Docker MTU + Docker tcpdump

I am troubleshooting an issue in a docker setup with some Arista cEOS where I can’t ping inside a VRF. First I though it was a MTU issue as when you use MPLS, there is an extra tag in the L2 frame.

…But my pings weren’t that big.

Still wanted to increase the MTU because that’s the expected thing to do in your WAN links if you run MPLS and want your users in different VRFs to be able to use the full 1500 bytes.

After some searching, It seems you can change the default value using the config file as per this link:

$ ip link show docker0
9: docker0: mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default
link/ether 02:42:be:73:8c:d3 brd ff:ff:ff:ff:ff:ff
$ cat /etc/docker/daemon.json
"data-root": "/home/somebody/storage/docker",
"mtu": 1600
$ sudo service docker restart
$ ip link show docker0
9: docker0: mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default
link/ether 02:42:fb:c0:cf:a2 brd ff:ff:ff:ff:ff:ff

And restart docker. But still had mtu 1500. Checking another link it seems I actually need to create a container so the bridge come up with the new value

$ docker run -d busybox top
9: docker0: mtu 1600 qdisc noqueue state UP mode DEFAULT group default
link/ether 02:42:fb:c0:cf:a2 brd ff:ff:ff:ff:ff:ff

Funny thing, once I started my lab again (using docker-topo) still got MTU 1500!!!

Will have to dig a bit why docker-topo doesnt take the docker mtu 1600 from the config file.

Solution: docker-topo is creating user-defined bridges, so it needs to be told that the mtu is different. The “mtu:1600” in the docker config it is only for the default bridge so when you start the busybox, it is attached to the default bridge and you see 1600.

The other thing I was curious was if I could tcpdump the networks created by docker.

Yes, you can!

# docker network ls

# ifconfig 

# tcpdump -i br-xxxx 


Sometime ago I tried these typical Italian rice bowls and like them. So I wanted to give it a go one day. The ones I tried had spinach and cheese inside. I quick search showed me videos for arancini but with meat. It looks good but wanted to try the spinach version so I went a bit free style.

These are the videos used as reference: link1 and link2

Ingredients for the rice balls

  • 1 and 1/2 cup of arborio/paella rice
  • 3 cups of boiling water
  • half onion chopped + splash of olive oil
  • pinch of sea salt
  • knob of butter
  • 1 tsp of tumeric


  • Fry the onion with the oil in a deep pan until soft
  • Add the rice and mix all together for a minute
  • Add the boiling water, salt, butter and tumeric
  • Cook at middle temperature and stir often
  • Once all liquid is absorbed, spread the rice in a tray to cool down.

Ingredients for the filling

  • 500g of washed spinachs
  • half onion chopped + splash of olive oil
  • 1 garlic clove
  • splash of milk
  • 1 big tsp flour


  • Fry the onion with the olive oil until soft.
  • Add the garlic and fry until golden
  • Add the spinach. They will reduce quite a lot.
  • Stir often and once the spinach are like a paste, add the milk and flour
  • Remove from heat and let it cool down

Frying the rice balls


  • 2 eggs
  • Breadcrumbs
  • sunflower oil (never through olive in the sink please!)
  • Cheese


  • Heat up a deep pan with the sunflower oil.
  • have a plate with the breadcrumbs and another with the mixed eggs
  • For making the balls, as per videos, wet your hands, make a decent ball, and make a hole with a finger.
  • Fill the hole with the cheese and spinach.
  • Cover the ball with a bit more rice and follow the technique to shape it like an egg
  • Pass the ball by the egg, then breadcrumbs and finally into the hot oil.
  • Fry until golden

This is my result:


To be honest, they look as I remembered but my spinach filling wasnt as great as the ones I tried.

I think I need to use mozarella cheese and add something else to the spinach mix (salt? nutmeg?)

Next time I will try to find the Arancini recipe with spinach.

As usual, with practice, comes mastery.

Will try again.