There are many reasons of OSPF to IS-IS migration, specifically for the Service Provider networks. Some of these reasons are shared later in the case study.
OSPFv2 by default setup only one adjacency over a single link. But this can be an issue some time and as a network designer you should understand the consequences and know the available solutions.
Placing a link in wrong OSPF area can create an OSPF sub optimal routing especially in hub and spoke topology.
In IS-IS or OSPFv3, this wouldn’t be an issue because IS-IS and OSPFv3 allow link to be placed in more than area or level. (In IS-IS, area is assigned to the router, not to the link. Thus I use level keyword)
Let’s look at the below network to understand what is the issue and how OSPF multi-area adjacency can solve the sub optimal routing problem. Read more
IS-IS Design considerations on MPLS backbone
Using IS-IS with MPLS require some important design considerations. IS-IS as a scalable link state routing protocol has been used in the Service Provider networks for decades.
In fact, eight of the largest nine Service Providers use IS-IS routing protocol on their network as of today. Read more
What is IS-IS Level 1 ? Why IS-IS level 1 is used ? What are the IS-IS levels ? What is the corresponding Area type in OSPF ?
IS-IS Level 1 is also called as IS-IS Level 1 sub domain. IS-IS is a link state routing protocol, similar to OSPF. You can read detail comparison of OSPF vs. IS-IS from here.
There is an Area concept in IS-IS as well but the purpose of IS-IS Areas are different than OSPF Areas.
In IS-IS Areas are used to create IS-IS Levels and IS-IS Levels are the similar logical concepts with OSPF Areas.
Two create IS-IS Level 1 adjacency between two routers, same IS-IS area number should be used. Otherwise only IS-IS Level 2 adjacency can be created. This is the first rule of IS-IS adjacency relationship.
Which OSPF Area type is similar to IS-IS Level 1 Sub domain ?
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
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.
Figure- 1 Why are dynamic routing protocols used?
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.
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.
OSPF LSA (link state advertisements) are used to create a logical network topology. But Why we have 11 different LSAs ? What are their purposes ? Most important questions many time is not asked by the engineers thus you can’t find many places on the Internet which provides these answers.
The reason of having 11 Type of OSPF LSA packet is Scalability. If the network only consist of small amount of routers ( routers, link and the physical topology is important to define the size ) then you would have maximum two type of LSA.
Let me explain the ospf lsa types and why we would only have maximum two type of LSA will be understood.
Note : In this article only OSPFv2 LSA Types explained.
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.
IS-IS, a link state routing protocol, requires careful attention during network design in order to avoid traffic blackholing. In the topology below, IS-IS routing protocol is used.
Flooding in full-mesh topology is a big concern for network-design experts, especially in large-scale OSPF deployments. When the link or node fails in an OSPF network, failure information is flooded everywhere in the same area. If Flat OSPF network design is used, then the problem gets bigger. Each router receives at least one copy of the new information from each neighbor.
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.
Figure – 1 EIGRP Hub and Spoke
If you design multi level IS-IS network and if you have more than one exit (L1-L2 routers) from the Level 1 domain, you will likely create a suboptimal routing. Multi-level IS-IS design is for large-scale network. What’s more, most of the real life networks use only flat Level 2 IS-IS as their interior gateway protocol (IGP).
I implore all my readers to always remember this topic: IGP LDP synchronization. It is important to use IGP LDP synchronization to avoid blackholing, especially when MPLS networks fails to function effectively.
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
OSPF Design – In the below picture, where should you place an OSPF ABR (Area Border Router) to scale OSPF design ? Why ?
Please share your thoughts in the comment box below.
First 5 correct answers will get my CCDE Preparation Workbook for free. Please subscribe to email list so I can see your email address for communication.
OSPF and MPLS is most commonly used two technologies in an MPLS VPN environment.
In this post I will share a mini design scenario with you and ask couple questions about the fictitious company architecture.
IS-IS is a link state routing protocol. Commonly used in Service Provider networks.
Back in old days, IS-IS routing protocol software was more stable and robust compare to OSPF, thus many service provider choose IS-IS as their interior routing protocol.
I collected the questions which I received from my students and readers related with IS-IS routing protocol and my answers in this post. Read more
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
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
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.
RFC 2547 defines standard MPLS VPN to carry customer prefixes over the MPLS backbone.
In February 2006 RFC 4364 was published for Inter AS VPNs which is known as Multi AS VPNs. RFC 4364 obsoleted RFC 2547 and defined many other applications for MPLS VPNs such as CSC which is also known as Carrier Supporting Carrier with the Cisco terminology and Carrier of Carrier with the Juniper definition.
With basic Layer 3 MPLS VPN , Enterprise customers can carry their prefixes from multiple sites over SP backbone . It is multi point to multipoint connection. With the ATOM based MPLS solution which is Cisco’s E-Line solution , customer sites are connected as point to point and with VPLS multipoint to multipoint.
Basic difference with the VPLS and IP/VPN from customer point of view , with VPLS all attached sites share the same L3 network.Service Provider acts as a big switch for the customer. IP/MPLS VPNs use different IP address at each site.
With the IP/VPN also known as BGP or L3 VPN , customer runs IP routing protocol or static route with the Service provider and Customer equipment which is known as CE don’t see other CE as connected like in VPLS or ATOM based MPLS.
Depending on expectations of the customer from the Service Provider , for the MPLS L3/VPN case, customer can run any of the IGP routing protocols including EIGRP, OSPF, IS-IS , BGP or static route. You may want to talk with your Service Provider before you decide since some Service Providers don’t service every routing protocols. Most of them if not all supports BGP.
If customer wants very granular policy control, dual homed site connectivity, and customer network stuff well trained , best choice would be BGP.
In the past fast convergence was an issue with BGP and maybe still with the vanilla BGP configuration, recent enhancements allow BGP to converge super fast thanks to BGP Fast Reroute Mechanism which is BGP PIC.
All IGP protocol’s metric information can be carried over SP MPLS backbone end to end. In this case SP core behaves differently. For OSPF there is Superbackbone and for ISIS there is L3 backbone concepts. This is out of the scope of this post so I will not explain further.But if you want to learn and interested please comment, so I definitely write about them.
One another caveat for PE-CE protocol , for almost all protocol , if customer has backdoor link to another customer site, loop or suboptimal path usage may occur. We prefer generally MPLS link when it is necessary to have low latency , secure , reliable connection compare to Internet based option.
If customer has backup Internet link (Not MPLS but maybe DSL, 3G/LTE, Satellite,Microwave, Cable) and its requirement is low latency , predictable delay variation which is called as jitter , reliable and secure ( Relative ) connection, probably wants to use MPLS connection as primary and Internet connection as a backup although LTE is much cheaper and provides very high bandwidth nowadays and started to take its place as a primary connection on some networks or part of the network such as remote offices.