Introduction to EIGRP

Lessons Discussion
184

Hello,
I am trying to understand the below paragraph.
“Sia Status (Stuck in Active): This is a bad one…it means that EIGRP has not received a reply to a query packet from one of the neighbors within the allowed time (about 3 minutes). When this happens, EIGRP will drop the neighbor adjacency, and it will be stuck in active. More on this later!”

Now my question is, If a network is unreachable in EIGRP then it will send a query packet to its neighbors. If the router does not receive any reply for that query then why would it drop its neighbor adjacency ???

185

The thing I struggled with is I assumed that this was the distance for each router to the destination. It is not about that. What this lesson is saying is that each of these routers is advertising a route with a certain feasible distance to R3 while following the split horizon rule. After all of that R3 looks at all of the routes for the destination that it has received from all of the routers and looks at the lowest feasible distance while following the feasibility condition.

one of the questions I had for a while was if split horizon blocks the interface on R1 from advertising wouldn’t it block it if we had 3 routers like this R1 → R2 → R3. It would not because split horizon just stops us from advertising a route out of the same interface it doesn’t stop us from receiving a route and adding our interface metric to it.

just wrote this just in case it helps anyone else. :smiley:

1 Like
186

Hello Aamir

This has been responded to here:

I hope this has been helpful!

Laz

187

Hi i have a doubt
I have the follow configuration af-interface (WHAT DO ? all interface ?)

router-eigrp xyz
address-family ipv6 autonomus-system 65000
af-interface default
passive interface 
shutdown

interface ethernet 0/0
no passive
no shut
188

Hello Ugo

The af-interface command is used to define user defaults to apply to EIGRP interfaces that belong to a particular address-family. The command can only be used with EIGRP’s named mode.

The command can be issued from the address-family configuration mode within the EIGRP router configuration. You can enter the configuration mode of all EIGRP interfaces on the router and issue commands for all of them, or you can choose individual interfaces to configure. For example, the following configuration sets all of the EIGRP interfaces to passive mode. Also, notice the options offered for the af-interface configuration mode command:

R1#configure terminal
R1(config)#router eigrp MyEIGRP_AS
R1(config-router)#address-family ipv4 vrf my_vrf autonomous-system 1
R1(config-router-af)#af-interface ?
  Async              Async interface
  Auto-Template      Auto-Template interface
  BDI                Bridge-Domain interface
  BVI                Bridge-Group Virtual Interface
  CDMA-Ix            CDMA Ix interface
  CTunnel            CTunnel interface
  Dialer             Dialer interface
  GMPLS              MPLS interface
  GigabitEthernet    GigabitEthernet IEEE 802.3z
  Group-Async        Async Group interface
  LISP               Locator/ID Separation Protocol Virtual Interface
  LongReachEthernet  Long-Reach Ethernet interface
  Loopback           Loopback interface
  Lspvif             LSP virtual interface
  MFR                Multilink Frame Relay bundle interface
  Multilink          Multilink-group interface
  Tunnel             Tunnel interface
  Vif                PGM Multicast Host interface
  Virtual-PPP        Virtual PPP interface
  Virtual-Template   Virtual Template interface
  Virtual-TokenRing  Virtual TokenRing
  default            Address-family default interface configuration
          
R1(config-router-af)#af-interface default
R1(config-router-af-interface)#passive-interface 
R1(config-router-af-interface)#

The result of the default keyword is that all EIGRP interfaces associated with that address family have been configured to be passive. The af-interface command also allows you to specify specific interfaces for which to apply EIGRP configuration parameters. The following commands change the EIGRP hold time for the GigabitEthernet0/1 interface:

R1#configure terminal
R1(config)#router eigrp MyEIGRP_AS
R1(config-router)#address-family ipv4 vrf my_vrf autonomous-system 1
R1(config-router-af)#af-interface gigabitethernet 0/1
R1(config-router-af-interface)#hold-time 10
R1(config-router-af-interface)#

You can achieve the same thing by going to each EIGRP interface and applying whatever EIGRP command you like, however, this makes it easier to do so directly from the configuration mode of the address family.

You can find out more information bout this command at the following Cisco command reference:

https://www.cisco.com/c/en/us/td/docs/ios/iproute_eigrp/command/reference/ire_book/ire_a1.html#wp1062438

I hope this has been helpful!

Laz

189

Absolutely one of the best explanations of EIGRP that I’ve came across.

190

Hello Terry

Thanks so much for your kind words! We work hard to make networking concepts as crystal clear as possible. I hope you find the rest of the lessons and the forum useful!

Laz

191

Hello, everyone!

When it comes to the Feasible and Advertised distances. These are not directly advertised by the routers, correct?

To be more specific, routers calculate both the FD and the RD based off the metric parameters the neighbor sent to them, correct? The neighbor doesn’t directly say “my FD is xxxx”, it instead provides the parameters which it used to calculate it. The receiving router then uses these parameters to locally calculate both the RD and its FD.

And one extra thing. So in order to consider a route a feasible successor, its RD must be less than the FD of the current successor. This is to prevent loops, correct? Is there an example of where these loops could occur or is it just the potential for loops to happen in the first place?

Thank you!

Kind regards,
David

192

Hello David

Feasible distance and advertised distances are all examined from the point of view of the local router. These comparisons are performed internally and their results are not advertised to other routers. They are used internally only to determine the successor and feasible successor. However, one thing to note is that the advertised distance by definition is actually advertised :wink: . It’s the value that a neighboring EIGRP router has reported as its distance to the destination in question. Now just to be clear, if you look at an EIGRP update in Wireshark, you will not see the actual metric value, but you will see the various components that comprise it, such as delay, bandwidth, reliability, load, and MTU. It is those values that are included in the update. When that is received by the local router, it will use the EIGRP formula using configured K values to calculate a specific number as the metric. That metric will then be used to compare with the feasible distance to determine if a path becomes a feasible successor or not.

Here is an example of the contents of an EIGRP update message that includes information about a particular prefix:

image
image690×520 12.7 KB

Note that you can see, under the heading “Legacy Metric” the delay, bandwidth, MTU, reliability and load that comprise the EIGRP metric.

Another way to say the feasibility condition is to say that the “candidate feasible successor router should be closer to the destination than the local router.” Yes, this is indeed used to prevent loops. A potential scenario where a loop could occur might be if a router incorrectly advertises a lower-cost path to a network that actually routes through the receiving router. This could cause the receiving router to start routing packets to that network through the advertising router, creating a loop.

I hope this has been helpful!

Laz