ROUTE Notes – EIGRP Neighbor Relationships

Or neighborships, as they call it in the book.  What a terrible word.

Study Questions

  • What settings must match between two routers in order to become EIGRP neighbors?

Both routers must be in the same primary subnet
Both routers must be configured to use the same k-values
Both routers must in the same AS
Both routers must have the same authentication configuration (within reason)
The interfaces facing each other must not be passive

  • What are the default hello and hold times in EIGRP?

On links with bandwidth > 1.544Mbps:
Hello:  5 sec
Hold:  15 sec

On links with bandwidth <= 1.544
Hello:  60 sec
Hold:  180 sec

  • How do you change the hello and hold times?

You set these values at the interface.

R1(config-if)#ip hello-interval eigrp 1 X
R1(config-if)#ip hold-tim eigrp 1 X

  • How do you keep an interface from being used for EIGRP discovery?

Don’t configure a network statement that includes that interface
Make the interface passive
Configure static neighbors for that interface

  • Why might NTP be a good thing to use in regards to EIGRP?

EIGRP uses key chains for authentication.  Key chains can be configured with a range of valid dates and times.  If the time on two routers was off by even a few seconds, some keys would expire, causing neighbor relationships to drop.

  • How do you configure EIGRP authentication?

In each interface participating in EIGRP, you configure the authentication mode and the key chain to use.

R1(config-if)#ip authentication mode eigrp 1 md5
R1(config-if)#ip authentication key-chain eigrp 1 KEYCHAIN1

  • What are the k-values that EIGRP uses?

k1 = bandwidth
k2 = load
k3 = delay
k4 = reliability
k5 = MTU

  • How does a router choose its router ID in EIGRP?

First, it looks for a configured router-id in the EIGRP configuration.  If none exists, it uses the highest (largest) IP address configured on a loopback interface.  If no loopbacks exist, it uses the highest IP configured on the other interfaces.

What Command Was That?

What command tells you…

  • …whether a neighbor was discovered or statically configured?

show ip eigrp neighbor detail

  • …what interfaces are involved in EIGRP?

show ip eigrp interfaces

  • …what k-values your router is using?

show ip protocols

  • …how long your router has been neighbored with another router?

show ip eigrp neighbors

  • …what your router ID is?

show ip eigrp topology
show ip eigrp accounting

  • …a summary of the configured network statements?

show ip protocols

  • …the configured hello interval?

show ip eigrp interface detail

  • …the configured hold time?

There’s isn’t a way to do it directly.  You have to check your neighbors several times over the course of a few seconds to see where the hold timers drop to before resetting.

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages. If you have any questions, the best way to contact me is through Twitter at @aconaway.

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5 comments for “ROUTE Notes – EIGRP Neighbor Relationships

  1. February 17, 2011 at 5:38 pm

    I am guessing the NTP authentication is for when you have the accept-lifetime command configured?

  2. February 17, 2011 at 7:44 pm

    NTP authentication isn't really directly related with the EIGRP keys; it's just a method to protect NTP from being used by unauthorized devices.  NTP itself is related to EIGRP in that it keep each device's time in sync to make sure they all start and expire keys at the same time.

  3. January 24, 2012 at 11:47 pm

    thanks for this EIGRP article

    your style of writing, which is neatly “configured” suit a reader like me…

    thanks for summed up this EIGRP thing (sory bad english)

  4. Josiah
    May 27, 2013 at 8:34 pm

    “Both routers must in the same AS”

    I believe you want:
    Both routers must BE in the same AS

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