End-to-End Connectivity

Lab Objectives

• Configure basic IPv4 addressing on the routers in the provided base topology and enable inter-router reachability.
• Trace an end-to-end packet path using diagnostic commands; identify where connectivity fails.
• Practice systematic troubleshooting: check interfaces, routing, and reachability to isolate faults.

Tip: End-to-end troubleshooting is a methodical process: start at the endpoints and work toward the network core, verifying each hop. Think of it like following footprints through a forest — each hop is a footprint that either exists (good) or is missing (problem).

BASE TOPOLOGY (use this EXACT topology for the lab)

                [Internet]
               203.0.113.1
                    |
               R1 (Gateway)
              Gi0/0: 10.10.10.1
              Gi0/1: 10.10.20.1
              Gi0/2: 10.10.30.1
              /     |     \
           R2      R3      R4

Gi0/0: 10.10.10.2 | Gi0/0: 10.10.30.2 Gi0/1: 10.10.40.1 | / \ | S1 S2 S3 / \ | /
PC1 PC2 PC3 PC4 PC5

IP SCHEME:

• 10.10.10.0/24 — R1-R2 link
• 10.10.20.0/24 — R1-R3 link
• 10.10.30.0/24 — R1-R4 link
• 10.10.40.0/24 — R2-S1 link
• 192.168.1.0/24 — VLAN 10 (Sales) — behind R2
• 192.168.2.0/24 — VLAN 20 (Engineering) — behind R2
• 192.168.3.0/24 — VLAN 30 (Management) — behind R3
• 203.0.113.0/24 — Public/Internet simulation

Lab Tasks (Try It Yourself First!)

Complete these tasks WITHOUT looking at the solution below. Use ? and show commands to figure it out.

Task 1: Configure Router-to-Router IPs

Assign the IPv4 addresses on each router interface that connects to other routers (use the Gi0/x names and IPs in the base topology). Ensure interfaces are enabled.

Task 2: Configure LAN Reachability

On R2 create static routes to the 192.168.1.0/24 and 192.168.2.0/24 networks (assume they are local to R2). On R3 create a static route to 192.168.3.0/24 (assume local to R3). Then configure R1 with static routes so it knows how to reach the 192.168.0/24 networks via R2 and R3.

Task 3: Trace and Verify End-to-End Path

From R2, trace a packet to a host in 192.168.3.0/24. Use traceroute (or equivalent) and show commands to find where the path breaks and which device is responsible.

Think About It: Why is it usually faster to find a routing problem by checking the routing tables (show ip route) on each hop than by only pinging endpoints?

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Lab Solution

Task 1 Solution: Configure Router-to-Router IPs

What we are doing: Bring up the inter-router interfaces with the IPv4 addresses from the topology so the routers can exchange packets.

! On R1
interface GigabitEthernet0/0
 ip address 10.10.10.1 255.255.255.0
 no shutdown
!
interface GigabitEthernet0/1
 ip address 10.10.20.1 255.255.255.0
 no shutdown
!
interface GigabitEthernet0/2
 ip address 10.10.30.1 255.255.255.0
 no shutdown
!
! On R2
interface GigabitEthernet0/0
 ip address 10.10.10.2 255.255.255.0
 no shutdown
!
interface GigabitEthernet0/1
 ip address 10.10.40.1 255.255.255.0
 no shutdown
!
! On R3
interface GigabitEthernet0/0
 ip address 10.10.20.2 255.255.255.0
 no shutdown
!
! On R4
interface GigabitEthernet0/0
 ip address 10.10.30.2 255.255.255.0
 no shutdown

What just happened:

• Each interface stanza selects the physical interface.
• ip address X X assigns the IPv4 address and mask.
• no shutdown administratively enables the interface (interfaces default to administratively down).
• This establishes Layer 3 adjacency between routers; without addresses and being up, routing is impossible.

Verify:

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.10.1      YES manual up                    up
GigabitEthernet0/1     10.10.20.1      YES manual up                    up
GigabitEthernet0/2     10.10.30.1      YES manual up                    up

R2# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     10.10.10.2      YES manual up                    up
GigabitEthernet0/1     10.10.40.1      YES manual up                    up

Task 2 Solution: Configure LAN Reachability

What we are doing: Add static routes so all routers know how to reach the LANs behind R2 and R3. Static routes are explicit and useful in small labs or in edge cases in production (e.g., stub networks).

