R1 has three neighbors toward 10.50.0.0/24. Walk me through which become successor and feasible successors.
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Say this first
I compute each neighbor's total metric, take the lowest as the feasible distance, then compare every other neighbor's reported distance with it. Here R2 gives 3328 with RD 3072, so R2 is the successor and FD is 3328. R3 gives 26368 but reports 3072, which is less than 3328, so R3 is a feasible successor. R4 also gives 26368, but it reports 26112, which is not less than 3328, so R4 fails the feasibility condition and is not a backup, even though its path is perfectly loop-free.
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The topology
R5 owns the gigabit LAN 10.50.0.0/24. R2, R3 and R4 each connect to R5. R1 connects to all three.
All interfaces are gigabit (delay 10 usec), but two links are 100 Mbps circuits with
bandwidth 100000 configured.
| Link | Bandwidth EIGRP uses | Delay |
|---|---|---|
| R1 Gi0/0/0 to R2 | 1000000 kbps | 10 usec |
| R1 Gi0/0/1 to R3 | 100000 kbps | 10 usec |
| R1 Gi0/0/2 to R4 | 1000000 kbps | 10 usec |
| R2 to R5 and R3 to R5 | 1000000 kbps | 10 usec |
| R4 to R5 | 100000 kbps | 10 usec |
| R5 LAN 10.50.0.0/24 | 1000000 kbps | 10 usec |
The math
| Via | Slowest link | BW term | Delay term | R1's metric | Neighbor's RD |
|---|---|---|---|---|---|
| R2 | 1 Gbps | 10 | 3 | 256 x 13 = 3328 | 256 x (10 + 2) = 3072 |
| R3 | 100 Mbps (R1 to R3) | 100 | 3 | 256 x 103 = 26368 | 256 x (10 + 2) = 3072 |
| R4 | 100 Mbps (R4 to R5) | 100 | 3 | 256 x 103 = 26368 | 256 x (100 + 2) = 26112 |
Applying DUAL
- Lowest metric is 3328 via R2. R2 is the successor, FD = 3328.
- R3: RD 3072 is less than 3328. Feasible successor.
- R4: RD 26112 is not less than 3328. Not a feasible successor.
R3 and R4 cost R1 exactly the same, yet only one is a backup. R3's slow link is on R1's side, so R3 itself is close to the destination. R4's slow link is on R4's side, so R4 reports a large distance.
What the router shows
R1# show ip eigrp topology all-links
EIGRP-IPv4 Topology Table for AS(100)/ID(10.255.0.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.50.0.0/24, 1 successors, FD is 3328, serno 14
via 10.0.12.2 (3328/3072), GigabitEthernet0/0/0
via 10.0.13.3 (26368/3072), GigabitEthernet0/0/1
via 10.0.14.4 (26368/26112), GigabitEthernet0/0/2Plain show ip eigrp topology lists only the first two lines.
Failures
- R2 fails: R1 promotes R3 at once. No queries are sent.
- R2 and R3 both fail: no feasible successor remains, so the route goes active. R1 queries R4, R4 replies with 26112, and R1 installs the path via R4 at 26368.
They will ask next
Is the path through R4 really a loop risk?
No, R4 goes straight to R5. But DUAL only sees R4's reported distance, and a distance of 26112 is bigger than R1's own 3328, so for all DUAL knows R4 could be routing through R1. It cannot prove the path safe, so it will not use it without a query.
How would you make R4 a feasible successor?
Make R4's own distance lower than 3328. If the R4 to R5 link were a full gigabit, R4 would report 3072 and pass. Raising R1's FD would also work in theory, but making the primary path worse just to create a backup is the wrong trade.
With variance 8, which paths carry traffic?
R2 and R3. 8 x 3328 = 26624, and R3's 26368 is below that and R3 is a feasible successor. R4 has the same metric but is never eligible for variance because it is not a feasible successor.
Traps
- Comparing the neighbor's RD with its own total metric through that neighbor. The comparison is RD against the successor's FD.
- Treating equal as passing. The RD must be strictly less than the FD.