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Spanning Tree Best Practices

Spanning Tree Protocol (STP) is a control plane mechanism for Ethernet. It is used to create a Layer 2 topology (a tree) by placing the root switch on top of the tree.

Since classical Ethernet works based on data plane learning and Ethernet frames don’t have TTL for loop prevention, loops are prevented by the STP blocking the links.

As you can see from the below figure, some links are blocked by the spanning tree. If Spanning Tree wouldn’t block those links, loop would occur in the below topology.

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What is flow-based load balancing ?

Flow-based load balancing is used mostly in layer 2 networks, although in Layer 3 routing, packets can be load balanced per packets or per flow, flow-based load balancing is commonly used with the Local area network, datacenter and datacenter interconnect technologies.

There are two important load balancing mechanisms in layer 2. Vlan-based load balancing and Flow-based load balancing. Understanding the differences of these two is important for network engineers, thus please read Vlan-based load balancing post from here as well.

Load-balancing is probably a wrong term though and load-sharing should be used for stateless devices and I explained the differences between load-balancing and load-sharing here.

Let’s look at below figure to understand flow-based load balancing.

flow-based load balancing

Figure -1 Flow based load balancing with basic switch topology

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What does PE-CE mean in MPLS ?

What does PE-CE mean in the context of MPLS ? What is CE , P and PE device in MPLS and MPLS VPN ?

These are foundational terms and definition in MPLS.

MPLS is one of the most commonly used encapsulation mechanism in Service Provider networks and before studying more advanced mechanisms, this article is must read.

In order to understand PE-CE, we need to understand first what are PE and CE in MPLS.

Let’s take a look at below figure.


Figure -1 MPLS network PE, P and CE routers Read more

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Why Are Dynamic Routing Protocols Used?

Why are dynamic routing protocols used is usually asked by newbies in the networking field, especially after they have heard about routing protocols. Besides that, they often asked this question: What is the difference between static routing and the dynamic routing protocols?

And the common answer is that dynamic routing protocols are scalable.

In other words, there is no need to configure a manual entry for each destination as well as specifying the next hop IP address or interface with the dynamic routing protocols. 

These are good reasons. But do we really have only such benefits? In very small networks, scalability is reasonable and correct. But for more sophisticated networks, there are other important reasons.

Before I explain the other reasons, let me clarify why static routing requires lots of manual configurations and why it is not scalable, compared to dynamic routing protocols.

why dynamic routing protocols are used

Figure- 1 Why are dynamic routing protocols used?

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HSRP VRRP GLBP Comparison– In this post I am going to cover the similarities and the differences between HSRP VRRP and GLBP protocols.

All these technologies provide first hop redundancy for the hosts.

I will use the below table for HSRP VRRP GLBP Comparison and the design attributes listed in it.

For the more technology comparison tables such as MPLS , Quality of Service , Multicast, VPNs , Security and more please click here.


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OSPF Area Types

OSPF Area Types – Different Areas in OSPF are used to create smaller fault domains. There are two OSPF area types in total.

OSPF Backbone area and OSPF non-backbone area

Backbone area in OSPF is Area 0. OSPF prevents loop by using backbone area concept.All the non-backbone areas should be connected to the Backbone area.

There are many Non-Backbone OSPF Area types. These are; Normal Area, Stub, Totally Stub, NSSA and Totally NSSA Areas.

In this article I will explain the non-backbone OSPF areas from the design point of view and share some caveats about the OSPF design.

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HSRP, VRRP and GLBP are the three commonly used first hop redundancy protocols in local area networks and the data center.

In this post, I will briefly describe them and highlight the major differences. I will ask you a design question so we will discuss in the comment section below.

hsrp vrrp glbp

source: Orhan Ergun CCDE Study Guide – Workbook

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What is OAM – Operation, Administration, Maintenance ?

OAM is a set of tools that have been used to provide network fault indication, performance information, fault localization, data and diagnosis functions.

In different standard bodies usage of OAM is slightly different thus it creates a confusion among the engineers.

IETF published a best practice RFC to clarify the OAM acronym. RFC 6291 ” Guidelines for the Use of the “OAM” Acronym in the IETF “.

Let’s take a look at How different standard bodies such as ITU-T, MEF and IEEE defines the OAM and then I will share IETF approach.

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Common Networking Protocols in LAN, WAN and Datacenter

Spanning Tree, Link Aggregation , VLAN and First Hop Redundancy protocols are used in Campus, Service Provider Access and Aggregation and in the Datacenter environment. There are definitely other protocols which are common across the Places in the Networks but in order to keep this article short and meaningful I choose these four. Read more

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OSPF protocol

OSPF Protocol – OSPF, Open shortest path first is a dynamic routing protocol which creates a topology between the routers to distribute routing information inside an Autonomous system.

If you are not familiar with OSPF, don’t worry ! In this article OSPF will be explained in great detail.

Are you interested in design aspect of OSPF, many OSPF design examples will be covered in the article.

Maybe OSPF network engineering interview question is what you are looking for. Read more

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Multicast Design – Podcast

Efficient data transfer, optimised resource usage , simplified configuration, optimum bandwidth usage are the main characteristics of Multicast.

In the diagram below; unicast and multicast flows are shown.

From the source, if same packet will be sent , with unicast transport you send two copies. Read more

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The difference between Load balancing and Load Sharing

It is important to understand the difference between load balancing and load sharing.

Routing protocols after calculating the routes from their databases , they automatically put equal cost routes into the routing table. Only exception to this behaviour is BGP.   Read more

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Route Redistribution Best Practices

You need route redistribution for many reasons.

In this post,the drivers for the route redistribution but more importantly the best practices for applying route redistribution will be explained in great detail. Read more

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Why you want to use particular first hop redundancy protocol. In this video Orhan Ergun is explaining Cisco specific HSRP ,GLBP and industry standard VRRP protocols.

What are the design considerations ?.

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Spanning Tree CST , PVST+ , RSTP and MST

In this video Orhan Ergun explains all the spanning tree modes and compare them from the design point of view. If you need scalability use MST , if you want fast convergence and flexibility use RSTP and so on. If you want me to share more spanning tree videos or explain specific technology please comment below.


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OSPF Area Border Router

OSPF ABR ( Area Border Router ) is the router that non backbone area routers use it as exit from their area. In the OSPF RFC it is stated that if one router has two interfaces on the two area it can be considered as an ABR.

But in the implementation , in order to have an ABR , router needs to have at least one interface is connected to area 0 which is backbone area.



In the figure, R3 is the backbone router but not an ABR since it has only connection to the Area 0. In order to be an ABR , router needs to have one interface in Area 0 and other at least one interface in different area.

R2 and R6 has a connection to both Area1 and Area 0 thus they are ABR. Also R4 is an ABR since it has a connection to both Area 0 and Area 2.

ABR aggregates topology information from one area to another area. What does it mean ?

In the figure although R3 seems only one router in the backbone area, let’s consider we have other routers in the area 0 as well. Their connections to each other and metric informations is not sent to other areas by the ABR.

Instead , ABR only sends the reachability information. The routers in different areas only know that they can reach each other but with the information provided by the ABRs.

Thats why , if you design multi area OSPF network , you can’t have end to end visibility. A router can have topology information about its own area only,