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Author Topic: Common Confusion within EIGRP and routing in General  (Read 2063 times)

rthurber

  • Guest
Common Confusion within EIGRP and routing in General
« on: June 02, 2016, 10:27:26 AM »

EIGRP

My notes on some often missunderstood EIGRP terms. I don't think my notes will make things any clearer for the general reader, but it helps me.

Feasible Distance is lower metric given one or more metrics (i.e. destination routes).

For the given destination, the two metrics are 258816 and 258916.

The FD is 258816

P 192.168.17.0/24, 1 successors, FD is 258816
        via 10.99.17.2 (258816/258560), Vlan17
        via 10.99.18.2 (258916/258560), Vlan18

Look at destination with a single route, same rule applies

!from R1
P 2.2.2.0/24, 1 successors, FD is 156160
        via 10.99.17.254 (156160/128256), Vlan17


Aside, Let's show the setup

!r1
   r1#show ip eigrp vrf VLAN17 topology
   EIGRP-IPv4 VR(VRF-AWARE) Topology Table for AS(99)/ID(192.168.18.1)
         Topology(base) TID(0) VRF(VLAN17)
   Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
          r - reply Status, s - sia Status

   P 192.168.17.0/24, 1 successors, FD is 258560
      via Redistributed (258560/0)
   P 10.99.17.0/24, 1 successors, FD is 28160
      via Connected, Vlan17
   P 172.16.0.0/23, 1 successors, FD is 28416
      via 10.99.17.254 (28416/2816), Vlan17
   P 2.2.2.0/24, 1 successors, FD is 156160
      via 10.99.17.254 (156160/128256), Vlan17
   P 10.99.18.0/24, 1 successors, FD is 28416
      via 10.99.17.254 (28416/2816), Vlan17
   P 192.168.18.0/24, 1 successors, FD is 258560
      via Redistributed (258560/0)
   P 1.1.1.0/24, 1 successors, FD is 128256
      via Connected, Loopback0

!sw1
   sw1#show ip eigrp topology 2.2.2.0/24
   EIGRP-IPv4 Topology Entry for AS(99)/ID(10.99.18.254) for 2.2.2.0/24
     State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256
     Descriptor Blocks:
     0.0.0.0 (Loopback0), from Connected, Send flag is 0x0
         Composite metric is (128256/0), route is Internal
         Vector metric:
      Minimum bandwidth is 8000000 Kbit
      Total delay is 5000 microseconds
      Reliability is 255/255
      Load is 1/255
      Minimum MTU is 1514
      Hop count is 0
      Originating router is 10.99.18.254


Explaining the values:

With the route above (on r1) , here are the values

!from R1
156160/128256

1st value is the FD, but what is the second value?

It's the metric that the advertiser has for the route. Referred to as the advertised distance.

It's the same value you would get if inspected the route on the remote neighbor (sw1)

Here's the topology for that same route from the neighbor

sw1#show ip eigrp topology 2.2.2.0/24
EIGRP-IPv4 Topology Entry for AS(99)/ID(10.99.18.254) for 2.2.2.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256
  Descriptor Blocks:
  0.0.0.0 (Loopback0), from Connected, Send flag is 0x0
      Composite metric is (128256/0), route is Internal
      Vector metric:
        Minimum bandwidth is 8000000 Kbit
        Total delay is 5000 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1514
        Hop count is 0
        Originating router is 10.99.18.254

sw1#show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(99)/ID(10.99.18.254)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status

P 192.168.17.0/24, 1 successors, FD is 258816
        via 10.99.17.2 (258816/258560), Vlan17
        via 10.99.18.2 (258916/258560), Vlan18
P 10.99.17.0/24, 1 successors, FD is 2816
        via Connected, Vlan17
P 192.168.0.0/16, 1 successors, FD is 258816
        via Summary (258816/0), Null0
P 172.16.0.0/23, 1 successors, FD is 2816
        via Connected, Vlan85
P 2.2.2.0/24, 1 successors, FD is 128256
        via Connected, Loopback0
P 10.99.18.0/24, 1 successors, FD is 2816
        via Connected, Vlan18
P 192.168.18.0/24, 1 successors, FD is 258816
        via 10.99.18.2 (258816/258560), Vlan18
        via 10.99.17.2 (258836/258560), Vlan17
P 1.1.1.0/24, 1 successors, FD is 130816
        via 10.99.17.2 (130816/128256), Vlan17


r1#show ip route vrf VLAN17
      2.0.0.0/24 is subnetted, 1 subnets
D        2.2.2.0 [90/156160] via 10.99.17.254, 00:04:39, Vlan17

Summary so far: when the EIGRP destination drops into the routing table the advertised
distance goes away, and a global Admin distance is now factored in.

