Chapter 2: BGP Path Manipulation with Weight and Local Preference Path Attributes

Chapter 2: BGP Path Manipulation with Weight and Local Preference Path Attributes


Objective

This lab focuses on manipulating BGP path selection using the Weight and Local Preference attributes to influence outbound traffic flow. Participants will learn how BGP selects the best path based on different attributes and how to modify them to achieve desired routing behavior.


Key Features Implemented

  • eBGP and iBGP session establishment
  • Full-mesh iBGP setup within autonomous systems (AS100 and AS200)
  • Configuring BGP Weight and Local Preference to influence outbound traffic

Topology

The lab utilizes a multi-AS BGP topology with ISPs and Enterprise networks, as depicted below. Enterprise-C (AS300) advertises prefixes, and Enterprises A (AS100) and B (AS200) use path attributes to influence how they reach these prefixes via ISP-1 (AS1) and ISP-2 (AS2).

BGP Path Manipulation Topology Diagram

Configuration Tasks

Step 1: IP Addressing and IGP Configuration

  1. Assign IP addresses as shown in the topology and detailed in the base configurations.
  2. Configure OSPF (Area 0) within AS100 (R4, R5, R6) for internal routing using appropriate OSPF process IDs (4 for R4, 5 for R5, 6 for R6).
  3. Configure OSPF (Area 0) within AS200 (R7, R8, R9) for internal routing using appropriate OSPF process IDs (7 for R7, 8 for R8, 9 for R9).
  4. Ensure Loopback0 interfaces (4.4.4.4/32, 5.5.5.5/32, 6.6.6.6/32 in AS100; 7.7.7.7/32, 8.8.8.8/32, 9.9.9.9/32 in AS200) are advertised via OSPF within their respective AS.

Step 2: Establishing BGP Sessions and Injecting Routes into BGP

  1. Configure eBGP sessions:
    • R1 (AS300) with R2 (AS1, neighbor 12.1.1.2) and R3 (AS2, neighbor 13.1.1.2).
    • R2 (AS1) with R1 (AS300, neighbor 12.1.1.1), R4 (AS100, neighbor 24.1.1.2), R5 (AS100, neighbor 25.1.1.2), and R7 (AS200, neighbor 27.1.1.2).
    • R3 (AS2) with R1 (AS300, neighbor 13.1.1.1), R5 (AS100, neighbor 35.1.1.2), R7 (AS200, neighbor 37.1.1.2), and R8 (AS200, neighbor 38.1.1.2).
    • R4 (AS100) with R2 (AS1, neighbor 24.1.1.1).
    • R5 (AS100) with R2 (AS1, neighbor 25.1.1.1) and R3 (AS2, neighbor 35.1.1.1).
    • R7 (AS200) with R2 (AS1, neighbor 27.1.1.1) and R3 (AS2, neighbor 37.1.1.1).
    • R8 (AS200) with R3 (AS2, neighbor 38.1.1.1).
  2. Configure iBGP Full Mesh:
    • AS100: R4, R5, and R6 must have full-mesh iBGP sessions using Loopback0 addresses. Remember to use update-source Loopback0 and next-hop-self where appropriate (e.g., R4 to R5, R4 to R6; R5 to R4, R5 to R6; R6 to R4, R6 to R5).
    • AS200: R7, R8, and R9 must have full-mesh iBGP sessions using Loopback0 addresses. Remember to use update-source Loopback0 and next-hop-self where appropriate.
  3. Inject Routes:
    • On R1 (AS300), inject 193.1.1.0/24 (Loopback1) and 193.1.2.0/24 (Loopback2) prefixes into BGP using network commands.

Step 3: Adjusting Weight to Influence Path Selection within AS 100

  • In AS 100:
    • Configure R4 to prefer ISP-1 (path via R2, neighbor 24.1.1.1) for reaching the 193.1.1.0/24 network by applying a route-map to set a higher weight (e.g., 200) for this prefix received from R2.
    • Configure R5 to prefer ISP-2 (path via R3, neighbor 35.1.1.1) for reaching the 193.1.2.0/24 network by applying a route-map to set a higher weight (e.g., 200) for this prefix received from R3.

