EIGRP load balancing over unequal-cost paths

Previous post covered about load balancing over equal-cost paths. This post will explain how to load balance traffics over UNequal-cost paths.
In the case there is only one route is installed into routing table, other feasible successor routes are kept in topology table and will be refered when successor route failed. To enable load sharing over unequal-cost paths, it requires the feasible successor route is installed into routing table. A value called VARIANCE is configured to achieve this goal.
Variance is a number in range 1 to 128, where 1 is default means load balancing over equal-cost paths. Variance will be multiplied by current best metric (successor's FD) and compare with feasible successor's FD to decide whether it is useful or not. The comparison is:

If feasible successor's FD < successor's FD * variance = route will be installed into routing table.

 In this figure, R1-R4-R5 is feasible successor route for R1 to reach 1.1.1.0/24.
Feasible successor's FD is entire metric calculated from R1-1.1.1.0/24, it means 27+7+12=46
Successor's FD is: 10+5+6=21.
With variance value is 3, route R1-R4-R5 will be installed into routing table because it satisfies the condition.

For demonstration, the bandwidth in R1's s0/0 is changed to 700. Now, R1 only uses s0/1 to forward traffic to 5.5.5.0/24

R1#show ip route | section 5.5.5
D       5.5.5.0 [90/2323456] via 2.2.2.2, 00:00:04, Serial0/1

 In topology table of R1, it still keeps route via R2 to reach 5.5.5.0/24 with higher metric:

R1#show ip eig to 5.5.5.0/24
...
  2.2.2.2 (Serial0/1), from 2.2.2.2, Send flag is 0x0
      Composite metric is (2323456/409600), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
       ...
  1.1.1.2 (Serial0/0), from 1.1.1.2, Send flag is 0x0
      Composite metric is (4322560/409600), Route is Internal
      Vector metric:
        Minimum bandwidth is 700 Kbit
       ...

To use unequal-cost load balancing over these routes, go to eigrp process and configure variance value is 2:

R1(config)#router eigrp 10
R1(config-router)#variance 2

  Now, both 2 routes are appeared in routing table:

R1#show ip route eig | section 5.5.5
     3.0.0.0/24 is subnetted, 1 subnets
D       5.5.5.0 [90/2323456] via 2.2.2.2, 00:01:08, Serial0/1
                [90/4322560] via 1.1.1.2, 00:01:08, Serial0/0

 R1#show ip route 5.5.5.5
Routing entry for 5.5.5.0/24
  Known via "eigrp 10", distance 90, metric 2323456, type internal
  Redistributing via eigrp 10
  Last update from 1.1.1.2 on Serial0/0, 00:02:17 ago
  Routing Descriptor Blocks:
    2.2.2.2, from 2.2.2.2, 00:02:17 ago, via Serial0/1
      Route metric is 2323456, traffic share count is 80
      Total delay is 26000 microseconds, minimum bandwidth is 1544 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
  * 1.1.1.2, from 1.1.1.2, 00:02:17 ago, via Serial0/0
      Route metric is 4322560, traffic share count is 43
      Total delay is 26000 microseconds, minimum bandwidth is 700 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

EIGRP load balancing over equal-cost paths

When router learned routes from other routing protocols, it will choose a best route to install to routing table. This route is the route with lowest  Administrative Distance, this value is assigned to each routing protocol.
In case there are multiple routes learned from a single routing protocol, metric will be considered and route with the lowest metric wins. If those routes have the same metric values, load balancing can be occur (EIGRP, OSPF). EIGRP even allows load balance over unequal-cost paths.

By default, EIGRP will load balance over 4 equal-cost paths and can be increased to 16 paths. EIGRP does not perform load balancing by itself, switching technologies handle it.

CEF is a layer 3 switching technology and enabled by default on IOS, and when CEF is enabling, load balancing will occur on per-destination basis. That means, all packets reach to a destination will use the same output interface.

In topology table:

R1#show ip eigrp to
IP-EIGRP Topology Table for AS(10)/ID(1.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status
...
P 3.3.3.0/24, 2 successors, FD is 435200
        via 10.0.0.2 (435200/409600), Ethernet0/0
        via 13.0.0.2 (435200/409600), Ethernet0/1
...

There are two successor routes to reach 3.3.3.0/24 from R1 with the same FD 435200. In this case, CEF is enabled on R1. so load balancing will occur in per-destination basis.

R1#show run | section cef
ip cef

 In per-destination basis, all traffics reach a destination will use the same output interface:

 When cef is disabled, packets are distributed over both two routes:

 Alternatetively, per-packet load balancing can be happen while CEF is enabled by configuring load sharing on outgoing interfaces.

R1(config)#ip cef
R1(config)#int e0/0
R1(config-if)#ip load-sharing per-packet
R1(config-if)#int e0/1
R1(config-if)#ip load-sharing per-packet

EIGRP Terminologies

In this post, I will explain some terminologies related to EIGRP, include:

• Feasible distance and reported/administrative distance (FD and AD).
• Successors and feasible successor routes.

1.Feasible Distance and Reported Distance - FD and RD


Feasible Distance (FD): is the metric of best route from local router to destination subnet. Best route is the route with the lowest metric.

Reported Distance (RD): is the metric advertised by the neighbor/next hop for that route. Local router knows this information because they exchange their topology tables together. 

In this example, let consider the route from R1 to 1.1.1.0/24.
There are two routes from R1 to reach 1.1.1.0/24:

1. R1-R2-R3-dest: metric is 10+15+7 = 32.
2. R1-R4-R5-dest: metric is 12+9+12 = 33.

In this scenario, R1 will choose 1st route with lower metric. The FD in here is 32 because it is the best metric.

Reported Distance is the value of next hop router to destination, in here, next hop router of R1 is R2, and RD of this route is the metric calculated from R2 to destination: 15+7 = 22.

2. Successor and Feasible successor routes

Successor route is the best route choosen to install into routing table, in above example is R1-R2-R3 route.
Feasible successor route is the backup route for successor route in case feasible successor route down.

By default, EIGRP only keeps one best route to install to routing table, but it still keeps the less-than-optimal routes in topology table.

To become a feasible route, a route must be qualified the stuff called feasibility condition. The condition is:

• The route must be loop-free. This means, the RD of that route from local router must less than the local best metric (FD).

So, in above diagram, what is the feasible successor route of the best route from R1 to 1.1.1.0/24?
As explained before, the best route from R1 to 1.1.1.0/24 is R1-R2-R3 with FD is 32. Another route to reach 1.1.1.0/24 is R1-R4-R5 with metric is 33.
If R1-R4-R5 route wants to become feasible successor route, its RD must be less than current FD of R1.
In this example, the RD of R1-R4-R5 is calculated from R4 to 1.1.1.0/24 so it is 21.

With this RD value (21 < 32), this route is feasible successor route.

Khái niệm Administrative Distance và Metric

I.Metrics

Routing protocols sẽ chọn route nào đc đánh giá là tốt nhất(best route) đến mối subnet. Việc lựa chọn này dựa vào việc route nào có metric càng nhỏ thì được đánh giá là càng tốt.
 Với mỗi giao thức thì lại có cách đánh giá riêng về metric. Ví dụ, với RIP thì đánh giá qua số lượng router(hop count) phải đi qua để đến subnet đích. Còn đối với OSPF, EIGRP thì lại khác.