EIGRP Neighbor and Topology Table Explained

(francesco r) #13

Hello Rene, In these days, I’m reading Eigrp from “Routing TCp/ip ver 1” and from
the new guide for ccie ver 5…
Now i cant’t able to anderstand the discuss of Fd…
If I do show ip eigrp topology (-all link) …the first value is Fd,( in the bracket , they call
computed distance reported distance, and now they say which is not always true that a router, during a topology change, choose that we call feasible successor…
what is the real story??

(Rene Molenaar) #14

Hi Francesco,

EIGRP will store all the possible paths in its topology table, between the brackets it stores the FD (feasible distance) and AD (advertised distance). For example:

(156160/128256)

156160 = feasible distance
128256 = advertised distance

Sometimes they use “computed distance” to refer to feasible distance and “reported” distance to refer to the advertised distance.

In the topology table you will find at least 1 successor and possible some feasible successors. To become a feasible successor, its AD has to be lower than the FD of the successor.

Hope this helps!

Rene

(francesco r) #15

Hello Rene,
I Know, I know
the problem now is this:

 Jack#show ip eigrp topology 
P 2.2.2.0/24, 1 successors, FD is 156160
        via 192.168.12.2 (156160/128256), FastEthernet0/0

I understand your explanation, is only that in the new guide they say about the value FD 156160 is an historical copy since a distance pass from active to passive.
This value is internal and never pass to neigbors, it could only decrease and is this that we check for FC…
they call (156160) first valure in the bracket …computed distance and this is that a router pass to neigbor…so they say that is not always true that if a router has more FS it chose that one meet the FC…

(Rene Molenaar) #16

Hi Francesco,

I’m not sure I get your question exactly :slight_smile: What page in the CCIEv5 book do you refer to?

Rene

(francesco r) #17

page 386 where they discuss about FD and 393 where they say about fs successor over successor :slight_smile:

(Rene Molenaar) #18

Hi Francesco,

I just took a look and now I get what you are referring to :slight_smile: This is interesting, the FD is indeed a historical copy while the CD is not. I’ll write a post on this.

Rene

(francesco r) #19

Wow… I’'wait for it…thanks :slight_smile:

(Hussein Samir) #20

Hi Rene,

I have three questions :-
1 - When Suddenly an route entry goes down the code for this entry will be Query or Active first ?
2 - When the route will become in update state ? do you mean it will become in update state when there is a change in the topology and it will become in update state till receiving ACK ? or when receiving an update for an entry so it will become in update state and in this case how long it will take for come back into passive state ? I’m really confused at this point ?
3 - When the route will become in reply state ? I have no Idea at this point ?

thanks.

(Hussein Samir) #21

I have one more question :-

can you explain the different between ( R - Reply & r - reply Status ) in detail ?

(Rene Molenaar) #22

Hi Hussein,

  1. The route will become “active” first and then the router will start sending query packets.
  2. The “U” for update means that an update packet has been sent to this destination.
  3. The “R” for reply means that an update packet has been sent to this destination.

You’ll have to be fast to actually see the update and reply status in the EIGRP topology table as this happens really fast, it will be easier to see this process by enabling a debug.

Rene

(Rene Molenaar) #23

Hi Hussein,

Did you see this post?

Rene

(Ahmed A) #24

thank you dear for your amazing explanation …it just like eating Italian pasta …delicious and useful

(Victor R) #25

Hi Rene,

How does EIGRP come up with the local metric for the interface it’s using to reach the destination network? And where can we see it. Thank you

(Rene Molenaar) #26

Hi Victor,

EIGRP uses this formula to calculate the metric:

Here is an example of a directly connected gigabit interface with a network that is advertised in EIGRP:

R1#show ip eigrp topology 192.168.12.0/24
EIGRP-IPv4 Topology Entry for AS(1)/ID(11.11.11.11) for 192.168.12.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2816
  Descriptor Blocks:
  0.0.0.0 (GigabitEthernet0/1), from Connected, Send flag is 0x0
      Composite metric is (2816/0), route is Internal
      Vector metric:
        Minimum bandwidth is 1000000 Kbit
        Total delay is 10 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 0
        Originating router is 11.11.11.11

You can see the metrics that are used for the formula (bandwidth and delay by default) and the FD which is 2816.

Rene

(Shantel - Networklessons.com) split this topic #27

19 posts were merged into an existing topic: EIGRP Neighbor and Topology Table Explained

(Vinod A) #28

Hi Rane,
I was walking across EIGRP Hello packet in wireshark ,i see there are value from K1 to K6 .

I read contents where we have K values are start from K1 to K5.

Its not indicating TOS as well becuase its always zero where as it is one in this case.

please refer attached . I am using topology given in your blog (lo --R1----R2–Lo)EIGRPHello

(Lazaros Agapides) #29

Hello Vinod

K6 is a new addition to the EIGRP protocol that was added in IOS version 15.2(2)T. See the following Cisco documentation for details:


Now K6 is restricted in its use since it can only be used under address family configuration mode. Because it is relatively new, and it is somewhat beyond the scope of CCNA or even CCNP, it’s not a concern until you get to the CCIE certifications, although I’m not sure that it has made its way into the curriculum yet.

Cisco’s RFC about the EIGRP protocol describes K6 like so:

K6 has been introduced with Wide Metric support and is used to allow for Extended Attributes, which can be used to reflect in a higher aggregate metric than those having lower energy usage. Currently there are two Extended Attributes, jitter and energy, defined in the scope of this document.

I hope this has been helpful!

Laz

(Frank I) #30

Hi Rene,
Thank you for the explaintions, is clear and easy to understand.

1 Like
(Andy K) #31

Great write-up as usual Rene. Quick question:

You said "Network 1.1.1.0 0.0.0.255 mean that I’m advertising the 1.1.1.0 network with wildcard 0.0.0.255. "

From what I’ve read elsewhere, the network EIGRP will advertise is the network that’s configured on the INTERFACE (not the mask configured under router eigrp). So, if you had a network statement:

network 10.0.0.0

but on the interface, it was:

10.10.10.1 255.255.255.0

it would advertise a /24 mask and not a /8, correct?

(Lazaros Agapides) #32

Hello Andy

It is true that EIGRP will advertise the network that’s configured on the interface, and not on the mask configured using the network command. What the network command does is it tells the router “if you find any interfaces with networks found within this range of addresses, advertise them.”

Let’s say you have a router with three interfaces configured with IP addresses 192.168.1.1/24 192.168.2.1/24 and 192.168.3.1/24. You can make all three networks participate in EIGRP by issuing the following commands under router eigrp configuration mode:

Router(config)# router eigrp 1 
Router(config-router)# network 192.168.1.0 0.0.0.255
Router(config-router)# network 192.168.2.0 0.0.0.255
Router(config-router)# network 192.168.3.0 0.0.0.255

or you could issue this command:

Router(config)# router eigrp 1 
Router(config-router)# network 192.168.0.0 0.0.3.255

In the second case, all three networks are contained within the network command. So this network command says “advertise all networks of interfaces that are found within the range specified in the network command.” What is advertised however is the mask of the interface, and not of the network command.

Now if you enter a network command without a wildcard mask, then the classful wildcard mask will be used. This doesn’t mean that the classful wildcard mask will be advertised. So in your case, if you have an interface with an IP address of 10.10.10.1 255.255.255.0, then a command of network 10.0.0.0 will include this interface’s network range. Thus, the interface network with its subnet mask will be advertised.

I hope this has been helpful!

Laz