Apr
29
Probably one of the most confusing topics for people studying BGP/MPLS VPNs are EIGRP SoO and Cost communities. Not the concepts themselves, but rather their purpose. Lack of clear documentation makes it hard to understand why and how the specific features are implemented. In this blog post we discuss the BGP Cost community and EIGRP SoO attribute. First, a brief overview of MP-BGP is given along with general concepts of route redistribution in MPLS/BGP VPNs. Next, the problems that arise when... Read More
Mar
17
Overview In this short blog post we are going to present a simple procedure for IPv4 prefix summarization. The procedure is based on the one found in “Optimal Routing Design” book by Russ White, Don Slice and Alvaro Retana, but differs in some respects. The process is three step, and require the use of Windows calculator for ease of computation. No binary breakdowns are involved, just some basic arithmetic. For the sake of simplicity, we skip all proofs, as those are trivial. The same approach... Read More
Jan
01
In this blog post we are going to discuss some OSPF features related to convergence and scalability. Specifically, we are going to discuss Incremental SPF (iSPF), LSA group pacing, and LSA generation/SPF throttling. Before we begin, let's define convergence as the process of restoring the stable view of network after a change, and scalability as the property of the routing protocol to remain stable and well-behaving as the network grows. In general, these two properties are reciprocal, i.e.... Read More
Nov
20
Every once in a while I come across a tip that is so exciting I want to share it with the world. I was recently going through one of the many posts I read, and saw the answer to a question that I have been wondering about for many years. Awesome job to Steve Shaw who came up with this. Here is the scenario. Read More
Nov
13
OSPF FA Overview This post is dedicated to one “esoteric” OSPF external route filtering method based on hiding OSPF Forwarding Address. Recall the meaning of OSPF FA. This is a special field used on OSPF type 5 and 7 LSAs to convey the information of the “external route source”. The purpose of FA is to optimize forwarding in situations where the external route source is connected to a shared segment. Here is an example: Read More
Sep
17
This is the follow up discussion for the post titled, "Have you seen my Router ID?" The underlying issue here was trying to get OSPF to bypass the usual selection process for Router ID. The normal selection order is: Manual router ID configured under ospf process Highest IP address of a loopback in the up state in the respective routing table Highest IP address of an interface of an up state in the respective routing table If there are no up interfaces and you have not manually configured a... Read More
Sep
17
In yesterday's post, titled "Have you seen my Router ID?", a challenge section was provided. This post will focus on scrutinizing the section itself, from a strategy / analysis point of view. From a high level overview, we have two devices peering OSPF over a FastEthernet link, with some loopback networks advertised by one side, and received on the other router.  If that was all that the section was asking for, then it should be a task that anyone at CCNA level could complete.  When looking at... Read More
Sep
16
There is more than one possible solution for this challenge. Feel free to post your proposed answer in the comments section. We will try to keep comments hidden from public view, so that the fun isn't spoiled for others. Also, don't feel bad if the answer(s) aren't immediately apparent. A number of very bright people have been puzzled by this scenario.  Answers will be posted on Friday, September 18th. Read More
Sep
15
The feature we are going to talk about today may look a bit convoluted, but it demonstrates core OSPF behavior: combining link-state and distance-vector behaviors. The command capability transit was introduced in IOS 12.3T and is on by default. However, the description is rather confusing and does not explain the underlying mechanics. We are going to give an in-depth look at this feature now. What is Transit Capability? In short, this is a special property of a non-backbone area that allows... Read More
Sep
04
We have added a lesson to the 10-day ATCOD on EIGRP for IPv6. This is a key CCIE R/S Version 4.x topic. Enjoy! Read More

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