EIGRP External Route Path Selection

Lessons Discussion
1

This topic is to discuss the following lesson:

2

Hi Rene
I have two notes about router one configuration :-
1 - In the configuration of external route ā€“ same AS number you need to advertising the 192.168.13.0 network or change the wildcard mask to 0.0.1.255 otherwise the router one and router three will not become neighbors.
2 - In the configuration of external route ā€“ different AS if you get rid of eigrp AS 1 we will lose the neighbor adjacency with router two.
please correct me if I wrong
Thanks a lot

3

Hi Hussein,

You are right, I left some errors in the configsā€¦oops! Just fixed them, thanks :slight_smile:

Rene

4

No thanks to the duty :slight_smile:

5

Hi,

Is this External routes behaviour the same for Internal routes , i.e 2 different internal AS processes?

Cheers,
Rob

6

Hello Robert

External routes are defined as routes learned by an EIGRP process via redistribution. This redistribution can also take place from one EIGRP AS to another just like in the lesson. In the lesson, you can see that these routes are marked with the EX designation, meaning they are indeed external routes.

This behaviour of choosing the lower EIGRP AS as better takes place only when you have multiple ASā€™es configured within your EIGRP topology, and when the same route is learned from two different EIGRP AS processes.

Now when you say ā€œinternal routes, from two different internal AS processesā€ Iā€™m not sure what you mean. All routes that are ā€œinternalā€ to a router, are those that are directly connected. Those are not learned via EIGRP, but are automatically placed within the routing table. All routes learned via EIGRP are learned from other routers. So you never have any ā€œinternalā€ routes learned via EIGRP.

I hope this has been helpful!

Laz

1 Like
7

Thanks for the explanation, I think I have my terminology mixed up here, I guess when there are 2 EIGRP ASā€™es, when they exchange ā€˜internalā€™ routes with each other, they will become ā€˜externalā€™ (as they are external to the AS, so what I mentioned is not actually possible in hindsight (after your explanation).

Cheers,

1 Like
8

Um so your saying we should use Vrf and if we needed to set this up where it would work?

9

Hello Brian

Hmm, Iā€™m not sure where you find that information. I donā€™t see any mention of VRFs in the lesson. Can you clarify? Thanks!

Laz

10

this is a post placed in wrong forums sorry can be deleted out. I dont remember the question as well which meant I was kind of just confirming something I already knew. However, does not go with this topic as I read through it three times just to make sure and this topic was about preference not about separating traffic or routing which is what VRF is.

Sorry!

1 Like
11

Hello Team,

May I know why it installs the lowest AS learned route despite of better metric having learned form higher AS
processā€¦is there any specific reason ?. Bcoz I labbed this by having higher AS with better metric and lower AS with worst metric still it chose the lowest AS learned route.

12

Hello Sathish

The administrative distance is a value used to determine which source of routing information is preferred. The AD will choose between various sources such as BGP, OSPF, EIGRP RIP, static routes, and directly connected routes, to name a few. The AD is always the first thing that is examined to determine if a route will enter the routing table. At this point, the metric is irrelevant.

For EIGRP, the AD plays a special role because different types of EIGRP routes are given different ADs. The externally learned EIGRP routes have an AD of 170 while internally learned routes have an AD of 90.

Based on the way the AD and the metric are used to populate the routing table, the AD is always taken into account first before the metric is even examined.

So it is for this reason that, regardless of what metric you have, the lower AD is always chosen first. Does that make sense?

I hope this has been helpful!

Laz

13

Thank you and Agreed, my question related to the statement in the lesson of below, not to the AD

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14

Hello Sathish

My apologies, thanks for the clarification! As stated in my previous post, the first thing that is looked at is the AD, regardless of the metric. The choice of the lowest AS only kicks in when there is a tie in the AD values. But thatā€™s still part of the process involved with the AD, before the metric comes into play.

So the process is:

  1. Check to see the source with the lowest AD
  2. If we have two EIGRP processes with the same AD, then we use the lowest AS to break the time regardless of the metric of either of those routes!
  3. Once we choose that source, only then will we take a look at the metric (in case there is another route that we learned from the same EIGRP AS).

Does that make sense?

I hope this has been helpful!

Laz