BGP VLAN neighbor preference

[gpfilgueira]

I am wrecking my head for a solution for this problem for weeks and cannot understand how to solve it.

I have a simple BGP topology with 6 routers. I will refer to R1 as the left one and R6 and the right one.

R6 has 4 vlans on subinterfaces and i need to find a way to make two of those vlans prefer using ine bgp neighbor to reach R1 and the other two vlans to prefer using the other neighbor to reach R1.

I tried ACLs, route-maps, prefix-lists, AS prepending, BGP communities, MED, local-preference, etc... And nothing seems to work!

It looks to be something so simple and yet i cannot discover how to do this.

I am using cisco c7200's, and (i don't know if it is relevant), i have ip sla with tracks and EEM's on R6 to manage bgp routing in cases of some interface failing.

Vlans 10.4.0.0 and 10.5.0.0 should get to PC1 using R1 gi2/0
Vlans 10.6.0.0 and 10.7.0.0 should get to PC1 using R1 gi1/0

But they should only prefer those routes and should be able to use the other route if needed.
 

[MHM Cisco World]

If this is gns3 can I see R6 and R1 config?

MHM

[gpfilgueira]

There are a bunch of leftover from things i tried but here it is:

# R1

R1#show run
Building configuration...

Current configuration : 3535 bytes
!
! Last configuration change at 00:43:06 UTC Thu Feb 6 2025
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
!
hostname R1
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
no ip icmp rate-limit unreachable
!
!
!
!
!
!
no ip domain lookup
ip cef
no ipv6 cef
!
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
ip tcp synwait-time 5
!
!
!
!
!
!
!
!
!
interface FastEthernet0/0
ip address 192.168.10.1 255.255.255.0
duplex full
!
interface GigabitEthernet1/0
ip address 192.168.0.1 255.255.255.252
negotiation auto
!
interface GigabitEthernet2/0
ip address 192.168.0.5 255.255.255.252
negotiation auto
!
interface GigabitEthernet3/0
no ip address
shutdown
negotiation auto
!
interface GigabitEthernet4/0
no ip address
shutdown
negotiation auto
!
interface GigabitEthernet5/0
no ip address
shutdown
negotiation auto
!
interface GigabitEthernet6/0
no ip address
shutdown
negotiation auto
!
router bgp 65529
bgp router-id 1.1.1.1
bgp log-neighbor-changes
network 192.168.0.0 mask 255.255.255.252
network 192.168.0.4 mask 255.255.255.252
network 192.168.10.0
redistribute connected
redistribute static
neighbor 192.168.0.2 remote-as 65530
neighbor 192.168.0.2 route-map TOP in
neighbor 192.168.0.6 remote-as 65531
neighbor 192.168.0.6 route-map BOTTOM in
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
!
ip prefix-list NET4 seq 5 permit 10.4.0.0/24
!
ip prefix-list NET5 seq 5 permit 10.5.0.0/24
!
ip prefix-list NET6 seq 5 permit 10.6.0.0/24
!
ip prefix-list NET7 seq 5 permit 10.7.0.0/24
!
ip prefix-list NETLOCAL seq 5 permit 192.168.10.0/24
!
route-map MET_BOTTOM permit 10
match ip address prefix-list NET6
set metric 50
!
route-map MET_BOTTOM permit 20
match ip address prefix-list NET7
set metric 50
!
route-map BOTTOM permit 10
match ip address prefix-list NET6
set local-preference 200
!
route-map BOTTOM permit 20
match ip address prefix-list NET7
set local-preference 200
!
route-map MED_BOTTOM permit 10
match ip address prefix-list NET6
set metric 50
!
route-map MED_BOTTOM permit 20
match ip address prefix-list NET7
set metric 50
!
route-map MED_BOTTOM permit 30
match ip address prefix-list NETLOCAL
!
route-map MED_BOTTOM permit 40
!
route-map PREPENDR2 permit 10
match ip address prefix-list NET6
set as-path prepend 65529 65529 65529 65529
!
route-map PREPENDR2 permit 20
match ip address prefix-list NET7
set as-path prepend 65529 65529 65529 65529
!
route-map PREPENDR3 permit 10
match ip address prefix-list NET4
set as-path prepend 65529 65529 65529 65529
!
route-map PREPENDR3 permit 20
match ip address prefix-list NET5
set as-path prepend 65529 65529 65529 65529
!
route-map MET_TOP permit 10
match ip address prefix-list NET4
set metric 50
!
route-map MET_TOP permit 20
match ip address prefix-list NET5
set metric 50
!
route-map TOP permit 10
match ip address prefix-list NET4
set local-preference 200
!
route-map TOP permit 20
match ip address prefix-list NET5
set local-preference 200
!
route-map MED_TOP permit 10
match ip address prefix-list NET4
set metric 50
!
route-map MED_TOP permit 20
match ip address prefix-list NET5
set metric 50
!
route-map MED_TOP permit 30
match ip address prefix-list NETLOCAL
!
route-map MED_TOP permit 40
!
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
privilege level 15
logging synchronous
stopbits 1
line aux 0
exec-timeout 0 0
privilege level 15
logging synchronous
stopbits 1
line vty 0 4
login
!
!
end

----------------

# R6

R6#show run
Building configuration...

Current configuration : 5050 bytes
!
! Last configuration change at 01:45:01 UTC Thu Feb 6 2025
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
!
hostname R6
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
no ip icmp rate-limit unreachable
!
!
!
!
!

