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1.1.e Spanning Tree Protocol

Since Ethernet Frames do not have TTL (Time To Live) like a IP-Datagram, they will live forever till they are processed. This could lead to many issues in case there is a loop in the topology. The packet will ends into an endless loop and cause a possible saturation of the link and resources of the devices that are trying to process this data.

After a defined process of exchanging specific frames (BPDU, Bridge protocol Data Unit) spanning-tree protocol creates a Layer-2 Tree topology by placing a root switch on the top of the tree, that all the members agreed before. The loop will be blocked by putting links in the tree into a blocking state in order to have a free loop topology. In case an uplink fails, the recalculation will start again and the blocking links may be unblock to prevent connectivity between devices in the upper layers. There is a process to select which ports are in blocking or forwarding state.

By putting some links into the blocking state, spanning-tree impide us to use multipaths to reach to the destination. Which means, that this mechanism impide us to use the fully amount of bandwith offered by the topology. Different versions/standards of the spanning-tree has been developed in order to solve this kind of issues with small difference between eachother. Let us have a look to some of them.

Here are some terms that will be discussed in this post:

  • Primary Root Bridge
  • Secondary Root Bridge
  • Bridge
  • BPDU
  • TCN
  • STP Timers

BPDU (Bridge Protocol Data Unit)

A BPDU is a spanning-tree message containing certain information about a device. The information found in this frame is pasted in the output below, which is related to the commong spanning tree. This can be read in every BPDU under the ‚Protocol Version Identifier‘ and the value set to it, in this case Spanning Tree (0) . It contains its protocol version identifier, type of BPDU, its flags, and information about the root bridge and the local brige. We can say that root and bridge identifier are different (00:1c:0e:87:78:00 and 00:1c:0e:87:85:00) which means the BPDU generated was not sent from the root brdige. We also found information about its Max Age, Hello Time and Forward Delay

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Eliminating Spanning-tree with FabricPath (MAC-in-MAC Routing)

Hello everyone out there. It’s me again.

For more than 30 years the Spanning-Tree Protocol accompanied us through thick and thin along the datacenters. Network requirements also increased together with business requirements, and they become very high. So high that a loss of connectivity of a few seconds (due Spanning-Tree convergence) may have a huge impact in the productivity of our environment. We also should not forget about the ports in blocking state becomming an unused port of bandwidth within the network.

This is how Spanning-Tree helped us to solve many of the problems we had in the network.

In the picture below you can see 15 links accross the network. But only five are being used, the rest of the links will be in blocking state at one end of the connection.

Therefore a few new drafts (Around 2010) has been increated in order to overcome this difficulties. One of them is FabricPath.

Some of the befenits of FP:

  • ECMP
  • No bandwidth restrictions due a sub-optimal path
  • more granular traffic engineering
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Cisco EVPN – VxLAN using unicast

Hello there! It has been a long time since my last post. Currently I am dealing with EVPN VxLAN and managed it to run a demonstration on my small modest virtual lab environment.

One of my first posts in this blog was about how to run VxLAN over an IPSEC Tunnel using a Fortigate firewall. This setup has some limitations when it comes to scalability or performance. In this case we are gonna see a very simple design and implementation using a spine & leaf architecture and unicast to replicate our data across the network.

There are still a lot of legacy DC running spanning-tree out there. It is not a crime doing it, since the network should meet customer requirements without adding unnecessary complexity. But… imagine you are running a financial network and your network flow relies on STP. You will face two challenges:

  1. The convergence time between failure and recovery are possibly to high.
  2. You will have some interfaces in the blocking state, which will impede you to use all the BW available in the network. More precisely you will end up with a lot of unused ports.
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Point-to-Point links with /31 – RFC-3021

Hello everyone! Today something old… but anyway interesting.

We know since last century that we will not have enough IPv4 addresses to satisfiy all our needs. Therefore some guys proposed twenty years ago in RFC-3021 a way to conserve the amount of public addresses. Link to RFC-3021.

Lets see a comparison for better understanding and to realize the purpose of it.

Bit prefixesAmount of IP-AddressesAmount of hostAmount of subnet within /24
/304264
/3122128

We clearly see that using a /31 we will be able to double the amount of point-to-point links that we can create. This demonstrate the inefficiency of running PTP-Links using a /30-bit prefix. This is very important to consider if you have a limited amount of public IPv4 addresses.

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