eigrp fd, ad, variance question

suelange · ‎01-25-2008

In my lab I have 2 routers connected to switch A, all running eigrp. The routers each constitute a valid path via which the switch can reach switch B on the other side of the two routers.

The metrics on the router interfaces leading to switch A are set to reflect one ISP with a bandwidth of 15MB and another ISP with a bandwidth of 6 MB.

The ip eigrp top all on Switch A shows only one router being feasible. The other router's AD is so much lower than the FD of the successor that it won't be considered, no matter the variance.

The problem is: other than exactly equal metrics, I can't put in a bandwidth/delay value pair on the 6MB router that creates an AD that allows it to ever become a feasible succesor. IF I set the metrics exactly equal, then it will balance "proportionately" over the two lines, which clearly would be a bad thing when there is such a differential in bandwidth rates.

Without adjusting the K values which we are warned never to do...how on earth do I make it proporionately load balance these two links?

I'm thinking that the feasibility requirement, necessary to prevent loops, pretty much makes the unequal load balance feature a null and void advantage...

Nagendra Kumar Nainar · ‎01-25-2008

Hi Shook,

For each destination, EIGRP uses the least BW and cumulative delay for calculation. So for the below mentioned topology,

1.1.1.0/24------R1--------R2--------R3(Your router)

2Mb 128Kb 6Mb

Dly 10 Dly 20 Dly 10

if you have a BW of 128Kbps for 1.1.1.0/24 network via 6 Mbps link, it will consider the 128Kbps as BW for calculation in R3 irrespective of the BW configured in your interface and the delay will be (10+20+10).

Try "show ip eigrp " and see what is the BW and delay via each link?

You probably can use the DELAY variable to get your task done.

Regards,

Nagendra

suelange · ‎01-25-2008

well the interface bw and delay are the metrics set in the redistribute command on the individual routers. They are set as follows:

6mb router:

router eigrp 64

redistribute bgp 6401 metric 6144 200 255 1 1500

network 10.64.0.0 0.0.255.255

no auto-summary

6 MB router interface:

serial0/1 is up, line protocol is up

Hardware is multilink group interface

Description: Sprint 46135253-6

Internet address is 172.17.1.1/30

MTU 1500 bytes, BW 6144 Kbit, DLY 100000 usec,

reliability 255/255, txload 1/255, rxload 1/255

on the 15 MB router the config is:

router eigrp 64

redistribute bgp 1 metric 15037 200 255 1 1500

network 10.64.0.0 0.0.255.255

no auto-summary

it's interace shows:

router eigrp 64

redistribute bgp 1 metric 15037 200 255 1 1500

network 10.64.0.0 0.0.255.255

no auto-summary

As you can see i've already fudged the delay on the 6 MB pipe considerably but still my EIGRP TOP shows:

P 10.1.0.0/16, 1 successors, FD is 221696, tag is 65000, serno 1431

via 10.64.1.3 (221696/221440), Vlan1

via 10.64.1.2 (467968/467712), Vlan1

To get the second (6mb) router to get anywhere near close enough AD value to be considered feasible I have tried every delay I can set and no help. I can set bandwidth to be 'wrong' but then the LB proprotionate will think it can send more over the slow link than it really can....

To say that delay is a cummulative value is correct but in this case we have only one real "hop" between so there isn't much to play with here....