Question - 1 Explain BGP Local -Preference attribute.
- Local preference is a Well-known and discretionary attribute.Local Preference is locally to Autonomous System, means it is globally within Autonomous System.
- It advertise into the local Autonomous System only, so it is sent to all IBGP routers in an AS, cannot be exchanged between EBGP routers.
- We can use local preference to select the outbound external BGP path. It will tell the Autonomous System about which path will exit the Autonomous System to reach a certain network.
- Always a path with highest Local Preference will be preferred.
- It is a numeric 32-bit number value, and can range from 0 to 4294967295. By default local preference value is 100 and it can be changed manually.
- You can be changed Local Preference via command bgp default local-preference [Local Preference value] or in a route-map using the command set local-preference [Local Preference value].
Question - 2 How can we configure Local-Preference attribute ?
- Router1 Configuration
Router1#show running-config
interface Loopback1
ip address 1.1.1.1 255.255.255.0
!
interface Loopback1
ip address 5.5.5.5 255.255.255.0
!
interface Ethernet1/0
ip address 192.167.26.1 255.255.255.0
half-duplex
!
interface Ethernet2/0
ip address 192.167.23.1 255.255.255.0
half-duplex
!
router bgp 10
no synchronization
bgp log-neighbor-changes
network 1.1.1.0 mask 255.255.255.0
neighbor 192.167.23.2 remote-as 20
neighbor 192.167.26.3 remote-as 20
no auto-summary
- Router2 Configuration
Router2#show running-config
interface Loopback1
ip address 2.2.2.2 255.255.255.0
!
interface Ethernet1/0
ip address 192.167.24.2 255.255.255.0
half-duplex
!
interface Ethernet2/0
ip address 192.167.23.2 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
neighbor 192.167.23.1 remote-as 10
neighbor 192.167.24.4 remote-as 20
no auto-summary
!
no ip http server
no ip http secure-server
!
ip route 3.3.3.0 255.255.255.0 192.167.24.4
ip route 4.4.4.0 255.255.255.0 192.167.24.4
ip route 192.167.25.0 255.255.255.0 192.167.24.4
- Router3 Configuration
Router3#show running-config
interface Loopback1
ip address 3.3.3.3 255.255.255.0
!
interface Ethernet1/0
ip address 192.167.26.3 255.255.255.0
half-duplex
!
interface Ethernet2/0
ip address 192.167.25.3 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
neighbor 192.167.25.4 remote-as 20
neighbor 192.167.26.1 remote-as 10
no auto-summary
!
no ip http server
no ip http secure-server
!
ip route 2.2.2.0 255.255.255.0 192.167.25.4
ip route 4.4.4.0 255.255.255.0 192.167.25.4
ip route 192.167.24.0 255.255.255.0 192.167.25.4
- Router4 Configuration
Router4#show running-config
interface Loopback1
ip address 4.4.4.4 255.255.255.0
!
interface Ethernet1/0
ip address 192.167.24.4 255.255.255.0
half-duplex
!
interface Ethernet2/0
ip address 192.167.25.4 255.255.255.0
half-duplex
!
router bgp 20
no synchronization
bgp log-neighbor-changes
neighbor 192.167.24.2 remote-as 20
neighbor 192.167.25.3 remote-as 20
no auto-summary
!
no ip http server
no ip http secure-server
!
ip route 2.2.2.0 255.255.255.0 192.167.24.2
ip route 3.3.3.0 255.255.255.0 192.167.25.3
ip route 192.167.23.0 255.255.255.0 192.167.24.2
ip route 192.167.26.0 255.255.255.0 192.167.25.3
Output -
So you can see in this output Router4 is using 192.167.23.1(Router2) to reach network 1.1.1.0/24 and 5.5.5.0/24
So you can see in this output Router4 is using 192.167.23.1(Router2) to reach network 1.1.1.0/24 and 5.5.5.0/24
Router3#configure terminal
Router3(config)#router bgp 20
Router3(config-router)#bgp default local-preference 500
Output -
Now you can see in this output, now Router4 is using 192.167.26.1(Router3) to reach network 1.1.1.0/24 and 5.5.5.0/24 because the local preference 500 is greater than 100
Question - 3 How can we configure Local-Preference attribute via Route-Map ?
Now clean up the previous local-preference configuration
Router3#configure terminal
Router3(config)#router bgp 20
Router3(config-router)#no bgp default local-preference 500
You can set the local preference via route-map also
Router3#configure terminal
Router3(config)#route-map LOCAL-PREF permit 20 -------------- Create the route-map
Router3(config-route-map)#set local-preference 600 -------------- Set the local preference
Router3(config-route-map)#exit
Router3(config)#router bgp 20
Router3(config-router)#neighbor 192.167.26.1 route-map LOCAL-PREF in --------- now put the ip address of neighbor (for which you want to set the local-preference value - 600) in route map for inbound
Router3(config-router)#exit
Router3(config)#clear ip bgp * ------------- we will clear the BGP session to apply the changes
Output -
Question - 4 How can we change the Local-Preference for certain prefix only ?
We will set the local-preference 700 for all prefix to reach the 5.5.5.0/24, and we will set the local-preference 100 for all prefix to reach the 1.1.1.0/24
Router3#configure terminal
Router3(config)#access-list 1 permit 5.5.5.0 0.0.0.255
Router3(config)#route-map LOCAL-PREF permit 10
Router3(config-route-map)#match ip address 1
Router3(config-route-map)#set local-preference 700
Router3(config-route-map)#exit
Router3(config)#route-map LOCAL-PREF permit 20
Router3(config-route-map)#set local-preference 600
Router3(config-route-map)#exit
Router3(config)#router bgp 20
Router3(config-router)#neighbor 192.167.26.1 route-map LOCAL-PREF in
Router3(config-router)#exit
Router3(config)#clear ip bgp *
Output -
Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.
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