Sunday, June 08, 2014

CCNA And CCNP In-Depth Lab:

The Three (Three?) ADs Of EIGRP

Your only cost for this huge CCNA and CCNP tutorial is a share or a Google +1.  Thanks!   -- Chris B.

A few basics of admin distance before we get EIGRP-specific:

The administrative distance of a route doesn't really have anything to do with "distance".  It's actually a measurement of the believability of a route source. 

The lower the AD, the more believable the source is.

The AD range is 0 - 255, with 255 indicating an untrustworthy source.

Simple enough, right?  Right!  

But.... (and you knew that was coming) .... there are some additional details that go along with ADs that we need to know for our CCNA and CCNP exams as well as avoiding confusion when working with real-world networks.

Take the admin distances of EIGRP routes, for example.  And yes, I said "distances", because there are multiple EIGRP admin distances -- and not just the two you might already know about.

Let's take a look at the two most common EIGRP admin distances in action on Cisco routers, and review some other important EIGRP concepts along the way.

In this lab, R1 and R2 have an EIGRP adjacency over the /24 network, and all routes we add to the network will be added to R1.  Autosummarization is disabled on both routers.

R1 has two loopbacks, /32 and /32, that are not shown, as they're not part of the lab -- yet. 


We'll add one loopback to EIGRP on R1 via the network command.   The all-zeroes wildcard mask means the single address indicated in the network command will be advertised by EIGRP.

R1(config)#router eigrp 100


Does the route appear in R2's EIGRP route table?

R2#show ip route eigrp is subnetted, 1 subnets
D [90/2297856] via, 00:00:06, Serial0

Yes, and it appears with the router table code "D", indicating an internal EIGRP route.   That route type has an admin distance of 90, as seen in the brackets.   The first number in the bracket is always the AD, and the second is the metric for the route.

We'll demo the second route type by using the redistribute connected and default-metric commands in R1's EIGRP config. EIGRP requires a seed metric for successful redistribution.  For you CCNPs-to-be, I'll use IOS Help to show each value you need for that command. This is good stuff for future CCNAs and CCENTs as well, but don't expect to see this command on your exams.

R1(config)#router eigrp 100

R1(config-router)#default-metric ?
  <1-4294967295>  Bandwidth in Kbits per second

R1(config-router)#default-metric 1500 ?

  <0-4294967295>  Delay metric, in 10 microsecond units

R1(config-router)#default-metric 1500 10 ?

  <0-255>  Reliability metric where 255 is 100% reliable

R1(config-router)#default-metric 1500 10 255 ?

  <1-255>  Effective bandwidth metric (Loading) where 255 is 100% loaded

R1(config-router)#default-metric 1500 10 255 1 ?

  <1-4294967295>  Maximum Transmission Unit metric of the path

R1(config-router)#default-metric 1500 10 255 1 1544

R1(config-router)#redistribute connected

What does R2's routing table show us now?

R2#show ip route eigrp is subnetted, 1 subnets
D [90/2297856] via, 00:06:52, Serial0 is subnetted, 1 subnets

D EX [170/2297856] via, 00:02:30, Serial0

The new route is in the table and is marked "D EX".  That indicates an EIGRP external route, and the AD of that route type is 170. 

Note the first route still has an AD of 90, even though it is a connected route on R1, which would fall under the "redistribute connected" command we just configured.

When a route is added to EIGRP via both the network command and the redistribute connected command, the network command takes precedence and the route will be an EIGRP internal route.

Those are the two main admin distances of EIGRP routes, and the ADs you'll see most often in real-world networking.

But I did mention a third, didn't I?  

We'll run a lab on the other AD values in just a moment -- but let me address another concern for a moment.

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See you there!   Right now, let's head back to the lab!

We can go beyond the default EIGRP admin distances of 90 and 170 and set the distance of these route types manually with the distance command.  

Please note: Even if you're familiar with this command, read the rest of this tutorial. There's a little trick with this command that a lot of network admins don't know about.

R2 has two routes in its EIGRP table right now, one with an AD of 90 and another with an AD of 170.

R2#show ip route eigrp is subnetted, 1 subnets
D [90/2297856] via, 00:00:15, Serial0 is subnetted, 1 subnets
D EX [170/2297856] via, 00:00:15, Serial0

Let's say we need to make these routes even more trustworthy, and lower the AD for each route type to 50.

There are two things we have to know to make this happen:

1.  What command do we need? 

2.  On which router or routers do we need this command?

We know the answer to #1 already -- but what about #2? Should we change the AD on R1, R2, or both?

Let's start with R1, along with a little IOS Help.

R1(config)#router eigrp 100
R1(config-router)#distance ?
  <1-255>  Administrative distance
  eigrp    IP-EIGRP distance

R1(config-router)#distance eigrp ?

  <1-255>  Distance for internal routes

R1(config-router)#distance eigrp 50 ?

  <1-255>  Distance for external routes

R1(config-router)#distance eigrp 50 50 ?


R1(config-router)#distance eigrp 50 50

17:15:08: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor (Serial0) is down: route configuration changed
17:15:08: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor (Serial0) is down: route configuration changed
17:15:26: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor (Serial0) is up: new adjacency

Two very important notes regarding this config:

Using the distance command will result in your local EIGRP adjacencies coming down.  If nothing else has been changed, they should come back up quickly, but this isn't something you want to do on a production network at a busy time of day.  In this example, the adjacencies came back in about 18 seconds.

Don't use the numeric option with the distance command in this situation. To change the admin distance for all your EIGRP routes, use distance eigrp. (The numeric option has a LOT of other mandatory values you'll need to configure.)

Now -- what effect did this have on the EIGRP routes on R2?

None whatsoever.

R2#show ip route eigrp is subnetted, 1 subnets
D [90/2297856] via, 00:00:01, Serial0 is subnetted, 1 subnets
D EX [170/2297856] via, 00:00:01, Serial0

Administrative distance is locally significant only.  When you change the AD of a route, that change isn't advertised.  It only takes effect locally.

Let's remove the distance command from R1 and add it to R2's config.

R1(config)#router eigrp 100

R1(config-router)#no distance eigrp 50 50

R2(config)#router eigrp 100

R2(config-router)#distance eigrp 50 50

The result on R2:

R2#show ip route eigrp is subnetted, 1 subnets
D [50/2297856] via, 00:00:04, Serial0 is subnetted, 1 subnets

D EX [50/2297856] via, 00:00:04, Serial0

Ta da!  The AD of both EIGRP route types on R2 is now 50!

Two more important notes concerning this command:

The network command is an "all or nothing" command.  You can't specify a specific EIGRP internal route for an AD change while leaving the other internal routes untouched.  Same for the external routes.

While you don't have to change both ADs, you do have to specify both in the command.  Let's say we wanted to set the AD of EIGRP internal routes to 100 while leaving the external route AD at 170.  Here's how we do that:

R2(config-router)#distance eigrp ?
  <1-255>  Distance for internal routes

R2(config-router)#distance eigrp 100 ?

  <1-255>  Distance for external routes

R2(config-router)#distance eigrp 100 170

R2#show ip route eigrp is subnetted, 1 subnets
D [100/2297856] via, 00:01:55, Serial0 is subnetted, 1 subnets
D EX [170/2297856] via, 00:01:55, Serial0

A lot going on with a simple one-word command, eh?  : ) 

Thanks for reading today's Cisco Command Discussion.  Right now, be sure to read these other important discussions, and I'll see you Monday -- and in my Video Boot Camps!

Chris B.


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