This used to throw me off alot because both values had a similar look (1234/1234)
- Advertised Distance is local to EIGRP (it's the second value with you view the topo)
- Administrative Distrance is global to routing (it's the 1st value when you view the routing table)

For EIGRP topoloty
(Fesible Dist/Adv Dist)
(Value1/Value2)

For IP Route table it can be a bit confusing but if you look carefully

[Admin Dist/Metric]
[90/156160]

!sw1 EIGRP topo
P 192.168.17.0/24, 1 successors, FD is 258816
        via 10.99.17.2 (258816/258560), Vlan17
        via 10.99.18.2 (258916/258560), Vlan18

!sw1 IP Route
D EX  192.168.17.0/24 [170/258816] via 10.99.17.2, 11:30:03, Vlan17

To some people this is self explanatory. But the rest of us (who are not born engineers).. You have to process these concepts a bit differently or your continually be confused. Most of use aren't in EIGRP day in and day out.




rthurber

  • Guest
Re: Common Confusion within EIGRP and routing in General
« Reply #1 on: June 02, 2016, 10:35:58 AM »
Along those lines. In my studies, I created a simple Perl script to help me understand how the EIGRP metric is calculated.

Link to the code on github:

You are not allowed to view links. Register or Login

Take the following route as an example:

sw1#show ip route 192.168.17.0
Routing entry for 192.168.17.0/24
  Known via "eigrp 99", distance 170, metric 258816, type external
  Redistributing via eigrp 99
  Last update from 10.99.17.2 on Vlan17, 11:48:05 ago
  Routing Descriptor Blocks:
  * 10.99.17.2, from 10.99.17.2, 11:48:05 ago, via Vlan17
      Route metric is 258816, traffic share count is 1
      Total delay is 110 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 1


sw1#show ip eigrp topology 192.168.17.0/24
EIGRP-IPv4 Topology Entry for AS(99)/ID(10.99.18.254) for 192.168.17.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 258816
  Descriptor Blocks:
  10.99.17.2 (Vlan17), from 10.99.17.2, Send flag is 0x0
      Composite metric is (258816/258560), route is External
      Vector metric:
        Minimum bandwidth is 10000 Kbit
        Total delay is 110 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 192.168.18.1 
        AS number of route is 100
        External protocol is OSPF, external metric is 2
        Administrator tag is 0 (0x00000000)
  10.99.18.2 (Vlan18), from 10.99.18.2, Send flag is 0x0
      Composite metric is (258916/258560), route is External
      Vector metric:
        Minimum bandwidth is 10000 Kbit
        Total delay is 113 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1
      External data:
        Originating router is 192.168.18.2 
        AS number of route is 101
        External protocol is OSPF, external metric is 0
        Administrator tag is 0 (0x00000000)


Let's take the BW and Delay of the first route (via Vlan17)

10000 Kbit
and 110 usec of delay




To go over the math.

((100000000 / 10000) + 110) * 256 = 2588160

Now this part is weird (maybe someone can explain it?) but I have to multiple it by .1

2588160 * .1 = 258816

If you read any of the Cisco press, common people will get pretty confused (most will just give up).. I even searched Google, and found a lot of blog postings, but I couldn't find any post that offered up the simple math (not to say it's not there and everyone learns differently).

If this is confusing (please let me know). :)
« Last Edit: June 02, 2016, 10:43:21 AM by rthurber »

Offline MC

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Re: Common Confusion within EIGRP and routing in General
« Reply #2 on: June 05, 2016, 10:27:05 PM »
+1 Thank you for sharing Mr. Thurber.

 

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