Packet Tracer 7.1.1 tutorial - BGP configuration

노션으로 옮김·2024년 1월 4일

BGP

BGP (Border Gateway Protocol) is a routing protocol used between Autonomous Systems.
BGP selects a path with the smallest counts of ASes to the destination.
Initially, it was designed for connections between External Autonomous Systems; however, it has since evolved to include both iBGP (Internal BGP) and eBGP (External BGP).

Topology Diagram

Since Internal BGP (iBGP) is not currently supported in Packet Tracer (see screenshot below), the topology should only consist of different ASes.

BGP Configuration

To cofigure BGP, it takes three steps.

Enter into BGP setup with an AS number
Set neighbors for calcuating the shortest path.
Set network addresses for advertisements to neighbors.

The entire BGP configuration is included in the running-config. Therefore, we will just examine the completed setup on each routers to understand BGP configuration set in this topology.

WAN - BGP(AS Number : 65000)

!
interface GigabitEthernet0/0/0
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface GigabitEthernet0/0/1
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface GigabitEthernet0/0/2
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface Serial0/1/0
 bandwidth 4000000
 ip address 156.12.1.6 255.255.255.252
 clock rate 4000000
!
interface Serial0/1/1
 bandwidth 1200
 ip address 8.8.8.2 255.255.255.0
!
interface Vlan1
 no ip address
 shutdown
!
router bgp 65000
 bgp log-neighbor-changes
 no synchronization
 neighbor 156.12.1.5 remote-as 10000
 neighbor 8.8.8.1 remote-as 20000
!

ISP1 - BGP (AS Number : 10000)

!
interface Loopback0
 ip address 156.100.1.1 255.255.255.0
!
interface Loopback1
 ip address 156.100.2.1 255.255.255.0
!
interface Loopback2
 ip address 156.100.4.1 255.255.254.0
!
interface GigabitEthernet0/0/0
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface GigabitEthernet0/0/1
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface GigabitEthernet0/0/2
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface Serial0/1/0
 bandwidth 4000000
 ip address 156.12.1.5 255.255.255.252
!
interface Serial0/1/1
 bandwidth 4000000
 ip address 10.1.1.1 255.255.255.0
 clock rate 4000000
!
interface Vlan1
 no ip address
 shutdown
!
router bgp 10000
 bgp log-neighbor-changes
 no synchronization
 neighbor 156.12.1.6 remote-as 65000
 neighbor 10.1.1.2 remote-as 20000
 network 156.100.4.0 mask 255.255.254.0
 network 156.100.2.0 mask 255.255.255.0
 network 156.100.1.0 mask 255.255.255.0
!

WAN2 - BGP 20000

!
interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface Loopback1
 ip address 1.1.2.1 255.255.255.0
!
interface Loopback2
 ip address 1.1.3.1 255.255.255.0
!
interface GigabitEthernet0/0/0
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface GigabitEthernet0/0/1
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface GigabitEthernet0/0/2
 no ip address
 duplex auto
 speed auto
 shutdown
!
interface Serial0/1/0
 bandwidth 4000000
 ip address 10.1.1.2 255.255.255.0
!
interface Serial0/1/1
 bandwidth 1200
 ip address 8.8.8.1 255.255.255.0
 clock rate 1200
!
interface Vlan1
 no ip address
 shutdown
!
router bgp 20000
 bgp log-neighbor-changes
 no synchronization
 neighbor 10.1.1.1 remote-as 10000
 neighbor 8.8.8.2 remote-as 65000
 network 124.243.1.0 mask 255.255.255.0
 network 124.243.2.0 mask 255.255.255.0
 network 124.243.3.0 mask 255.255.255.0
 network 1.1.1.0 mask 255.255.255.0
 network 1.1.2.0 mask 255.255.255.0
 network 1.1.3.0 mask 255.255.255.0
!

Check Points

Just display the routes table on WAN - BGP 65000:

WAN1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     1.0.0.0/24 is subnetted, 3 subnets
B       1.1.1.0/24 [20/0] via 8.8.8.1, 00:00:00
B       1.1.2.0/24 [20/0] via 8.8.8.1, 00:00:00
B       1.1.3.0/24 [20/0] via 8.8.8.1, 00:00:00
     8.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C       8.8.8.0/24 is directly connected, Serial0/1/1
L       8.8.8.2/32 is directly connected, Serial0/1/1
     156.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
C       156.12.1.4/30 is directly connected, Serial0/1/0
L       156.12.1.6/32 is directly connected, Serial0/1/0
     156.100.0.0/16 is variably subnetted, 3 subnets, 2 masks
B       156.100.1.0/24 [20/0] via 156.12.1.5, 00:00:00
B       156.100.2.0/24 [20/0] via 156.12.1.5, 00:00:00
B       156.100.4.0/23 [20/0] via 156.12.1.5, 00:00:00

Notice that there are three destination addresses 1.1.X.0/24 routed through 8.8.8.1 on the WAN2 router, not through 156.12.1.5 on the ISP router.

This is because the path through 8.8.8.1 has fewerAS counts than the path through 156.12.1.5.

Also notice that there are three destination addresses 156.100.1,2.0/24, 156.100.4.0/23 routed through 156.12.1.5 on the ISP router, not through 8.8.8.1 on the WAN2 router.

This is because the calculation for this path was completed before connecting with 8.8.8.1. Updates for BGP synchronization are done at regular intervals, not on live.