Chapter 1: BGP Transit AS

Chapter 1: BGP Transit AS


Objective

This lab focuses on configuring BGP to establish transit AS connectivity between different autonomous systems. The goal is to ensure connectivity between AS 1 and AS 3 through AS 2 while implementing both eBGP and iBGP sessions.


Key Features Implemented

  • IP Address Configuration
  • eBGP Peering Between ASes
  • IGP Configuration Within AS 2
  • iBGP Peering Within AS 2
  • Injecting Routes into BGP
  • Verification of End-to-End Connectivity

Topology

The lab uses a multi-AS BGP topology as depicted in the diagram below. AS 2 acts as a transit AS for AS 1 and AS 3.

BGP Transit AS Topology Diagram

Configuration Tasks

  1. Assign IP Addresses:
    • Configure the IP addresses on all routers as shown in the topology.
  2. Configure eBGP Between AS 1 and AS 2:
    • Establish eBGP neighbor relationship between R5 (AS 1) and R1 (AS 2).
  3. Configure eBGP Between AS 2 and AS 3:
    • Establish eBGP neighbor relationship between R4 (AS 2) and R6 (AS 3).
  4. Configure IGP Within AS 2:
    • Implement OSPF (Area 0) within AS 2 (R1, R2, R3, R4). OSPF process IDs are 1 for R1, 2 for R2, 3 for R3, and 4 for R4.
  5. Configure iBGP Within AS 2:
    • Establish a full mesh of iBGP peerings between all routers in AS 2 (R1, R2, R3, R4) using their Loopback0 addresses (1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4).
  6. Advertise Networks into BGP:
    • On R5 (AS 1), inject the 195.1.1.0/24 network (Loopback1) into BGP.
    • On R6 (AS 3), inject the 196.1.1.0/24 network (Loopback1) into BGP.
  7. Ensure Connectivity Between AS 1 and AS 3:
    • Verify that AS 1 (R5) and AS 3 (R6) can reach each other's advertised Loopback1 networks (ping 196.1.1.1 from R5, ping 195.1.1.1 from R6).

Base Configurations

Ready to get started? Here, you'll find the initial configurations for each device to begin this lab.
hostname R1

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
interface Ethernet0/0
 ip address 12.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 13.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/2
 ip address 15.1.1.1 255.255.255.252
 no shutdown
hostname R2

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
interface Ethernet0/0
 ip address 12.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 24.1.1.1 255.255.255.252
 no shutdown
hostname R3

interface Loopback0
 ip address 3.3.3.3 255.255.255.255
interface Ethernet0/0
 ip address 13.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 34.1.1.1 255.255.255.252
 no shutdown
hostname R4

interface Loopback0
 ip address 4.4.4.4 255.255.255.255
interface Ethernet0/0
 ip address 24.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 34.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/2
 ip address 46.1.1.1 255.255.255.252
 no shutdown
hostname R5

interface Loopback1
 ip address 195.1.1.1 255.255.255.0
interface Ethernet0/0
 ip address 15.1.1.2 255.255.255.252
 no shutdown
hostname R6

interface Loopback1
 ip address 196.1.1.1 255.255.255.0
interface Ethernet0/0
 ip address 46.1.1.2 255.255.255.252
 no shutdown

Verification Commands

  • show ip bgp summary
    • Check BGP Peering Status.
  • show ip bgp
    • Verify Route Advertisements.
  • show ip route bgp
    • Check BGP routes in the routing tables.
  • ping 196.1.1.1 source loopback1 (From R5 to R6's Loopback1)
  • ping 195.1.1.1 source loopback1 (From R6 to R5's Loopback1)
    • Reachability checks across ASes.

Expected Outcomes

  • Successful BGP neighbor relationships (eBGP and iBGP).
  • Routes are learned across ASes (AS 1 learns AS 3 routes, AS 3 learns AS 1 routes, AS 2 learns both).
  • Connectivity between AS 1 (network 195.1.1.0/24) and AS 3 (network 196.1.1.0/24).
  • Full reachability within AS 2 through the IGP (OSPF).

