EIGRP Feasible Distance & Successor

Greetings All,

I'm having some difficulty understanding the difference between Feasible Distance and Successor. My current understanding is that both FD and Successor are the best route to a remote network, if that is the case, then what is the difference?

Any help would be GREATLY appreciated.

Thank you for you knowledge.

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notun1

tester654321 wrote: »

Greetings All,

I'm having some difficulty understanding the difference between Feasible Distance and Successor. My current understanding is that both FD and Successor are the best route to a remote network, if that is the case, then what is the difference?

Any help would be GREATLY appreciated.

Thank you for you knowledge.

FD is calculated metric from a router to reach that Network. The router can have several calculated Metric to reach that network. But the best metric is count as FD for that route and put it as a successor in the routing table.

However to reach a network there can be two or more FD shown in routing table as successor if you are using Variance.

Please have a look here
===================

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 7 subnets
D 10.1.3.0 [90/2172416] via 10.1.6.2, 00:22:10, Serial0/1
[90/2684416] via 10.1.4.1, 00:22:10, Serial0/0
D 10.1.2.0 [90/2172416] via 10.1.4.1, 00:22:10, Serial0/0
C 10.1.1.0 is directly connected, FastEthernet1/0
D 10.1.7.0 [90/2681856] via 10.1.6.2, 00:22:10, Serial0/1
C 10.1.6.0 is directly connected, Serial0/1
D 10.1.5.0 [90/2681856] via 10.1.6.2, 00:22:10, Serial0/1
[90/2681856] via 10.1.4.1, 00:22:10, Serial0/0
C 10.1.4.0 is directly connected, Serial0/0
D 192.168.0.0/21 [90/2809856] via 10.1.6.2, 00:22:10, Serial0/1
=====================
to reach Network 10.1.3.0 this router has 2 FD 2172416 and 2684416 in routing table..that was done using Variance .

and Network 10.1.5.0 has only one FD 2681856 in routing table for Equal cost load balancing.

hope this will help you..

Zartanasaurus

It's confusing because of the naming. Most people see the word "successor" and think backup. Like the VP is the successor to the President if something happens. In EIGRP think of successor as "success". The successor route is the one that was successful in being the best route to the remote network. The route with the lowest feasible distance becomes the successor route. The "backup" route is the feasible successor. In order to become a feasible successor, the reported distance must be less than the feasible distance.

Feasible distance is the calculated metric to get to the remote network from the POV of the router itself (call this router 0).

Reported (advertised) distance is the metric to get to the remote network from the POV of the router neighbor telling router 0 about the network.

Administrative distance is a tie breaking number among routing protocols for the router to determine which path to take if it knows about a route via two different protocols. IOW Static route >> EIGRP >> OSPF >> RIP, etc.

shaurya.gujjar

Hi

It would be great if any of you can answer to my question,

As you said Feasible Distance (FD) is Reported Distance(RD) value received from neighbor + cost to get to the advertising router. Which cost are you talking about, is it the end to end cost or just cost to get to the advertising neighbor?
How cost to advertising neighbor is calculated?

Also if a topology looks like three routers R1,R2,R3 connected to each other via serial links. On R3 and Fast Ethernet interface with an ip address 192.168.2.0/24 is advertised through EIGRP network command.

In my calculation for FD on Router 2: i.e FD = RD received from R3 + Cost to R3

From below sample o/p my Que is:

If the RD received from R3 is 28160 how Feasible Distance (FD) is being calculated as on R2.

On R2 = [(10,000,000 / Bandwidth) + ((sum of delays) /10)] * 256

On R2 = ((10,000,000/ 1544) + ((20000 + 100) /10)) *256
On R2 = ((6476 + 2010) * 256)
On R2 = (8486 * 256)
On R2 = 2172416

According to your statement and my calculation WHY I AM NOT ADDING THE RD VALUE TO GET THE FD.

The following is the O/P on the following routers:

R2#show ip eigrp topology 192.168.2.0
EIGRP-IPv4 Topology Entry for AS(1)/ID(172.17.0.1) for 192.168.2.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416
Descriptor Blocks:
20.0.0.2 (Serial1/1), from 20.0.0.2, Send flag is 0x0
Composite metric is (2172416/28160), route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 20100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
Originating router is 172.18.0.1

R1#show ip eigrp topology 192.168.2.0
EIGRP-IPv4 Topology Entry for AS(1)/ID(172.16.0.1) for 192.168.2.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2684416
Descriptor Blocks:
10.0.0.2 (Serial1/0), from 10.0.0.2, Send flag is 0x0
Composite metric is (2684416/2172416), route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 40100 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 172.18.0.1

pham0329

I think of the successor as the next hop router, and the FD as the metric to the successor.

@shaurya.gujjar

Routers doesn't simply add the AD(or RD) of the neighbor to the cost from itself to the neighbor to find the FD. In an update message, the neighbor sends information about the prefix being advertised, including information about that prefix such as the delay, bandwidth, mtu, reliability, load

From that, the receiving router calculates the distance from the neighbor to the final destination, as well as its FD through that neighbor. For example, suppose R1 advertises is connected to 10.0.0.0/24, when it advertise this to R2, the update message will look something like

Route: 10.0.0.0/24
Delay: 100
Bandwidth: 100,000
Reliability: 255
Load: 1
MTU: 1500

**The actual bandwidth value is actually 10^7/bandwidth * 256, and delay is (delay/10) * 256. I'm using 100000/100 here to simply the example.

When R2 receives this information, it'll calculate R1's metric to the 10.0.0.0/24 network. It'll also calculate it's metric to 10.0.0.0/24 using R2 as the next hop.

R1 will compare it's bandwidth to R2 against R2's bandwidth to 10.0.0.0/24, and use whatever is lower in its calculation. It'll look at R2's delay, and add it's own delay to it.

So, going back to the example. Suppose R1 has a T1 link to R2. By default, a T1 has a bandwidth of 1544, and delay of 2000.

So in R1 compare R2's bandwidth to the 10.0.0.0/24 network to R1's bandwidth to R2. Since R1's bandwidth to R2 is only 1544, it uses that in its calculation.

R1 then takes the delay of R2, and add that to the delay from itself to R2. So the final equation looks like 1544 + 2100 = FD

Here's a packet capture of an update message. Note that the message only contains the route information, not the actual calculated AD/RD

Untitled.png