OSPF Load Balancing is to place multiple next-hops into the Routing and Forwarding table for a given IP destination prefix. In this post, we will look at OSPF Load Balancing, OSPF Load Sharing, OSPF ECMP, OSPF UCMP, where we should use it, where we shouldn’t use it, and what can be dangerous if we have OSPF Load balancing will be explained.

OSPF Equal Cost Load Balancing – OSPF ECMP

What is OSPF Equal Cost Load Balancing let’s have a look at the below topology and let’s try to understand?

In the above topology, the 192.168.0.0/24 network is connected to Router D.

As a link-state routing protocol, OSPF routers in the network would know that the 192.168.0.0/24 subnet is connected to Router D.

And they would run SPF/Dijkstra algorithm to calculate the shortest path to this destination.

In the above topology, Interface costs are shown.

When we look at Router A to 192.168.0.0/24 subnet, we have two paths. A-B-D and A-C-D.

Both of the paths’ total cost is 10+10 = 20.

Thus, Router A can do load balancing for that destination prefix.

When OSPF has two paths, we don’t need to Continue reading

BGP AS Path Prepending or BGP prepend is a common technique for incoming path manipulating. When we want to engineer the traffic coming from another BGP AS to our BGP AS, BGP AS prepending is one of the most common mechanisms. There are cases BGP AS Prepend doesn’t work and shouldn’t be used as well, and in this post, we will look at them too by using the below topology.

In the above topology, we have two BGP Autonomous Systems. AS 200 is Customer BGP AS, and AS 100 is Provider BGP AS.

As a customer, AS 200 wants AS100 to send the traffic over the left path as a Primary path and the right path as a backup path as is depicted in the above topology.

BGP AS Path Prepend

When we want to have Primary and Backup Paths as it is depicted in the above topology. BGP AS Path Prepending technique is used to influence upstream BGP Autonomous Systems’ decision.

BGP Prepend means, adding our BGP AS to the AS-path multiple times. In the above topology, 10.0.10.0/24 network’s BGP AS 200 is advertised with 3 AS prepend. By default when the prefix is advertised to Continue reading

In this post, we will compare EIGRP and OSPF. We will look at some of the important aspects when we compare EIGRP vs OSPF. From scalability, standardization, working on different topologies and many aspects will be compared in this most detailed comparison blog post on the Internet.

We prepared the above comparison chart for EIGRP vs OSPF comparison. We will look at some of those important Comparison criteria from a design point of view.

EIGRP vs OSPF Scalability

OSPF supports two layers of Hiearchicy. OSPF Backbone areas and OSPF Non-backbone areas. EIGRP on the other side supports as many as you want. You can summarize EIGRP prefixes at every hop. This capability provides a scale advantage to EIGRP. In EIGRP, we don’t need an ABR node for summarization for example.

EIGRP vs OSPF in Full Mesh, Ring and Hub and Spoke Topologies

The full mesh may require a lot of logical connections, OSPF with Mesh-group feature can scale but it can be a scaling problem for the EIGRP networks. If we think that in real-life networks, EIGRP is usually used in Hub and Spoke topologies most of the time, expecting EIGRP to run on Full-mesh topologies is not Continue reading

In this post, we will compare BGP and OSPF. We will look at some of the important aspects when we compare BGP vs OSPF. Although OSPF is used as an IGP and BGP is used mainly as an External routing protocol, we will compare from many different design aspects. Also, BGP can be used as an Internal IGP protocol as well and we will take that into consideration as well.

We prepared the above comparison chart for BGP vs OSPF comparison. We will look at some of those important Comparison criteria from a design point of view.

BGP vs OSPF Scalability

One of the biggest reasons we choose BGP, not OSPF is Scalability. BGP is used as a Global Internet routing protocol and as of 2022, the Global routing table size for IPv4 unicast prefixes is around 900 000. So almost a million prefixes we carry over BGP on the Internet.

So, proven scalability for BGP we can say. OSPF usually can carry only a couple of thousands of prefixes, this is one of the reasons, OSPF is used as an Internal dynamic routing protocol, not over the Internet.

BGP vs OSPF in Full Mesh, Ring and Hub and Continue reading

BGP Weight Attribute is used in Cisco routers. In this post, with the below topology, we will look at why the BGP weight attribute is used, why it BGP weight shouldn’t be used, advantages and disadvantages of the BGP weight attribute.

Cisco BGP Weight Attribute

Let’s first define what is BGP Weight attribute. BGP selects the best path based on the BGP path attributes. Weight is considered a very important tie-breaker in BGP’s best-path selection.

When there are two paths to any BGP destination prefix, the BGP Weight attribute is compared before BGP Local Preference and many other BGP Path attributes.

Since this is not BGP’s best path selection post, and assuming you already know the process, please note, Weight attribute is compared before even BGP Local Preference.

But, let’s have a look at the below topology to understand it better.

In the above topology, we want to use the left path for the prefixes in AS1, thus we have a higher BGP Local preference value.

As the BGP Local preference value is exchanged internally between all IBGP neighbors, both left and right routers in AS65000, use the left exit point, which is Local Pref 100 to reach the Continue reading

BGP Interview questions and answers are shared here. In this post, we will look at some of the important BGP questions that are asked in the Interviews and some of the certification exams. You can consider this as a BGP Quiz and test your BGP knowledge.

Which of the below option is the reason to run IBGP? (Choose Two)

A. It is used for the reachability between PE devices in the MPLS network

B. It is used to carry EBGP prefixes inside an Autonomous System

C. It is used with Route Reflectors for the scalability reason in large scale networks

D. It is used to prevent failures outside your network from impacting your internal network operation

Answer: One of the correct answers to this question is to carry EBGP prefixes inside an Autonomous system. IGP is used for the reachability between PE devices in an MPLS network. Option C is valid but not the correct answer, because; the question is asking about the reasons, not the best practices.

Option D is one of the correct answers as well because, with IBGP, the internal network is protected from outside failures by separating the local failure domains.
That’s why; the answers to Continue reading