Step 4: Adjusting Local Preference to Influence Path Selection within AS 200

  • In AS 200:
    • Configure R7 to prefer ISP-1 (path via R2, neighbor 27.1.1.1) for reaching the 193.1.1.0/24 network by applying a route-map to set a higher local preference (e.g., 200) for this prefix received from R2.
    • Configure R8 to prefer ISP-2 (path via R3, neighbor 38.1.1.1) for reaching the 193.1.2.0/24 network by applying a route-map to set a higher local preference (e.g., 200) for this prefix received from R3.

Base Configurations

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

interface Loopback1
 ip address 193.1.1.1 255.255.255.0
interface Loopback2
 ip address 193.1.2.1 255.255.255.0
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
hostname R2

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
interface Ethernet0/2
 ip address 25.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/3
 ip address 27.1.1.1 255.255.255.252
 no shutdown
hostname R3

interface Ethernet0/0
 ip address 13.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 37.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/2
 ip address 38.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/3
 ip address 35.1.1.1 255.255.255.252
 no shutdown
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
 no shutdown
interface Ethernet0/1
 ip address 46.1.1.1 255.255.255.252
 ip ospf 4 area 0
 no shutdown

router ospf 4
hostname R5

interface Loopback0
 ip address 5.5.5.5 255.255.255.255
 ip ospf 5 area 0
interface Ethernet0/0
 ip address 25.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 56.1.1.1 255.255.255.252
 ip ospf 5 area 0
 no shutdown
interface Ethernet0/2
 ip address 35.1.1.2 255.255.255.252
 no shutdown

router ospf 5
hostname R6

interface Loopback0
 ip address 6.6.6.6 255.255.255.255
 ip ospf 6 area 0
interface Ethernet0/0
 ip address 46.1.1.2 255.255.255.252
 ip ospf 6 area 0
 no shutdown
interface Ethernet0/1
 ip address 56.1.1.2 255.255.255.252
 ip ospf 6 area 0
 no shutdown

router ospf 6
hostname R7

interface Loopback0
 ip address 7.7.7.7 255.255.255.255
 ip ospf 7 area 0
interface Ethernet0/0
 ip address 37.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 79.1.1.1 255.255.255.252
 ip ospf 7 area 0
 duplex auto
interface Ethernet0/2
 ip address 27.1.1.2 255.255.255.252
 duplex auto

router ospf 7
hostname R8

interface Loopback0
 ip address 8.8.8.8 255.255.255.255
 ip ospf 8 area 0
interface Ethernet0/0
 ip address 38.1.1.2 255.255.255.252
 duplex auto
interface Ethernet0/1
 ip address 89.1.1.1 255.255.255.252
 ip ospf 8 area 0
 duplex auto

router ospf 8
hostname R9

interface Loopback0
 ip address 9.9.9.9 255.255.255.255
 ip ospf 9 area 0
interface Ethernet0/0
 ip address 79.1.1.2 255.255.255.252
 ip ospf 9 area 0
 duplex auto
interface Ethernet0/1
 ip address 89.1.1.2 255.255.255.252
 ip ospf 9 area 0
 duplex auto

router ospf 9

Verification Commands

  • show ip bgp summary
    • Check BGP neighbor relationships.
  • show ip bgp
    • Check BGP best path selection (observe AS_PATH, Weight, Local Preference).
  • show ip bgp 193.1.1.0/24 (and 193.1.2.0/24)
    • Detailed route view to verify Weight and Local Preference attribute influence on routers in AS100 and AS200.
  • show ip route 193.1.1.0/24 (and 193.1.2.0/24)
    • Check Routing Table to verify which paths are preferred and installed.

Expected Outcomes

  • Entire AS100 to prefer ISP-1 (path via R2/R4) for 193.1.1.0/24 due to Weight manipulation on R4.
  • Entire AS100 to prefer ISP-2 (path via R3/R5) for 193.1.2.0/24 due to Weight manipulation on R5.
  • Entire AS200 to prefer ISP-1 (path via R2/R7) for 193.1.1.0/24 due to Local Preference manipulation on R7.
  • Entire AS200 to prefer ISP-2 (path via R3/R8) for 193.1.2.0/24 due to Local Preference manipulation on R8.
  • The routing table reflects the expected best paths based on configured attributes.
  • All BGP sessions are established and stable.

