I thought this would be the perfect opportunity for a useful blog post. You should have a copy of Doyle’s Routing TCP/IP Volume 1 to be able to really follow this post, as it refers directly to figure 7-7 in that book. I am not attempting to explain everything that Jeff has explained. I just want to clarify what is going on in the figure. Jeff and the people at Cisco Press have graciously given me permission to reproduce the figure in this post.
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Now, maybe I am a little slow, but it took me a some time to wrap my head around this one. First of all, you need to look at both the Figure 7-7 and Table 7-2 together (the figure in this post is mislabeled because it is from the First Edition). When you first look at the diagram, all you see is a series of circles and ellipses with arrows pointing around and between them. You see references to input events. You see circles with text in them. It’s all very confusing. To make sense of it, you have to understand what the diagram is trying to represent.
When a route goes in to an active state, it can either get stuck in active, or eventually return back to a passive state. The diagram is illustrating the various input events that can cause a route to go from active to passive, vice versa, or to stay in its current state, and it shows how DUAL keeps track of the state with various flags.
Looking at the diagram, there is only 1 passive state. Passive is good. When the route is passive, r=0 and O=1. Everything is working. However, there are 4 different types of active states where: (r=1, O=0), (r=1, O=1),(r=1,O=2),(r=1,O=3).
To understand, look at the bottom left circle and the ellipse below it. What the ellipse with the text IE4 and the arrow around it is really saying is that “if we receive Input Event 4 (any input event other than last reply or a query from the successor), we are going to stay in an active state where r=1 and O=0. As long as we keep receiving IE4, we will stay in this state. However, if we receive IE11 (Last reply received; FC met with current FD), then we will return to a passive state. If we receive IE9, we will become active( r=1,O=1). If we receive IE10, we will become active (r=1,O=2).
It’s that simple. If you have any corrections, don’t be shy. I’m just learning this stuff too. FWIW this level of detail is probably overkill for the BSCI exam, however I’ve decided to study in such a way that I’m almost ready for the CCIE written by the time I finish my CCNP. That means i’ll be spending as much time in Doyle’s book as the BSCI Cert Guide. The extra detail and complexity will hopefully give me ideas for useful posts, so you don’t have to spend too much time reading about what kind of cheese I like.
2 Comments
thank you so much. I was facing hard time in understanding this. Finally, got this topic.
Howz you prep going for CCIE ? I am preparing for CCIP.
Cheers,
Saran
It’s in progress. I’ll be taking BSCI in a couple of weeks, depending on if my order from amazon arrives quickly. I’m waiting on some books, because the official cert guide for bsci is inadequate, and I didn’t realize just how inadequate it was until I started getting in to it. I’m making steady progress though, even though some weeks I don’t get much done because of work. That is coming to an end soon, however, so I will be able to accelerate the pace.
Good luck with the CCIP