Guided buildadvanced7 steps~25 min5 devices

EIGRP across three routers

Join two sites through a hub with EIGRP, find why one router learns nothing, then add a direct link and watch EIGRP choose speed over hop count.

What you'll be able to do: Three routers in one EIGRP autonomous system: both sites reach each other through the hub, a router typed into the wrong AS has been found by its empty neighbour table and fixed, and a slow direct link sits ready while EIGRP keeps traffic on the faster two-hop path.

Start this build in the lab

Topics: EIGRP · Dynamic routing · Routing tables · Wildcard masks · WAN links

What you'll build

North — a router, the north site router
PC-North — a pc, a workstation at the north site
South — a router, the south site router
PC-South — a pc, a workstation at the south site
Hub — a router, the hub router both sites connect through

Step by step

1. Stand up the north site
Drag a router and a PC onto the canvas, cable the PC into the router's Gi0/0, then name the router North and give Gi0/0 the LAN's gateway address.
- Cable North Gi0/0 ↔ PC-North Eth0
On North — Name the router and make Gi0/0 the north LAN's gateway
```
enable
configure terminal
hostname North
interface Gi0/0
ip address 192.168.10.1 255.255.255.0
no shutdown
exit
end
```
On PC-North — Name the workstation, address it, and point it at its gateway
```
hostname PC-North
ipconfig Eth0 192.168.10.10 255.255.255.0 192.168.10.1
```
Check: run show ip route on North and look for C 192.168.10.0/24 is directly connected, Gi0/0.
Why: The C line is the only route a router gets for free. Everything else in this build's routing tables will be learned by EIGRP.
2. Stand up the south site
The same again for the second site, in its own subnet: a router named South with 192.168.20.1 on Gi0/0, and a PC behind it.
- Cable South Gi0/0 ↔ PC-South Eth0
On South — Name the router and make Gi0/0 the south LAN's gateway
```
enable
configure terminal
hostname South
interface Gi0/0
ip address 192.168.20.1 255.255.255.0
no shutdown
exit
end
```
On PC-South — Name the workstation, address it, and point it at its gateway
```
hostname PC-South
ipconfig Eth0 192.168.20.10 255.255.255.0 192.168.20.1
```
Check: run show ip route on South and look for C 192.168.20.0/24 is directly connected, Gi0/0.
Why: Two sites, two subnets, and no link between them yet: each router's table holds its own LAN and nothing else.
3. Hang both sites off a hub
Drag a third router between the sites and name it Hub. Cable North's Gi0/1 to Hub's Gi0/0 (10.0.1.0/30) and Hub's Gi0/1 to South's Gi0/1 (10.0.2.0/30), and address all four ends. Hub has no LAN of its own: it is the transit router both sites' traffic will cross.
- Cable North Gi0/1 ↔ Hub Gi0/0
- Cable Hub Gi0/1 ↔ South Gi0/1
On North — Take the north end of the first hub link, 10.0.1.1
```
enable
configure terminal
interface Gi0/1
ip address 10.0.1.1 255.255.255.252
no shutdown
exit
end
```
On Hub — Name the hub and take its end of both links
```
enable
configure terminal
hostname Hub
interface Gi0/0
ip address 10.0.1.2 255.255.255.252
no shutdown
exit
interface Gi0/1
ip address 10.0.2.1 255.255.255.252
no shutdown
exit
end
```
On South — Take the south end of the second hub link, 10.0.2.2
```
enable
configure terminal
interface Gi0/1
ip address 10.0.2.2 255.255.255.252
no shutdown
exit
end
```
Check: run show ip interface brief on Hub and look for Gi0/1 10.0.2.1 YES manual up up.
Why: Every router can now reach its direct neighbours and nothing further. North has never heard of 10.0.2.0/30 on the far side of Hub, let alone the south LAN.
4. Start EIGRP on North and Hub
`router eigrp 100` starts EIGRP in autonomous system 100 — a number every router in the design has to agree on. The `network` lines take a wildcard, exactly like OSPF's, and switch EIGRP on for each interface they match. The moment Hub joins, the two become neighbours and North installs a D route to 10.0.2.0/30, a link it has never been cabled to.
On North — Run EIGRP AS 100 on the north LAN and the hub link
```
enable
configure terminal
router eigrp 100
network 192.168.10.0 0.0.0.255
network 10.0.1.0 0.0.0.3
end
```
On Hub — Run EIGRP AS 100 on both of Hub's links
```
enable
configure terminal
router eigrp 100
network 10.0.1.0 0.0.0.3
network 10.0.2.0 0.0.0.3
end
```
Check: run show ip route on North and look for D 10.0.2.0/30 [90/3072] via 10.0.1.2, Gi0/1.
