Orhan Ergun No Comments


Finally, informational EIGRP RFC 7868 has been published.It is not anymore Cisco’s EIGRP, it is an open standard. Without a most critical feature of EIGRP,can we really say that? Why Cisco doesn’t share the most important feature which can help in large scale EIGRP design although industry has been asking from them for a long time ?

EIGRP RFC 7868 specifies EIGRP Dual Algorithm, EIGRP Packets such as Update, Query and Reply, EIGRP Operation, and EIGRP Metrics (K1,K2,….K6).

And since EIGP is RFC anymore, other vendors can legally implement EIGRP. There was couple of open source EIGRP implementations already,but with the RFC status, seeing new implementations among the big vendors would not be a big deal.

In addition to EIGRP packet types and metric values, there are a couple of important things to understand about EIGRP. Read more

Orhan Ergun No Comments

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?

Read more

Orhan Ergun 3 Comments

EIGRP Feasible Successor

One of the advantages of EIGRP Feasible Successor is that it speeds up the EIGRP. In fact, if there is a Feasible Successor in the EIGRP network, such network converges faster than OSPF or IS-IS.

But what is EIGRP Feasible Successor and how can we find EIGRP Feasible Successor?

Or, if there is EIGRP Feasible Successor, how does EIGRP converges faster than OSPF or ISIS?

In this post, I will explain the answers to the above questions.

EIGRP Feasible Successor is a backup node that can satisfy the EIGRP feasibility condition.

Feasibility condition simply means that the backup router should be loop-free.

Let’s examine the topology shown below (Figure-1) to understand how EIGRP finds loop-free alternate/backup node.
Read more

Orhan Ergun 2 Comments


EIGRP vs OSPF  – Below comparison table is your primary resource for the OSPF and EIGRP routing protocols when you compare them from the design point of view.

Knowing and understanding these design practices will not only help you for the real life network design but also will help for the any design certification exams.

If you have any question regarding the parameters in the comparison chart, please share in the comment so I can provide more information.

Read more

Orhan Ergun No Comments

EIGRP Hub and Spoke Design

EIGRP Hub and Spoke – EIGRP is the best protocol in Hub and Spoke topologies because of many reasons. In this article, I will explain the design consideration for EIGRP in a Hub and Spoke topology, also known as star topologies.


eigrp hub and spoke

Figure – 1 EIGRP Hub and Spoke

Read more

Orhan Ergun 4 Comments

Routing design

Understanding everything about routing design is no brainer, especially if you have the chart below on your wall.

The table below highlights the pros and cons of each routing protocol. Of course, you need to consider the design attributes shown in Figure A before embarking on routing design.

Should you like the comparison of the routing protocols illustrated in the table below or should you want to see similar comparison for other technologies, feel free to add your comment in the comment section.

Another boon for all my readers!

If you are interested in network design or considering CCDE, CCDP, or CCDA certification, you can subscribe for membership here so that you can peruse all the design resources (Videos , Tests , Case Studies , and E-Books).

Figure A: Comparison of Routing Protocols

Orhan Ergun 1 Comment

Network Design – Physical Topology Matters

Short time ago I published a video on my youtube channel about Triangle vs Square Network Topology and I highly recommended triangle topology whenever it is possible.

I received couple of questions about the topologies and wanted to explain one of them in this post for everyone.

I used below topology in the video;



Left picture illustrates the triangle physical topology and right one for the square topology.

Distribution layer devices are advertising the same networks in both topology. It says router but it could be the Multilayer switch as well.

Assume we are running OSPF but using triangle instead of square applies to any other IGP protocol ( EIGRP , IS-IS , even RIP ).

The reason you want to use triangle topology is high availability.In the left topology if the link between core and distribution layer fails, will not be any routing protocol convergence  since the core devices will do the ECMP ( Equal Cost Multi Path) towards distribution, and distribution will do ECMP towards core thus all the links will be in the RIB and FIB so will be used actively. ( Flow based load-balancing ).

For the square topology; if the same link fails , since the left core device to destination prefix through the other core device metric is higher than the direct (failed) path , there is no equal cost and unless you enable Unequal cost multi path with EIGRP , you can’t place two routes for that prefix in the FIB. ( You may want to check OSPF Optimized Multipath draft ).

Question : In real life deployment , would we announce the same prefix from the two different distribution switches as depicted in the picture ?.

