May
10
Edit: Thanks for playing! You can find the official answer and explanation here. Read More
Apr
04
Hi Brian, What is the major difference in using an E1 route over an E2 route in OSPF? From what I’ve observed, if you redistribute a route into OSPF either E1 or E2, the upstream router will still use the shortest path to get to the ASBR regardless of what is shown in the routing table. The more I read about this, the more confused I get. Am I missing something? Matt Hi Matt, This is actually a very common area of confusion and misunderstanding in OSPF. Part of the problem is that the vast... Read More
Mar
30
OSPF and MTU Mismatch Dear Brian, What is the difference between using the “system mtu routing 1500” and the “ip ospf mtu-ignore” commands when running OSPF between a router and a switch? Thanks, Paul Hi Paul, Within the scope of the CCIE Lab Exam, it may be acceptable to issue either of these commands to solve a specific lab task. However, it is key to note that there is a difference between ignoring the MTU for the purpose of OSPF adjacency and matching the MTU within a real production... Read More
Jan
03
Continuing my review of titles from Petr’s excellent CCDE reading list for his upcoming LIVE and ONLINE CCDE Bootcamps, here are further notes to keep in mind regarding EIGRP. About the Protocol The algorithm used for this advanced Distance Vector protocol is the Diffusing Update Algorithm. As we discussed at length in this post, the metric is based upon Bandwidth and Delay values. For updates, EIGRP uses Update and Query packets that are sent to a multicast address. Split horizon and DUAL form... Read More
Jan
01
This document is presented as a series of Questions and Answers, discussing various aspects of OSPF protocol designed to prevent inter-area routing loops and related issues. The discussion covers ABR functions, Virtual-Links, OSPF Super-backbone, OSPF Sham-Links, BGP Cost Community. Reader is assumed to know these concepts already, as this publication focuses on complex interaction features arising in MPLS/BGP VPN scenarios. The discussion is culminated by analyzing a number of issues arising... Read More
Dec
30
To start my reading from Petr's excellent CCDE reading list for his upcoming LIVE and ONLINE CCDE Bootcamps, I decided to start with: EIGRP for IP: Basic Operation and Configuration by Russ White and Alvaro Retana I was able to grab an Amazon Kindle version for about $9, and EIGRP has always been one of my favorite protocols. The text dives right in to none other than the composite metric of EIGRP and it brought a smile to my face as I thought about all of the misconceptions I had regarding... Read More
Sep
03
I enjoyed Petr's article regarding explicit next hop.  It reminded me of a scenario where a redistributed route, going into OSPF conditionally worked, depending on which reachable next hop was used. Here is the topology for the scenario: Here is the relevant (and working :)) information for R1. Read More
Sep
02
Abstract This publication briefly covers the use of 3rd party next-hops in OSPF, RIP, EIGRP and BGP routing protocols. Common concepts are introduced and protocol-specific implementations are discussed. Basic understanding of the routing protocol function is required before reading this blog post. Overview Third-party next-hop concept appears only to distance vector protocol, or in the parts of the link-state protocols that exhibit distance-vector behavior. The idea is that a distance-vector... Read More
Jun
02
This goal of this post is brief discussion of main factors controlling fast convergence in OSPF-based networks. Network convergence is a term that is sometimes used under various interpretations. Before we discuss the optimization procedures for OSPF, we define network convergence as the process of synchronizing network forwarding tables after a topology change. Network is said to be converged when none of forwarding tables are changing for "some reasonable" amount of time. This "some" amount... Read More
May
03
The purpose of event dampening is reducing the effect of oscillations on routing systems. In general, periodic process that affect the routing system as a whole should have the period no shorter than the system convergence time (relaxation time). Otherwise, the system will never stabilize and will be constantly updating its state. In reality, complex system have multiple periodic processes running at the same time, which results is in harmonic process interference and complex process spectrum.... Read More

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