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EIGRP load balancing using non-parallel T1s

Posted on 2008-10-21
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Last Modified: 2012-06-27
             LAN 10.100.0.0/16
               L0 192.168.254.1/32
                       A
                       |\
                       | \10.105.1.0/30
                       |  \
10.105.1.8/30 |   C LAN 10.205.0.0/16  L0 192.168.254.3/32
                       |  /
                       | /10.105.1.4/30
                       |/
                       B
         LAN 10.110.0.0/16
         L0 192.168.254.2/32

All three links between routers are point-to-point T1s.

EIGRP is properly configured on all routers.

Is there a way to load balance traffic between router A's LAN and router B's LAN by utilizing the T1s going to router C and the T1 between router A and B?

(not sure if it is relevent, but I added the loopback interface for each router in the above diagram, along with its LAN subnet)
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Question by:DougR73
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5 Comments
 
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Expert Comment

by:Don Johnston
ID: 22775446
If the routes are not equal, the easiest way would be to raise the variance multiplier so unequal cost multipath is used.

This example assumes that the alternate route is no worse than 2 times the best route. Which given your example should be the case.



router eigrp
 variance 2
 

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Expert Comment

by:Don Johnston
ID: 22775489
Although, since cost is based on minimum bandwidth along the path and cumulative delay along the path, you could ignore the delay. You will need to change the K values on all routers in order for this to work.

BTW, this assumes that the bandwidth is correctly set on all the routers.




router eigrp 1
 metric-weight 0 1 0 0 0 0

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Author Comment

by:DougR73
ID: 22779577
I'd rather not change EIGRP's metric calculation since, in this case, the only K value which is inhibiting Router A from populating its routing table with 10.110.0.0/16 through Router C is K3 (delay). (ALL T1 interfaces have "bandwidth 1544" in its config)

I added "variance 2" to routers A and B and made sure router C is advertising via EIGRP the 10.100.0.0/16 and 10.110.0.0/16 subnets.

The routing table on Router B does not include a second route for the 10.100.0.0/16 subnet.  Below is the output of "sh ip eigrp topo 10.100.0.0 255.255.0.0"

From Router B's perspective:
Router B --> Router A delay to 10.100.0.0/16 subnet is 20100
Router B --> Router C is connected so the metric is 0
Router C --> Router A delay to 10.100.0.0/16 subnet is 40100

With the "variance 2" command in place, shouldn't the second route to 10.100.0.0/16 through Router C be populated in Router B's routing table since the entire delay through router C is less than 2x the delay going directly to Router A???

show ip eigrp topology 10.100.0.0 255.255.0.0
IP-EIGRP (AS 1): Topology entry for 10.100.0.0/16
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416
  Routing Descriptor Blocks:
  10.105.1.9 (Serial0/1/0), from 10.105.1.9, 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 32/255
        Minimum MTU is 1500
        Hop count is 1
  10.105.1.6 (Serial0/3/0), from 10.105.1.6, 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 102/255
        Minimum MTU is 1500
        Hop count is 2

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Accepted Solution

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Don Johnston earned 500 total points
ID: 22779754
The problem is that in order for a higher cost route to be eligible for unequal cost load balancing, it must be a feasible successor.

To be a feasible successor, the route must have an advertised distance which is lower than the feasible distance of the best route.

(FD/AD)
using Router A (2172416/28160)
using Router C (2684416/2172416)

Notice how the advertised distance of the Router C path is not lower (it's the same) than the feasible distance of the Router A path?

That is what's keeping it from working.

You'll either have to reduce the delay of the Router C path, make bandwidth more of a factor (raise K1 to a "2"), or just stop considering delay in the calculation.



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Author Comment

by:DougR73
ID: 22780319
On Router C, I configured the serial interface to A and serial interface to B with "delay 1000".

The routing table on Router A and B now show two routes to each other's LAN (one going straight to the other router, and the other going through Router C).

Thanks, Don!!!
sh ip eigrp topology 10.100.0.0 255.255.0.0
IP-EIGRP (AS 1): Topology entry for 10.100.0.0/16
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172416
  Routing Descriptor Blocks:
  10.105.1.9 (Serial0/1/0), from 10.105.1.9, 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 32/255
        Minimum MTU is 1500
        Hop count is 1
  10.105.1.6 (Serial0/3/0), from 10.105.1.6, Send flag is 0x0
      Composite metric is (2428416/1916416), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 30100 microseconds
        Reliability is 255/255
        Load is 192/255
        Minimum MTU is 1500
        Hop count is 2

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