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Dynamic Routing Lab: OSPF (Internal) & eBGP (External) in Packet Tracer

Theory prerequisites

Read Packets & IP Addressing and Static Routing, OSPF & BGP. Return to the Lab-Aligned Learning Path after verification.

This lab bridges the gap between Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP). You will build a multi-router topology where:

  • OSPF (Open Shortest Path First - Area 0): Dynamically routes internal enterprise traffic between branch and core routers within Autonomous System 65001.
  • eBGP (External Border Gateway Protocol): Establishes an exterior peering session between your enterprise edge router (AS 65001) and an upstream ISP (AS 65002).
  • Default Route Injection (default-information originate): Injects an exit path into OSPF so all internal PCs can seamlessly reach the internet/ISP.

Topology

Enterprise OSPF Area 0 connected through an eBGP edge to an ISP autonomous system and public server

Protocol Comparison: OSPF vs. BGP

Feature OSPF (Open Shortest Path First) BGP (Border Gateway Protocol)
Protocol Type Interior Gateway Protocol (IGP) Exterior Gateway Protocol (EGP)
Scope Inside a single organization / AS Between different organizations / ISPs
Algorithm / Metric Link-State (Dijkstra SPF) / Metric: Cost (Bandwidth) Path-Vector / Path Attributes (AS-Path, Weight, Local Pref)
Transport / Port Raw IP Protocol 89 TCP Port 179
Primary Goal Fast convergence and shortest internal path Policy control, scalability, and loop-free internet routing

Addressing Plan

Device Interface IP Address Subnet Mask Routing Protocol Role
R1-Core Gig0/0 10.1.1.1 255.255.255.0 OSPF Area 0 Enterprise Client LAN Gateway
R1-Core Gig0/1 10.1.12.1 255.255.255.252 OSPF Area 0 Internal Core-to-Edge Transit Link
R2-Edge Gig0/0 10.1.12.2 255.255.255.252 OSPF Area 0 Internal Transit to Core
R2-Edge Gig0/1 203.0.113.1 255.255.255.252 eBGP (AS 65001) External eBGP Peering to ISP
ISP-Router Gig0/0 203.0.113.2 255.255.255.252 eBGP (AS 65002) ISP Peering Interface
ISP-Router Gig0/1 198.51.100.1 255.255.255.0 BGP Network Public Cloud Gateway
PC0 Fa0 10.1.1.10 255.255.255.0 Static (Gateway 10.1.1.1) Enterprise Client PC
Cloud-Server0 Fa0 198.51.100.2 255.255.255.0 Static (Gateway 198.51.100.1) Remote Public Server

1. Place and Cable Devices

Devices Required

  • 3x Router (1941: R1-Core, R2-Edge, ISP-Router)
  • 2x Switch (2960-24TT: Switch0, Switch1)
  • 1x Generic PC (PC0)
  • 1x Generic Server (Cloud-Server0)

Cabling (Copper Straight-Through)

  • PC0 <—> Switch0
  • Switch0 <—> R1-Core (Gig0/0)
  • R1-Core (Gig0/1) <—> R2-Edge (Gig0/0)
  • R2-Edge (Gig0/1) <—> ISP-Router (Gig0/0)
  • ISP-Router (Gig0/1) <—> Switch1
  • Switch1 <—> Cloud-Server0

2. R1-Core Configuration (OSPF Area 0)

Open R1-Core CLI:

enable
configure terminal
hostname R1-Core

! 1. Interface IP Configuration
interface GigabitEthernet0/0
 description Enterprise LAN
 ip address 10.1.1.1 255.255.255.0
 no shutdown
exit

interface GigabitEthernet0/1
 description Transit Link to R2-Edge
 ip address 10.1.12.1 255.255.255.252
 no shutdown
exit

! 2. Configure OSPF Process 1 in Area 0
! Note: Wildcard mask is inverted subnet mask (0.0.0.255 for /24, 0.0.0.3 for /30)
router ospf 1
 router-id 1.1.1.1
 network 10.1.1.0 0.0.0.255 area 0
 network 10.1.12.0 0.0.0.3 area 0
exit

end
write memory

3. R2-Edge Configuration (OSPF + eBGP + Default Route)

Open R2-Edge CLI:

enable
configure terminal
hostname R2-Edge

! 1. Interface IP Configuration
interface GigabitEthernet0/0
 description Internal Link to R1-Core
 ip address 10.1.12.2 255.255.255.252
 no shutdown
exit

interface GigabitEthernet0/1
 description External WAN to ISP
 ip address 203.0.113.1 255.255.255.252
 no shutdown
exit

