Build the idea first, then use the next activity to check it.
EIGRP and the feasible successor
A protocol that keeps a spare route ready before it needs one, and the condition a spare has to satisfy to be safe.
What you will be able to do
- Describe what EIGRP exchanges and how it differs from OSPF
- Explain what a successor and a feasible successor are
- State the feasibility condition and why it prevents loops
- Explain what variance does and when it is worth using
Before this: You should understand what a routing protocol exchanges and be comfortable reading a routing table - see "Why static routing stops scaling".
Why it matters: EIGRP's design answers a question OSPF answers differently: how do you fail over instantly without risking a loop? The feasibility condition is the clearest small piece of routing theory you will meet, and it explains why a backup route sometimes is not accepted.
OSPF gives every router the whole map and lets each one calculate. EIGRP does something different: routers tell their neighbours what distances they can reach destinations at, and nobody holds a topology map at all. Each router knows what its neighbours claim, and that turns out to be enough - provided one condition is checked carefully.
Two distances
For each destination, a router tracks two numbers per neighbour, and keeping them straight is most of understanding EIGRP.
| Term | What it measures |
|---|---|
| Reported distance | What the neighbour says it costs THEM to reach the destination |
| Feasible distance | What it costs ME to reach it through that neighbour |
The feasible distance is the reported distance plus the cost of the link to that neighbour. The route with the lowest feasible distance is installed, and the neighbour it goes through is called the successor.
The spare that is ready in advance
Any other neighbour offering a path to the same destination is a candidate backup. EIGRP does not accept all of them. A candidate becomes a feasible successor only if it passes this test:
The reason is loop prevention. If a neighbour's own cost to reach the destination is lower than mine, that neighbour cannot be routing through me - it would have to be further away than me if it were. So switching to it cannot create a loop, and can be done instantly with no calculation and no consultation.
One candidate accepted, one refused
A router reaches the warehouse floor through its first neighbour at a feasible distance of 3000. A second neighbour reports it can reach the floor at 1500, so its reported distance is below 3000 and it becomes a feasible successor. A third reports 3200 - higher than the current feasible distance - so it is refused, even though the path through it might be perfectly good. EIGRP would rather ask again than risk a loop.
What happens when the successor fails
If a feasible successor exists, EIGRP installs it immediately. No timer, no recalculation, no messages to anyone. This is the whole payoff of the design and it is why EIGRP converges as fast as it does.
If there is no feasible successor, the router has no route it can prove is safe. It marks the destination active and queries its neighbours, waiting for replies before installing anything. That is slower, and a query that goes unanswered is the origin of EIGRP's one notorious failure mode.
Turning it on
router eigrp 100
network 10.30.0.0 0.0.0.3
network 10.30.40.0 0.0.0.255
passive-interface GigabitEthernet0/1
redistribute static
The number after the protocol name is the autonomous system number. It is the autonomous system number, and it is real: two routers with different numbers will not form a neighbour relationship - which is the first thing to check when an EIGRP adjacency does not come up. A classful statement such as `network 10.0.0.0` covers the major network; use the wildcard form when you want a narrower range.
Variance and unequal-cost paths
In EIGRP theory, variance allows feasible successors whose feasible distance falls within a multiple of the best path to be considered for unequal-cost installation. The console accepts and stores `variance`, but this lab platform has not verified that it actually produces unequal-cost load sharing, so treat the effect as theory here.
router eigrp 100
variance 2
Variance is a multiplier applied to the best feasible distance. In the EIGRP model, a feasible successor within that multiple can be considered for installation. The word "feasible" is doing real work there: variance never promotes a path that failed the feasibility condition. Whether this platform installs and load-shares that additional path is not verified.
Reading the state
show ip eigrp neighbor
# IP-EIGRP neighbors for process 100
# H Address Interface Hold Uptime SRTT RTO Q Seq
# 0 10.30.0.2 Gi0/0 12 00:14:02 10 5000 0 7
show ip eigrp topology
# IP-EIGRP Topology Table for AS(100)/ID(10.30.0.1)
# P 10.30.40.0/24, 1 successors, FD is 3072
# via 10.30.0.2 (3072/3000), GigabitEthernet0/0
show ip route eigrp
# D 10.30.40.0/24 [90/3072] via 10.30.0.2, 00:14:02, Gi0/0
Terminology
- Reported distance
- The cost a neighbour advertises for reaching a destination.
- Feasible distance
- My total cost to a destination through a given neighbour.
- Successor
- The neighbour on the installed, lowest-cost path.
- Feasible successor
- A backup neighbour whose reported distance is below my current feasible distance, and which can therefore be used instantly.
- Variance
- A multiplier that, in EIGRP theory, allows feasible successors within that factor of the best cost to be considered for installation. Its unequal-cost effect is not verified on this lab platform.
Key takeaways
- EIGRP exchanges distances to destinations, not a map of the topology.
- Reported distance is the neighbour's cost; feasible distance is mine through them.
- A feasible successor's reported distance must be strictly below my feasible distance.
- That condition guarantees the backup is not routing through me, so failover is instant.
- With no feasible successor the router must query its neighbours and wait.
- Variance is an accepted configuration whose unequal-cost load-sharing effect is unverified here; it can only consider routes that were already feasible.
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