Im watching some free vLectures from IPexpert here, the one im watching is the one about Spanning-Tree. Its pretty good so far. I do hope however, that their video-on-demand products are alot better quality. Im still waiting to take the plunge and buy their BLS (Blended Learning Solution) until I know where I stand both financially and knowledge wise (written).

Anyways, its good information/knowledge, by very smart people, so I suggest you head over there and check it out!

So im finishing up some troubleshooting exercises in Routing TCP/IP. They are mainly about the metrics used by EIGPR. Here’s a review of the functionality of EIGRP and how it chooses the best path to a destination.

First of, EIGRP is an Enhancement of the proprietary IGRP protocol, which is almost but phased out by now. EIGRP is still a distance vector protocol, since it routes by rumor. Each router does not have a full view of the entire network topology and can therefor not base any decision on

Lets nail down how the metric itself is calculated:

EIGRP (and IGRP for that matter) is based on a range of values. Bandwidth,  load, delay, reliability and MTU. This is called a composite metric. Each of these are called a K-value. K1=Bandwidth, K2=Load, K3=Delay, K4=Reliability, K5=MTU. At the moment, only Bandwidth and Delay has any influence in the composite metric. Therefor K1=K3=1.

The bandwidth in this calculation, is the minimum bandwidth along the path. The delay is the cummulative delay along the entire path.

The bandwidth is inverse to 10^7, and the delay is divided by 10.

Lets take an example, here’s the topology:

What we want to calculate here, is the metric from R1 to the subnet 10.10.10.0/30.

First of, whats the minimum bandwidth configured. That would be the serial link between R1 and R2. It is only 56kbits.

Lets jot that one down:

Bw = 56 = 10^7/56 = ~178571.

Next up is the delay: 20000 + 7000 + 1000 = 28000, out delay is then:

Dly = 28000 / 10 = 2800.

To get the total metric, we add these two numbers: 178571 + 2800 = 181371. Now EIGRP multiplies the metric by 256, so we get: 46430976.

This is our total metric from R1 to the net 10.10.10.0/30.

Now some terms that used in the EIGRP lingo.

Advertised distance = The metric as reported by a neighbor.

Feassible distance = The metric currently used as the best path.

Successor = The next hop router currently used by this router.

Feassible successor = A next hop router that can immediately be used if the primary (successor) fails.

Feasibility Condition = A condition that states that an advertised distance must be less than the current feassible distance.

To choose a successor, which is basically the path (vector) to use for next hop. The router chooses the advertised distance from each neighbor + the local link metric, and the best one wins. Simple enough. Besides this, if any advertised distance meets the feasibility condition (advertised distance < feasible distance), it will be entered into the topology table as a feasible successor.

Now I mentioned the topology table, this is the table of sucessors along with feassible succesors for each route. There’s also a neighbor table, which lists the routers immediate neighbors (next to on data-link).

When everything is stable and fully converged, a route is said to be in a passive state. If something happens, a local computation occurs. If the router has a feassible successor, this one is choosen immediately, and everything is again stable. The router never puts the route into active. If the router does not have a feassible successor, heaven and earth is set into motion. The router puts the route into an active state, queries are sent to search for a better route to this destination/route. If the neighboring router doesnt have a feassible successor, it will also send out queries. When all replies have been received by all routers, the destination/route is put into the passive state (if a route can be reached at all).

A quick note about timers and adjacencies about EIGRP.

Hello’s = On broadcast networks, the hello timer is every 5 seconds. On serial links and links slower than T1, the hello’s are sent every 60 seconds.

Hold Downs = Set to 3 times the hello interval. This means 15 seconds for broadcast and 180 on serial links and slower than T1 links.

An adjacency is defined as two EIGRP speaking routers sharing the same data-link, with common required attributes. Common attributes are the K-values and the subnet, along with authentication options.

The last thing I want to mention is the SIA (Stuck In Active) concept. It is a theoretical concept that queries sent out to neighbors are further replicated downstream to other routers, which again send them further because they dont have a different feasible successor. There’s a timer called the active timer, which is set to 3 minutes. If the router does not receive a reply within that period, it is declared SIA, the route is tagged unreachable from that neighbor, and the neighbor is declared dead. This will cause all routes from which the neighbor is the next hop, to be declared unreachable, causing the route to be locally computed again and maybe causes the other routes to be set into the active state, and queried itself. This can cause a slow convergence of multiple routes. There are two extra types of packets that EIGRP uses. The SIA query and SIA reply. After 90 seconds a SIA query is sent from the originating router, to make sure its still “alive” on that route, the neighbor router then further propagates this query. If the SIA reply is received, the router wont be declared dead for 3 times the SIA query is sent out, making the other network routes more stable by not putting them into active. The total time a router can be “held up”, is then 6 minutes (90 seconds before first SIA query, another 90 before the next, and another 90), all in all 6 minutes. This is push come to shove, and hopefully a well designed network wont take that long to converge.