Answer : Yes we do. If we have distribution layer as depicted in the picture, which mean we have access layer as well. If Access layer is layer 3 which mean, default gateway for the devices is the access layer switch, then access and distribution layer would be running routing protocol.And from the design point of view you would want to run OSPF since between distribution and core is also OSPF and you don’t want to have more than one IGP in your topology unless you have to.

I used layer 3 access as an example for the simplicity but, we announce the prefixes from both distribution layer devices with multilayer access design ( Access-Distribution Layer 2 ) with or without MLAG ( VSS , VPC , MLAG with ICCP ). If you are using MLAG based solution, it is a matter of the number OSPF neighbour ship counts. I would want to see your comment if you know/guess the reason.

Orhan Ergun No Comments

Network Design Mistakes

Orhan Ergun prepared a topology for the CCDE students. A topology has full of design mistakes and 2 CCDE candidates will try to find the issues. Orhan will help them but let’s see if they can find all the mistakes. Enjoy !

Orhan Ergun 3 Comments

Is OSPF Distance Vector Protocol ?

No but.

When an ABR receive type 1 router and type 2 network LSAs within the area , it will only send the reachability information to another area. ABR is the choke point where topology information hides and only reachability information sends between the areas.
When ABR sends a summary type 3 LSAs into another area , it says I can reach network , etc and you can reach these networks through me. But ABR will not send with summary LSA , if you want to reach send first to me and I will send packet to Router A , Router A will send packet to Router B and so on. Which mean is ABR will hide topology information.

Internal routers within area will believe what their ABR says , they cannot calculate end to end path since they dont know full topology. This is distance vector behavior. Is not the same with EIGRP ? At every hop EIGRP router calculate the best path which is feasible distance to destination and sends it to next router.

Here one thing is important ; calculate and then send !. First calculation is done so , other routers wait this router to finish its job and send the route with its the best path , receiving router puts these information from all the sending router to EIGRP topology database and then run DUAL.

But with link state protocols both OSPF and IS-IS , when they receive LSA , they first floods the LSAs to their neighbor and starts to run SPF, of course here we are talking about msec level.But within large environment with the lots of links these can be a scaling issue.

Likewise, if an ABR receives multiple Network Summary LSAs from other ABRs across the
backbone, the original ABR will choose the lowest cost advertised amongst them and create NEW summary LSA and send into attached areas. 2 things are important in this sentence :

1. It will create new summary LSA , because unlike Type 5 external LSA , Type 3 summary LSA is limited to Area , when ABR receive summary LSA from another ABR , It creates new summary and sends to attached areas , this can be scaling issue.

2.ABR needs to create new summary Type 3 LSA for every attached area. This is also can be a scaling issue. From here we could discuss How many ABR per area ? or how many router per OSPF domain but lets keep this for another blog post.

Lets create one scenerio where ABR connects Area0 backbone area and Area1 internal area. First lets talk when we have just one ABR and then redundant ABRs.

If there is only ABR and we cannot add second ABR or high availability is not a big concern at this time , hiding topology and reachability information is not a big issue. Only concern would be within Service Provider environment which if They put their PE into Area 1 and Area 1 does not have specific routes to exit from Area 1.

Because PEs needs to know each other loopback interface as /32 host route to create LSP for transport label. So the solution might be leaking PEs loopback into Area1. If ABR in this situation goes down , all internal Area 1 routers will lose their connectivity to the domain.

To better idea to put more than one ABR per area for high availibility. In this case still route leaking for special purpose is needed but also there are at least two more caveats.
First , if we hide reachability information ( By default we hide topology information ) internal Area 1 routers will chose the best path according to their cost to ABR but since those internal routes dont know the entire topology of OSPF domain, after ABR to the destination in the Ospf area 0 can be a sub optimal. This if from edge to core sub optimality consideration, it may or may not be an issue for your network and its application. But if you have tight latency requirements this may affect your design choice.

Second is blackholing could be a problem if we are also hiding reachability information from Area 1 to backbone area . In this case , from one of the ABR to one of thr internal Area 1 router link goes down , traffic still can reach for the network behind that internal router to the ABR which failed link. And traffic could be blackhole.

General idea and solution when we summarize from two ABR like in this situation , put the link between the ABRs and sends the routing information over that link. Here another big consideration is coming to play , Which area would you put that inter ABR link ? Area 0 ? Area 1 ? Lets keep this to nother blog post.