Final Configurations

Want to take a look for yourself? Here, you will find the final configurations of each device required for this lab.
hostname R1

interface Loopback0
 ip address 1.1.1.1 255.255.255.255
 ip ospf 1 area 0
interface Ethernet0/0
 ip address 12.1.1.1 255.255.255.252
 ip ospf 1 area 0
 no shutdown
interface Ethernet0/1
 ip address 13.1.1.1 255.255.255.252
 ip ospf 1 area 0
 no shutdown
interface Ethernet0/2
 ip address 15.1.1.1 255.255.255.252
 no shutdown

router ospf 1

router bgp 2
 neighbor 2.2.2.2 remote-as 2
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 next-hop-self
 neighbor 3.3.3.3 remote-as 2
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 3.3.3.3 next-hop-self
 neighbor 4.4.4.4 remote-as 2
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 4.4.4.4 next-hop-self
 neighbor 15.1.1.2 remote-as 1
hostname R2

interface Loopback0
 ip address 2.2.2.2 255.255.255.255
 ip ospf 2 area 0
interface Ethernet0/0
 ip address 12.1.1.2 255.255.255.252
 ip ospf 2 area 0
 no shutdown
interface Ethernet0/1
 ip address 24.1.1.1 255.255.255.252
 ip ospf 2 area 0
 no shutdown

router ospf 2

router bgp 2
 neighbor 1.1.1.1 remote-as 2
 neighbor 1.1.1.1 update-source Loopback0
 neighbor 1.1.1.1 next-hop-self
 neighbor 3.3.3.3 remote-as 2
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 3.3.3.3 next-hop-self
 neighbor 4.4.4.4 remote-as 2
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 4.4.4.4 next-hop-self
hostname R3

interface Loopback0
 ip address 3.3.3.3 255.255.255.255
 ip ospf 3 area 0
interface Ethernet0/0
 ip address 13.1.1.2 255.255.255.252
 ip ospf 3 area 0
 no shutdown
interface Ethernet0/1
 ip address 34.1.1.1 255.255.255.252
 ip ospf 3 area 0
 no shutdown

router ospf 3

router bgp 2
 neighbor 1.1.1.1 remote-as 2
 neighbor 1.1.1.1 update-source Loopback0
 neighbor 1.1.1.1 next-hop-self
 neighbor 2.2.2.2 remote-as 2
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 next-hop-self
 neighbor 4.4.4.4 remote-as 2
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 4.4.4.4 next-hop-self
hostname R4

interface Loopback0
 ip address 4.4.4.4 255.255.255.255
 ip ospf 4 area 0
interface Ethernet0/0
 ip address 24.1.1.2 255.255.255.252
 ip ospf 4 area 0
 no shutdown
interface Ethernet0/1
 ip address 34.1.1.2 255.255.255.252
 ip ospf 4 area 0
 no shutdown
interface Ethernet0/2
 ip address 46.1.1.1 255.255.255.252
 no shutdown

router ospf 4

router bgp 2
 neighbor 1.1.1.1 remote-as 2
 neighbor 1.1.1.1 update-source Loopback0
 neighbor 1.1.1.1 next-hop-self
 neighbor 2.2.2.2 remote-as 2
 neighbor 2.2.2.2 update-source Loopback0
 neighbor 2.2.2.2 next-hop-self
 neighbor 3.3.3.3 remote-as 2
 neighbor 3.3.3.3 update-source Loopback0
 neighbor 3.3.3.3 next-hop-self
 neighbor 46.1.1.2 remote-as 3
hostname R5

interface Loopback1
 ip address 195.1.1.1 255.255.255.0
interface Ethernet0/0
 ip address 15.1.1.2 255.255.255.252
 no shutdown

router bgp 1
 network 195.1.1.0
 neighbor 15.1.1.1 remote-as 2
hostname R6

interface Loopback1
 ip address 196.1.1.1 255.255.255.0
interface Ethernet0/0
 ip address 46.1.1.2 255.255.255.252
 no shutdown

router bgp 3
 network 196.1.1.0
 neighbor 46.1.1.1 remote-as 2
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