Thursday, December 5, 2013

Provider Bridge and Provider Edge

"The essence of planning a packet network is to understand the flow of traffic first, not only from a level of 40,000 ft but also on ground......."

                                                                                               After meeting various people

My Dear Friends of Transmission Fraternity, 

Packet network planning is all about knowing the flow of traffic and the entire in and out of traffic while the transport is being planned. When we talk about a packet network we usually talk in the terms of L2/ MPLS/ L3. The L3 is not actually a transport solution but more of a terminal solution to the traffic, so we can consider the L2 and the MPLS to be a kind of a technology that actually helps in efficient transport of the traffic.

Two basic terminologies come into the being in realizing such transport systems.

1. Provider Bridge.
2. Provider Edge.

While these are the two major technologies that are actually involved in the realization of the packet network we need to first understand one word very clearly "PROVIDER"

So who or what is called as a provider?

A provider entity is any system that helps in interchange and exchange of any traffic that may be Unicast, Broadcast or Multicast.
This is a simple example and definition of  a provider. So this means that any network that is shipping the traffic content from one point to another point with or without intelligence may be referred to as a provider. A provider can be a telco or it can be any network entity in the system that transports traffic.

This provider can act as a Bridge or as an Edge. The term Bridge and Edge is actually used in the context be the way the traffic is flowing through the network elements and the location or the point at which the MAC address is learnt.


A L2 Provider Bridge Network is actually realized by connecting various L2 switches together in the network. As known the L2 switches work on the principle of Mac learning and forward packets through the ports in the manner the source mac addresses are learnt. Every L2 switch entity is thus a Mac learning entity in the system.

So if a L2 provider bridge network is selected then at each and every point actually the traffic is forwarded after the Mac address is learnt.

Let us see the following example in the picture.

The figure actually shows how the traffic flow happens in a provider bridge. As we can see that at every transit point the traffic is actually bridged. Bridging means reforwarding of traffic by means of learning mac addresses. So if a traffic has to enter Node 1 and go out of Node-4 to a particular destination mac address, the destination device address should be learnt in all the NEs in the entire network so that the traffic can be bridged.

Each and every flow instance is called as a bridge and since the traffic is always having to pass through these bridges as they transgress the Network elements these are called as Provider Bridge elements.

Limitations of a Provider Bridge network:

  1. Mac address to be known at every point so there cannot be any kind of full point to point services in the network that is not requiring to learn mac. 
  2. As and when the customers are increasing in one of the endpoints the vFIB capacity of all the network elements are to be upgraded. 
  3. The whole system and all the NEs in the network are to be upgraded in configuration as and when the number of users are increasing. 
  4. When there are more number of NEs introduced in the transit there will be a considerable addition to the delay latency time that it takes for the traffic to flow. 

A provider edge network tends to eliminate all the limitation aspects that are actually found in the Provider Bridge network. The Provider Edge network actually works on the principle of tunneling of traffic from one point to another point. So if a packet is to be sent from say Node-1 to Node-4 there is a tunnel that is created from Node-1 to Node-4 and on this the packet is actually put on. 

The below picture will clear this:

As shown in the picture this is actually a mechanism where the traffic is tunneled across the intermediate node so that the intermediate nodes do not need to know at all about the Mac Addresses.

This principle makes the network more scalable and more agnostic to mac learning. The mac learning concept is used if and only if there are multiple endpoints in a service. The intermediate points are not a part of the service integration but only points where the traffic is made in and out. 

Since the tunnel is a point to point entity there needs to be actually no realization of MAC in the system and the traffic is sent end to end. 

This can be done without the learning of the MAC. What actually happens in the transit point is that the traffic enters with a "Label" and goes out with another LABEL. 

that is the reason why the tunnel is also called as an LSP (Label Switched Path). 

The Label Switched path can have its own protection as we discussed in the MPLS section also. 

So what should be remembered by my friends of the Tx Fraternity:

1. Discuss and realize how you want to scale up the network. 
2. Understand and think which design is suitable PB or PE. It is not that PB should always be rejected. In smaller networks PE can be a major overkill. 
3. Know the service, whether it needs mac learning or not. Unnecessary bridging can kill processes in the card. 

Every process in telecom transmission is a function of "Science" and not a result of "Trend". Remember you can be a lamb and follow the trend like following the herd, or you can be scientific and make your network so well that it actually sets the Trend. 

In the war between Science and Trend.... Science has and will always win. 

Till then, 



1 comment:

  1. nice blog thanks a lot. So in conclusion if my equipment have the abaility of the following ports
    Costumer bridge port = connecting to a bridge costumer divide any kind of switch
    Costumer edge port = connecting to and end device a PC for example
    is that correct for you?