Why: In [90/3072], 90 is EIGRP's administrative distance — trusted above OSPF's 110 and RIP's 120 — and 3072 is its metric: 256 × (10,000,000 ÷ the slowest bandwidth on the path in kbit/s + the total delay in tens of microseconds). Two gigabit interfaces: 256 × (10 + 2) = 3072.
5. Bring South up in the wrong AS
Configure South the way a tired engineer might: every `network` line right, but `router eigrp 10` where the design says 100. Both processes run, and nothing happens. Open `show ip eigrp neighbors` on South — the table is empty, no D route arrives in either direction, and PC-North still cannot reach PC-South.
On South — Start EIGRP with the AS number mistyped as 10
```
enable
configure terminal
router eigrp 10
network 192.168.20.0 0.0.0.255
network 10.0.2.0 0.0.0.3
end
```
Check: run show ip eigrp neighbors on South and look for EIGRP-IPv4 Neighbors for AS(10).
Why: The AS number is not a label. It travels in every EIGRP packet, and a router ignores hellos from a different AS, so the two processes never meet. Nothing prints an error — the empty neighbour table is the symptom to learn to spot.
6. Fix the AS number
An AS number cannot be edited in place: remove the process with `no router eigrp 10`, then build it again under 100. The adjacency with Hub forms at once, and North learns the south LAN two hops away.
On South — Replace the AS 10 process with the right one, AS 100
```
enable
configure terminal
no router eigrp 10
router eigrp 100
network 192.168.20.0 0.0.0.255
network 10.0.2.0 0.0.0.3
end
```
On PC-North — Cross both hub links to the south LAN
```
ping 192.168.20.10
```
Check: run show ip route 192.168.20.10 on North and look for Known via "eigrp 100", distance 90, metric 3328.
Why: The metric grew by one more gigabit hop's delay: 256 × (10 + 3) = 3328. EIGRP adds up delay along the path and keeps only the slowest bandwidth, so every hop costs something, but a fast hop costs very little.
7. Add a slow shortcut and watch EIGRP ignore it
Cable North's Se0/0/0 straight to South's Se0/0/0, address it as 10.0.3.0/30, and add it to EIGRP on both routers. Counting hops, as RIP would, the direct link wins — one hop instead of two. EIGRP keeps the path through Hub: over the serial line — 1,544 kbit/s and 20,000 µs of delay — the south LAN costs 2,170,112, against 3,328 through two gigabit hops.
- Cable North Se0/0/0 ↔ South Se0/0/0 (serial)
On North — Address the north end of the serial link and run EIGRP on it
```
enable
configure terminal
interface Se0/0/0
ip address 10.0.3.1 255.255.255.252
no shutdown
exit
router eigrp 100
network 10.0.3.0 0.0.0.3
end
```
On South — Address the south end and run EIGRP on it too
```
enable
configure terminal
interface Se0/0/0
ip address 10.0.3.2 255.255.255.252
no shutdown
exit
router eigrp 100
network 10.0.3.0 0.0.0.3
end
```
Check: run show ip route on North and look for D 192.168.20.0/24 [90/3328] via 10.0.1.2, Gi0/1.
Why: EIGRP builds its metric from the slowest bandwidth on the path and the total delay, so it sees link speed where RIP only counts routers. The serial line is not wasted: South is an EIGRP neighbour across it too, ready to carry the traffic if the path through Hub ever fails.

The theory behind it

Static routes across a WAN — Join three sites with point-to-point WAN links and route between them by hand, one line at a time.
RIP: routing by counting hops — Chain three routers over two serial links and let RIP version 2 teach every router the way to every network, counting the hops as it goes.
Two routers that learn the network — Join two sites over a WAN link, then let OSPF fill in the routing tables that you would otherwise type by hand.
A backup link that waits its turn — Run two sites over a fast primary link, park a floating static route on a slow standby line, then pull the primary and watch the backup carry the traffic.
Steer OSPF with link cost — Give OSPF two paths between two sites, watch it choose the cheaper one over the shorter one, then re-rate a link and watch the route move.
Share one default route with OSPF — Give the edge router a default route to the provider, then let OSPF hand it to the rest of the campus instead of typing it on every router.

Build it for real

The lab walks you through these steps and ticks each one off as your network starts working.

Open in the lab

What you'll build

Step by step

1. Stand up the north site

2. Stand up the south site

3. Hang both sites off a hub

4. Start EIGRP on North and Hub

5. Bring South up in the wrong AS

6. Fix the AS number

7. Add a slow shortcut and watch EIGRP ignore it

The theory behind it

More in Routing

Build it for real