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 Loopback1
 ip address 193.1.1.1 255.255.255.0
interface Loopback2
 ip address 193.1.2.1 255.255.255.0
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

router bgp 300
 network 193.1.1.0
 network 193.1.2.0
 neighbor 12.1.1.2 remote-as 1
 neighbor 13.1.1.2 remote-as 2
hostname R2

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
interface Ethernet0/2
 ip address 25.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/3
 ip address 27.1.1.1 255.255.255.252
 no shutdown

router bgp 1
 neighbor 12.1.1.1 remote-as 300
 neighbor 24.1.1.2 remote-as 100
 neighbor 25.1.1.2 remote-as 100
 neighbor 27.1.1.2 remote-as 200
hostname R3

interface Ethernet0/0
 ip address 13.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 37.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/2
 ip address 38.1.1.1 255.255.255.252
 no shutdown
interface Ethernet0/3
 ip address 35.1.1.1 255.255.255.252
 no shutdown

router bgp 2
 neighbor 13.1.1.1 remote-as 300
 neighbor 35.1.1.2 remote-as 100
 neighbor 37.1.1.2 remote-as 200
 neighbor 38.1.1.2 remote-as 200
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
 no shutdown
interface Ethernet0/1
 ip address 46.1.1.1 255.255.255.252
 ip ospf 4 area 0
 no shutdown

router ospf 4

ip prefix-list match-193-1-1-0-net seq 5 permit 193.1.1.0/24
! ip prefix-list match-193-1-2-0-net seq 5 permit 193.1.2.0/24 ! Not used by R4 as per specific task

route-map set-weight-path-attribute-prefer-ISP-1 permit 10
 match ip address prefix-list match-193-1-1-0-net
 set weight 200
route-map set-weight-path-attribute-prefer-ISP-1 permit 20

! route-map set-weight-path-attribute-prefer-ISP-2 permit 10 ! Not used by R4
!  match ip address prefix-list match-193-1-2-0-net
!  set weight 200
! route-map set-weight-path-attribute-prefer-ISP-2 permit 20

router bgp 100
 neighbor 5.5.5.5 remote-as 100
 neighbor 5.5.5.5 update-source Loopback0
 neighbor 5.5.5.5 next-hop-self
 ! neighbor 5.5.5.5 route-map set-weight-path-attribute-prefer-ISP-2 in ! Route-map applied to eBGP neighbor
 neighbor 6.6.6.6 remote-as 100
 neighbor 6.6.6.6 update-source Loopback0
 neighbor 6.6.6.6 next-hop-self
 neighbor 24.1.1.1 remote-as 1
 neighbor 24.1.1.1 route-map set-weight-path-attribute-prefer-ISP-1 in
hostname R5

interface Loopback0
 ip address 5.5.5.5 255.255.255.255
 ip ospf 5 area 0
interface Ethernet0/0
 ip address 25.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 56.1.1.1 255.255.255.252
 ip ospf 5 area 0
 no shutdown
interface Ethernet0/2
 ip address 35.1.1.2 255.255.255.252
 no shutdown

router ospf 5

! ip prefix-list match-193-1-1-0-net seq 5 permit 193.1.1.0/24 ! Not used by R5 as per specific task
ip prefix-list match-193-1-2-0-net seq 5 permit 193.1.2.0/24

! route-map set-weight-path-attribute-prefer-ISP-1 permit 10 ! Not used by R5
!  match ip address prefix-list match-193-1-1-0-net
!  set weight 200
! route-map set-weight-path-attribute-prefer-ISP-1 permit 20

route-map set-weight-path-attribute-prefer-ISP-2 permit 10
 match ip address prefix-list match-193-1-2-0-net
 set weight 200
route-map set-weight-path-attribute-prefer-ISP-2 permit 20

