CCNP TSHOOT EIGRP TUTORIAL:

Being Passive Isn't Always Bad

(Good Reading For CCNAs-To-Be, Too!)

Chris Bryant, CCIE #12933!

As a current Bulldog and a future CCNP, you're an aggressive student, and you really go after what you want.

In other words -- you're Active, not Passive.

(I have a feeling I capped those words for a reason.)

But being Passive isn't always bad -- and when it comes to our EIGRP routes, it's generally a good thing.

Let's take a look at a typical EIGRP Topology table to see what the heck I'm talking about...

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status

P 3.3.3.3/32, 1 successors, FD is 2297856

via 172.12.123.3 (2297856/128256), Serial0
via 172.12.123.2 (2323456/409600), Serial0

P 2.2.2.2/32, 1 successors, FD is 2297856

via 172.12.123.2 (2297856/128256), Serial0
via 172.12.123.3 (2323456/409600), Serial0

We know that EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate routes.

We also know that DUAL calculates backup routes (Feasible Successors) before they're actually needed and then places them into the Topology table. This is why EIGRP is able to cut over to a backup route so quickly when needed -- because it's already been calculated.

That's great, right? Right!

But what if there is no backup route?

DUAL will mark the route as Active, indicating that the route is being recalculated and cannot be used to route data. DUAL will then send out a Query message to all of that router's EIGRP neighbors.

A DUAL Query is basically one neighbor asking another, "Hey, do you know how to get to this network I just lost my route to?"

If that neighbor has a route, the query will be answered with that route. If the neighbor doesn't have such a route, that neighbor will ask its neighbors. The process continues until one of two things happens...

A downstream router responds to the query with a valid, loop-free route, which will quickly be marked as Passive in the EIGRP topology table.

No downstream router responds to the query with a valid, loop-free route, and the original route remains marked as Active, and is therefore unusable.

An EIGRP route in an Active state sounds great, but as you see, it's what we don't want to see.

Now we come to a good news / bad news situation --

The good news is that you can go a long time without even seeing an EIGRP go Active, and even if you happen to see that scarlet "A" in your Topology table, it likely will not be there by the time you can repeat show ip eigrp topology - even if you use the Up arrow to repeat the command instead of typing it again.

The bad news is that t-shooting these SIA suckers can be more than a little difficult, since there's no one "magic reason" that a route goes SIA.

We'll take a detailed look at SIA routes in the CCNP TSHOOT DVD and Study Guide, and more free CCNP TShoot videos and tutorials are on the way!

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Chris Bryant

"The Computer Certification Bulldog"

CCIE #12933

chris@thebryantadvantage.com