! 2. Configure OSPF (Area 0) and Inject Default Route
router ospf 1
 router-id 2.2.2.2
 network 10.1.12.0 0.0.0.3 area 0
 ! Injects a default route (0.0.0.0/0) into OSPF so R1-Core forwards internet traffic to R2
 default-information originate
exit

! 3. Configure eBGP with ISP
router bgp 65001
 bgp router-id 2.2.2.2
 neighbor 203.0.113.2 remote-as 65002
 neighbor 203.0.113.2 description Peering-to-ISP
 ! Advertise enterprise summary prefix to the ISP
 network 10.1.0.0 mask 255.255.0.0
exit

! 4. Static Summary Route (Required for BGP network command to advertise)
ip route 10.1.0.0 255.255.0.0 Null0

end
write memory

4. ISP-Router Configuration (eBGP AS 65002)

Open ISP-Router CLI:

enable
configure terminal
hostname ISP-Router

! 1. Interface IP Configuration
interface GigabitEthernet0/0
 description Peering Link to Enterprise R2-Edge
 ip address 203.0.113.2 255.255.255.252
 no shutdown
exit

interface GigabitEthernet0/1
 description Public Cloud Server Subnet
 ip address 198.51.100.1 255.255.255.0
 no shutdown
exit

! 2. Configure eBGP Peering and Advertise Public Network
router bgp 65002
 bgp router-id 3.3.3.3
 neighbor 203.0.113.1 remote-as 65001
 neighbor 203.0.113.1 description Peering-to-Enterprise-R2
 network 198.51.100.0 mask 255.255.255.0
exit

end
write memory

5. End Device Configuration

  • PC0:
  • IP Address: 10.1.1.10
  • Subnet Mask: 255.255.255.0
  • Default Gateway: 10.1.1.1
  • Cloud-Server0:
  • IP Address: 198.51.100.2
  • Subnet Mask: 255.255.255.0
  • Default Gateway: 198.51.100.1
  • Under Config > Services > HTTP: Ensure HTTP is On.

6. Verification and Troubleshooting

Step 1: Verify OSPF Adjacency (Inside AS 65001)

On R1-Core CLI, run: 

show ip ospf neighbor
Expected Status: State should show FULL/DR or FULL/BDR with Neighbor ID 2.2.2.2.

Check the OSPF routing table on R1-Core: 

show ip route ospf
Expected Output: You will see O*E2 0.0.0.0/0 [110/1] via 10.1.12.2, confirming R1-Core received the injected default route from R2-Edge.

Step 2: Verify BGP Peering (Between AS 65001 and AS 65002)

On R2-Edge CLI, run: 

show ip bgp summary
Expected Output: 
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
203.0.113.2     4 65002      15      15        4    0    0 00:08:22        1
(Notice the State/PfxRcd column shows 1 received prefix, indicating an active BGP session).

Inspect learned BGP routes on R2-Edge: 

show ip bgp
Shows the learned prefix 198.51.100.0/24 with Next Hop 203.0.113.2 and AS-Path 65002 i.

Step 3: End-to-End Connectivity Test

From PC0 Command Prompt, ping the remote cloud server across both routing protocols: 

ping 198.51.100.2
From PC0 Web Browser, browse to: 
http://198.51.100.2

7. What to Observe (Packet Journey Across OSPF & BGP)

Hop / Segment Protocol in Control Routing Decision
PC0 → R1-Core Default Gateway PC forwards frame to default gateway 10.1.1.1.
R1-Core → R2-Edge OSPF (IGP) R1-Core looks up 198.51.100.2, matches the OSPF O*E2 default route, and forwards across 10.1.12.0/30.
R2-Edge → ISP-Router eBGP (EGP) R2-Edge checks its BGP routing table for 198.51.100.0/24, finding next-hop 203.0.113.2 via AS 65002.
ISP-Router → Cloud-Server Direct Connected ISP-Router delivers frame to Cloud-Server0 on its local subnet.