router bgp 100
 neighbor 4.4.4.4 remote-as 100
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 4.4.4.4 next-hop-self
 ! neighbor 4.4.4.4 route-map set-weight-path-attribute-prefer-ISP-1 in ! Route-map applied to eBGP neighbor
 neighbor 6.6.6.6 remote-as 100
 neighbor 6.6.6.6 update-source Loopback0
 neighbor 6.6.6.6 next-hop-self
 neighbor 25.1.1.1 remote-as 1
 neighbor 35.1.1.1 remote-as 2
 neighbor 35.1.1.1 route-map set-weight-path-attribute-prefer-ISP-2 in
hostname R6

interface Loopback0
 ip address 6.6.6.6 255.255.255.255
 ip ospf 6 area 0
interface Ethernet0/0
 ip address 46.1.1.2 255.255.255.252
 ip ospf 6 area 0
 no shutdown
interface Ethernet0/1
 ip address 56.1.1.2 255.255.255.252
 ip ospf 6 area 0
 no shutdown

router ospf 6

router bgp 100
 neighbor 4.4.4.4 remote-as 100
 neighbor 4.4.4.4 update-source Loopback0
 neighbor 5.5.5.5 remote-as 100
 neighbor 5.5.5.5 update-source Loopback0
hostname R7

interface Loopback0
 ip address 7.7.7.7 255.255.255.255
 ip ospf 7 area 0
interface Ethernet0/0
 ip address 37.1.1.2 255.255.255.252
 no shutdown
interface Ethernet0/1
 ip address 79.1.1.1 255.255.255.252
 ip ospf 7 area 0
 duplex auto
interface Ethernet0/2
 ip address 27.1.1.2 255.255.255.252
 duplex auto

router ospf 7

ip prefix-list match-193-1-1-0-net seq 5 permit 193.1.1.0/24

route-map set-LP-path-attribute-prefer-ISP-1 permit 10
 match ip address prefix-list match-193-1-1-0-net
 set local-preference 200
route-map set-LP-path-attribute-prefer-ISP-1 permit 20

router bgp 200
 neighbor 8.8.8.8 remote-as 200
 neighbor 8.8.8.8 update-source Loopback0
 neighbor 8.8.8.8 next-hop-self
 neighbor 9.9.9.9 remote-as 200
 neighbor 9.9.9.9 update-source Loopback0
 neighbor 9.9.9.9 next-hop-self
 neighbor 27.1.1.1 remote-as 1
 neighbor 27.1.1.1 route-map set-LP-path-attribute-prefer-ISP-1 in
 neighbor 37.1.1.1 remote-as 2
hostname R8

interface Loopback0
 ip address 8.8.8.8 255.255.255.255
 ip ospf 8 area 0
interface Ethernet0/0
 ip address 38.1.1.2 255.255.255.252
 duplex auto
interface Ethernet0/1
 ip address 89.1.1.1 255.255.255.252
 ip ospf 8 area 0
 duplex auto

router ospf 8

ip prefix-list match-193-1-2-0-net seq 5 permit 193.1.2.0/24

route-map set-LP-path-attribute-prefer-ISP-2 permit 10
 match ip address prefix-list match-193-1-2-0-net
 set local-preference 200
route-map set-LP-path-attribute-prefer-ISP-2 permit 20

router bgp 200
 neighbor 7.7.7.7 remote-as 200
 neighbor 7.7.7.7 update-source Loopback0
 neighbor 7.7.7.7 next-hop-self
 neighbor 9.9.9.9 remote-as 200
 neighbor 9.9.9.9 update-source Loopback0
 neighbor 9.9.9.9 next-hop-self
 neighbor 38.1.1.1 remote-as 2
 neighbor 38.1.1.1 route-map set-LP-path-attribute-prefer-ISP-2 in
hostname R9

interface Loopback0
 ip address 9.9.9.9 255.255.255.255
 ip ospf 9 area 0
interface Ethernet0/0
 ip address 79.1.1.2 255.255.255.252
 ip ospf 9 area 0
 duplex auto
interface Ethernet0/1
 ip address 89.1.1.2 255.255.255.252
 ip ospf 9 area 0
 duplex auto

router ospf 9

router bgp 200
 neighbor 7.7.7.7 remote-as 200
 neighbor 7.7.7.7 update-source Loopback0
 neighbor 8.8.8.8 remote-as 200
 neighbor 8.8.8.8 update-source Loopback0
 neighbor 8.8.8.8 next-hop-self
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