI Model Layer 2 data network.
The MTU size depends on the physical properties of the communications media.

For most Ethernet networks, this is set to 1500 bytes by default.
More recently, Ethernet frames with over 1500 bytes of payload are becoming popular (they are called Jumbo Frames) and they carry 9000 bytes of payload.

The issue with increasing the MTU over the default value of 1500 is that all network devices along the path must support the increased MTU value.

On the other hand, having VPN/PPPoe/firewall/etc, might cause the MTU on the WAN link to be lower than 1500.

In both cases, we must ensure that the MTU is consistent over all the path between the RecoverPoint clusters, otherwise unexpected issues can appear such as:

• Replication impact (possible DRU, link resets, and so forth)
• Inability of clusters to communicate
• Clusters unknown across system

Fluctuating MTU is also not supported as the RPAs support fixed MTU values only and having Fluctuating MTU may end up in communication issues.

To check the Maximum Transmission Unit (MTU) in RecoverPoint, log in to the Installation menu as an admin (or boxmgmt for RecoverPoint Classic) role user.

From the admin main menu select:

[2] Setup
[1] Modify settings
[7] MTU configuration
[1] View MTU values

Selecting 1 display result such as:
|------------|
| MTU values |
|------------|
| WAN | 1500 |
| LAN | 1500 |
|-----|------|

To test the MTU using ping:

Varies by version but get to the Run Internal Command prompt (menu buttons 3 then 5 from boxmgmt main menu in version 4.1 and later).
[5] Run internal command
This is the list of commands that you are allowed to use: arp arping date ethtool kps.pl netstat ping ping6 ssh su telnet top uptime

Enter internal command:
For internal command to test ping, use the following as an example when testing to get the best value for MTU:

ping -I eth0 -M do -s <packet size> <destination IP>

-I Argument forces ping traffic out the specified interface (eth0 in this instance, the WAN port).
-s Argument specifies the packet size to test.

Example results:
Enter internal command:

ping -I eth0 -M do -s 1472 192.168.0.105
PING 192.168.0.105 (192.168.0.105) from 192.168.0.108 eth0: 1472(1500) bytes of data.
1480 bytes from 192.168.0.105: icmp_req=1 ttl=64 time=0.164 ms
1480 bytes from 192.168.0.105: icmp_req=2 ttl=64 time=0.148 ms
1480 bytes from 192.168.0.105: icmp_req=3 ttl=64 time=0.144 ms
1480 bytes from 192.168.0.105: icmp_req=4 ttl=64 time=0.125 ms
1480 bytes from 192.168.0.105: icmp_req=5 ttl=64 time=0.151 ms
1480 bytes from 192.168.0.105: icmp_req=6 ttl=64 time=0.144 ms

If the test is unsuccessful, test lower MTU values. A guideline to testing is to use values that RecoverPoint has preset (see below) but subtract 28 from the value to account for the packet header. In the above example, tested 1500 MTU by pinging 1472 as packet size to account for the 28-bit overhead on the ping test. If testing 1492 as MTU, you would use 1464 as your -s size. Testing 1430, you would use 1402 and so on.

To change the MTU value:

[2] Setup
[1] Modify settings
[7] MTU configuration
[2] Configure MTU values

Selecting 2 here allows you to pick the interface to modify:
1. WAN
2. LAN

Select the interface type to modify (1 to 2): 1
The current WAN MTU value is 1500. Do you want to modify it? (y/n)? y
1. 9000 bytes (Recommended when using iSCSI)
2. 1500 bytes (Default value)
3. 1492 bytes (Recommended when using PPPoE)
4. 1430 bytes (Recommended when using VPN)
5. Other (For EMC personnel use only)
Select MTU value: (1 to 5) (Default '2'):

Enter the correct value based on the ping tests results.

|------------|
| MTU values |
|------------|
| WAN | 1500 |
| LAN | 1500 |
|-----|------|

Do you want to apply these configuration settings now (y/n)? y
The RPA receives these changes, but it may take some time for them to be applied to the entire system.

For more information, see MTU, and packet fragmentation searches on the Internet.