Changing MTU in EVE-NG (allowing Jumbo frames!)

EVE-NG rules. As far as network simulation software goes, it’s the best.

When studying or otherwise, EVE-NG is the way I prefer to try things out. One thing that happens, however, when using virtualised networks, is you obscure some underlaying things – one of them being MTU. In a previous post, I went through how the base OS that EVE-NG runs on virtualises the links between routers and switches, here I will show a way to boost the MTU these virtual network links use, so that we can throw proper jumbos across the network.

In this topology, I have 2 routers, connected with dual Ethernet links, configured in a LAG. This doesn’t affect MTU at all, I just thought I’d mention it so it’s not confusing.

Topology of this little lab

The link between these routers (ae0) is set to a layer-2 MTU of 9192, which is the maximum for the platform (Juniper vSRX 3.0). This means that we should be able to send an IP packet (like a ping) of over 9000 bytes.. And yet – we can’t:

root@R1> ping size 9001 do-not-fragment
PING ( 9001 data bytes

--- ping statistics ---
4 packets transmitted, 0 packets received, 100% packet loss

So – let’s first see why, then fix it.

If we SSH onto our EVE-NG server, and take a look at the virtual network that makes up the link between ge-0/0/1 on R1 and ge-0/0/1 on R2, as well as ge-0/0/2 on both as well (since we’re in a 2 member LAG). See previous post for more detail on how to track down which link is which and what they’re called inside EVE-NG’s OS:

root@eve-ng-2:~# ifconfig vunl0_26_2
vunl0_26_2 Link encap:Ethernet  HWaddr d6:41:90:79:cb:60
          RX packets:405 errors:0 dropped:0 overruns:0 frame:0
          TX packets:868 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:66286 (66.2 KB)  TX bytes:88080 (88.0 KB)

There it is. At layer-2, we have MTU 9000. As the virtual routers are assuming a piece of wire with no MTU at all connecting them to the other side, there is a problem. I think I’m MTU 9126, but in reality I’m down to 9000 on the wire. Which explains why I can send a ping that’s nearly 9000 bytes:

root@R1> ping size 8976 do-not-fragment
PING ( 8976 data bytes
8984 bytes from icmp_seq=0 ttl=64 time=1.150 ms
8984 bytes from icmp_seq=1 ttl=64 time=1.028 ms
8984 bytes from icmp_seq=2 ttl=64 time=1.066 ms
8984 bytes from icmp_seq=3 ttl=64 time=1.073 ms

Oh yeah. 8976 bytes at layer 3, plus IP and ethernet headers = 9000.

So – let’s isolate the 4 network interfaces that EVE-NG has generated to be my ge-0/0/1’s and ge-0/0/2’s, then set the MTUs to something high – like 9200:

root@eve-ng-2:~# ifconfig vunl0_26_2 mtu 9200
root@eve-ng-2:~# ifconfig vunl0_26_3 mtu 9200
root@eve-ng-2:~# ifconfig vunl0_27_2 mtu 9200
root@eve-ng-2:~# ifconfig vunl0_27_3 mtu 9200

When EVE-NG creates a ‘wire’ between two network devices, it really creates 2 virtual network interfaces and assigns each of them to a Linux bridge. In my case, with 4 total interfaces, I have two bridges. They inherit the MTU of the smallest member, so both of my bridges now should be 9200:

root@eve-ng-2:~# ifconfig vnet0_37
vnet0_37  Link encap:Ethernet  HWaddr 0a:c8:48:ad:34:71
          RX packets:30596 errors:0 dropped:28163 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:3289806 (3.2 MB)  TX bytes:0 (0.0 B)

Alright, we can see MTU on one of the bridges is now 9200 (it was 9000 before as both contributing interfaces were 9000) and we can also see a lot of dropped packets (because all my test pings at above 8976 bytes were crashing on the digital rocks).

So.. Can we send big pings now? Man. I hope so.

root@R1> ping size 9100 do-not-fragment
PING ( 9100 data bytes
9108 bytes from icmp_seq=0 ttl=64 time=1.374 ms
9108 bytes from icmp_seq=1 ttl=64 time=0.764 ms
9108 bytes from icmp_seq=2 ttl=64 time=0.910 ms
9108 bytes from icmp_seq=3 ttl=64 time=1.022 ms

Oh yes we can.


Note – this is not a permanent fix – it will last only as long as the virtual interfaces (or a server reboot) – and you need to do it for every link that is created. Perhaps it’s something to submit as a feature request to the EVE-NG team – but for now this will let us live in jumbo-harmony. Network links are ephemeral in EVE-NG, so statically setting this with a config file isn’t a great solution – but on the other hand we don’t drop our labs all that often – do we?