! On R2 (R2 is directly connected to 192.168.1.0/24 and 192.168.2.0/24)
ip route 192.168.1.0 255.255.255.0 GigabitEthernet0/1
ip route 192.168.2.0 255.255.255.0 GigabitEthernet0/1

! On R3 (directly connected to 192.168.3.0/24)
ip route 192.168.3.0 255.255.255.0 GigabitEthernet0/0

! On R1 (provide reachability to those LANs via R2 and R3)
ip route 192.168.1.0 255.255.255.0 10.10.10.2
ip route 192.168.2.0 255.255.255.0 10.10.10.2
ip route 192.168.3.0 255.255.255.0 10.10.20.2

What just happened:

• ip route <net> <mask> <next-hop> installs static routes into the routing table.
• On R1 we pointed the 192.168.x.0/24 networks to the appropriate next-hop router (R2 or R3).
• In production, static routes are used for small stable networks or as a fallback; dynamic routing protocols are preferred for larger, changing topologies.

Verify:

R1# show ip route 192.168.3.0
Routing entry for 192.168.3.0/24
  Known via "static", distance 1, metric 0
  Tag 0
  * 10.10.20.2, via GigabitEthernet0/1

R2# show ip route 192.168.1.0
Routing entry for 192.168.1.0/24
  Known via "connected", distance 0, metric 0
  Not advertised to any peer
  via GigabitEthernet0/1

Task 3 Solution: Trace and Verify End-to-End Path

What we are doing: Use traceroute from R2 toward a host in 192.168.3.0/24 to observe each hop and identify where forwarding fails.

R2# traceroute 192.168.3.10
Type escape sequence to abort.
Tracing the route to 192.168.3.10
  1  10.10.10.1  1 ms  1 ms  1 ms
  2  10.10.20.1  2 ms  2 ms  2 ms
  3  10.10.20.2  3 ms  3 ms  3 ms
  4  192.168.3.10  4 ms  4 ms  4 ms

What just happened:

• traceroute shows the sequence (hops) the packet traverses. Each line is a router that decremented IP TTL and returned ICMP TTL-exceeded (or forwarded to final host).
• If a hop times out (* * *) the packet either was filtered by an ACL, the device does not respond to TTL-exceeded, or routing to the next hop is broken.

Verification commands (if traceroute fails at hop 2):

R2# show ip route
! Look for routes to 10.10.20.0 and 192.168.3.0

R2# ping 10.10.10.1
! Tests reachability to R1

R1# show ip route
! Confirm R1 has route to 192.168.3.0

Real-world note: In production, traceroute helps quickly locate whether a failure is on the local LAN, at the customer edge, or in the service-provider core. It is often the first tool used before collecting device logs.

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Troubleshooting Scenario

Scenario: One static route on R1 is misconfigured

Symptom: Traceroute from R2 to 192.168.3.10 times out after reaching R1 (it does not reach R3).

Your task: Find and fix the issue.

Hint: Check the next-hop configured on R1 for the 192.168.3.0/24 route.

Solution:

• On R1 verify the route:

R1# show running-config | include ip route
ip route 192.168.3.0 255.255.255.0 10.10.30.2

• The route incorrectly points to 10.10.30.2 (R4) instead of 10.10.20.2 (R3).
• Fix it:

R1(config)# no ip route 192.168.3.0 255.255.255.0 10.10.30.2
R1(config)# ip route 192.168.3.0 255.255.255.0 10.10.20.2

• Verify traceroute now reaches the destination.

Why this matters: A single wrong next-hop causes traffic to be black-holed. Systematically checking routing tables and next-hops is the fastest way to find such errors.

Verification Checklist

• All inter-router interfaces configured and in the up/up state.
• R1, R2, R3 routing tables contain routes for the 192.168.x.0/24 networks.
• Traceroute from R2 to 192.168.3.10 completes to the destination.

Common Mistakes

Symptom	Cause	Fix
traceroute stops at R1	R1 has no route or wrong next-hop to the destination network	Add or correct static route on R1 (point to 10.10.20.2 for 192.168.3.0)
ping to neighbor fails despite interface up	Interface has wrong IP/mask or missing no shutdown	Reconfigure IP/mask; run no shutdown
intermittent connectivity	Asymmetric routing or incorrect mask causing overlap	Check masks and routing for overlaps; correct masks/next-hops

Challenge Task

Configure a simple default route on R2 and R3 pointing to R1 (as the Internet gateway) and verify hosts behind R2 and R3 can reach the simulated Internet address 203.0.113.1. Do this without step-by-step commands — use your troubleshooting methodology to validate and fix any issues.