I hope some of this info is of use to somebody, but at least it made me rethink everything

At the bottom of this post is a link to a small article by Jared over at IPexpert about the books he recommends for the CCIE written exam. Anyone who looked at the recommended book list from Cisco is blown away. I believe theres 21 books on that list, most of which, I suspect is no less than 600 pages. Jareds list is only one third of that. This is a guy with 3x CCIE’s under his belt, so I take his words as the definite truth :

I am still reading through the Routing TCP/IP Vol. 1, to get a good feel for the routing protocols (all except IS-IS because its not on the blue-print, and it generally annoys me). After that I think I will follow his advice and go with the Exam Guide to put everything into context.

Here’s the famous link: http://ipexpert.ccieblog.com/2008/11/21/so-much-information-part-one/

So im reading about EIGRP’s DUAL algorithm, and how it works, and MAN, its confusing. After the example in Routing TCP/IP, even they mention it might require several readings. Basically it boils down to the fact if a router being notified of a bad link has a feasible successor or not. If it doesnt, it goes into an active state and sends out queries to all of its neighbors. It will stay in active until it has received a reply from all of its neighbors it has queried. However, if it does have a feassible successor, it will switch to that one, and notify its neighbors of its change.

Here is where im on shaky grounds. If a router receives an update, via a path its using as a the successor, it will always perform a local calculation, and if the FD is higher than the current FD, it will look for a feassible successor, and if no one is found, it will query neighbors.

The issue in tracing this is the events of things, and how they all fit together. An update might cause a route change. A query must nessecitate a reply. All of this, and we are not even talking about SIA (Stuck In Active) yet. When you dig down into this, you really see how complex some protocols are.

A good thing about this though, Routing TCP/IP (which im beginning to believe is THE authority on igrp’s) states that EIGRP is a distance vector protocol, and not the hybrid stuff that previous material was keen to make it out to be . It is still a send this route to this neighbor, and then it will send the route to the next neighbor. The entire routing “belief” system is based on what its peers are “believing”. This in stark contrast to link-state, where everyone has to make up their own mind about the topology.

Okay, im off to relax for a bit. Heavy stuff this DUAL.

I wanted to post a link to a great site which I have started using called flashcarddb.com, what it does is pretty simple, but it does its job in a slick fashion.

There is a system called the leitner system, which in all its simplicity is making you remember things by repeating them in periods with different intervals. If you miss a question, it is scheduled to be repeated more often, until you get it right every time. This I think will come in handy with some things I simply cant remember. I will be using this along with normal note taking procedures. I want to enter in as many as required. Right now reading through Routing TCP/IP Vol. 1, I am entering in some must-remember facts, which I can then later on practice. So its not a replacement for notes, but a supplement to make sure you get facts straight.

I have a cold at the moment, so progress is slow, want to read an hour before I head off to bed.

Im having some “fun” with static routes. Its an okay topology, 4 routers connected. The task asked for having some floating static routes, along with some load-balancing routes. Since they are CEF based, it will be load-shared by default to destination load sharing. Note to self: if incomming interface is CEF, what is the output switching method going to be? — Thats right, CEF.

One thing that strike’s me, is the complexity of static routes in a relatively small network. The use of a dynamic routing protocol in even small topologies become quite evident. The task also required you to write static routes in their full length, with the use of summarization. Then you REALLY have a mess to work with. Default routes on stub routers will save alot of typing/head scratching.

Anyways, good stuff to read and review.

So like many others finishing their CCNP’s, ive been looking at the CCIE R&S track. There are two obstacles to become one of the prestigious ones with CCIE digits. One is the written exam, and the second is the lab exam. Of these two, the lab exam seems to be the big one of the two. I would like to add a third however. That one is the study path to passing the other two.

What I mean by this is how you layout a complete plan on how you want to obtain the CCIE, from the beginning to the finish. There are alot of seperate things on both the written and the lab blueprint, with very little guidance on what sort of gap to expect from CCNP to CCIE (R&S). It is therefor pretty hard to determine what books to read, what labs to do, what to read up on, in what order to do things and so on and so forth. I have asked around about this, and it is very individual how people tackle this issue. Some people will prepare for the lab exam with full vendor material along with books to learn the technologies, and during this course they will do the written. Another approach is to treat the written for itself and after that is done, study for the lab. I am at a toss-up on what is the right choice, should I ever embark on the journey. My beef with doing the whole thing at once, is whether or not the theoritical part is there to do the vendor material, and with the written-then-lab approach, it is that the written will be all theory and not much hands-on to back it up.

In the end, I am 100% convinced that a plan of any sort is nessecary for any chance of success, so this is not something one can just skip by and hit the lab from the get go. A vendor offered plan for this sort of thing would be in order I think. They have alot of success-stories and will therefor be able to form a clear and consise opinion on how to start planning out your journey.