R6#show run
Building configuration...

Current configuration : 5050 bytes
!
! Last configuration change at 01:45:01 UTC Thu Feb 6 2025
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
!
hostname R6
!
boot-start-marker
boot-end-marker
!
!
!
no aaa new-model
no ip icmp rate-limit unreachable
!
!
!
!
!
!
no ip domain lookup
ip cef
no ipv6 cef
!
!
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
!
ip tcp synwait-time 5
!
track 1 ip sla 1 reachability
delay down 10 up 10
!
track 2 ip sla 2 reachability
delay down 10 up 10
!
!
!
!
!
!
!
!
!
interface FastEthernet0/0
no ip address
duplex full
!
interface FastEthernet0/0.4
encapsulation dot1Q 4
ip address 10.4.0.1 255.255.255.0
ip access-group VLAN45 in
!
interface FastEthernet0/0.5
encapsulation dot1Q 5
ip address 10.5.0.1 255.255.255.0
ip access-group VLAN45 in
!
interface FastEthernet0/0.6
encapsulation dot1Q 6
ip address 10.6.0.1 255.255.255.0
ip access-group VLAN67 in
!
interface FastEthernet0/0.7
encapsulation dot1Q 7
ip address 10.7.0.1 255.255.255.0
ip access-group VLAN67 in
!
interface GigabitEthernet1/0
ip address 192.168.0.22 255.255.255.252
negotiation auto
!
interface GigabitEthernet2/0
ip address 192.168.0.26 255.255.255.252
ip access-group TRACK_10_4_TRAFFIC out
negotiation auto
!
interface GigabitEthernet3/0
no ip address
shutdown
negotiation auto
!
interface GigabitEthernet4/0
no ip address
shutdown
negotiation auto
!
interface GigabitEthernet5/0
no ip address
shutdown
negotiation auto
!
interface GigabitEthernet6/0
no ip address
shutdown
negotiation auto
!
router bgp 65535
bgp log-neighbor-changes
network 10.4.0.0 mask 255.255.255.0
network 10.5.0.0 mask 255.255.255.0
network 10.6.0.0 mask 255.255.255.0
network 10.7.0.0 mask 255.255.255.0
network 192.168.0.20 mask 255.255.255.252
network 192.168.0.24 mask 255.255.255.252
neighbor 192.168.0.21 remote-as 65534
neighbor 192.168.0.25 remote-as 65534
!
ip forward-protocol nd
!
!
no ip http server
no ip http secure-server
!
ip access-list standard VLAN_4_5
permit 10.4.0.0 0.0.0.255
permit 10.5.0.0 0.0.0.255
ip access-list standard VLAN_6_7
permit 10.6.0.0 0.0.0.255
permit 10.7.0.0 0.0.0.255
!
ip access-list extended VLAN45
permit ip 10.4.0.0 0.0.0.255 any
permit ip 10.5.0.0 0.0.0.255 any
ip access-list extended VLAN67
permit ip 10.6.0.0 0.0.0.255 any
permit ip 10.7.0.0 0.0.0.255 any
!
!
ip prefix-list VLAN10-4 seq 5 permit 10.4.0.0/24
!
ip prefix-list VLAN10-5 seq 5 permit 10.5.0.0/24
!
ip prefix-list VLAN10-6 seq 5 permit 10.6.0.0/24
!
ip prefix-list VLAN10-7 seq 5 permit 10.7.0.0/24
ip sla 1
icmp-echo 192.168.0.1 source-interface GigabitEthernet1/0
frequency 10
ip sla schedule 1 life forever start-time now
ip sla 2
icmp-echo 192.168.0.5 source-interface GigabitEthernet2/0
frequency 10
ip sla schedule 2 life forever start-time now
!
route-map PREFERDOWN permit 10
match ip address prefix-list VLAN10-6 VLAN10-7
set local-preference 200
!
route-map PORBAIXO deny 10
match ip address prefix-list VLAN10-4 VLAN10-5
set local-preference 200
!
route-map PREFERUP permit 10
match ip address prefix-list VLAN10-4 VLAN10-5
set local-preference 200
!
route-map PREFER_R5 permit 10
match ip address VLAN_6_7
set local-preference 200
!
route-map PREFER_R4 permit 10
match ip address VLAN_4_5
set local-preference 200
!
route-map PORCIMA deny 10
match ip address prefix-list VLAN10-6 VLAN10-7
set local-preference 200
!
route-map VIA_R5 permit 10
match ip address VLAN67
set local-preference 200
!
route-map VIA_R5 permit 20
!
route-map VIA_R4 permit 10
match ip address VLAN45
set local-preference 200
!
route-map VIA_R4 permit 20
!
route-map NO_PREFER_R5 permit 10
match ip address VLAN_6_7
set local-preference 100
!
route-map NO_PREFER_R4 permit 10
match ip address VLAN_4_5
set local-preference 100
!
!
!
control-plane
!
!
line con 0
exec-timeout 0 0
privilege level 15
logging synchronous
stopbits 1
line aux 0
exec-timeout 0 0
privilege level 15
logging synchronous
stopbits 1
line vty 0 4
login
!
event manager applet DOWN_SHUT
event track 2 state down
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "router bgp 65535"
action 4.0 cli command "no neighbor 192.168.0.25 remote-as 65534"
action 5.0 cli command "end"
event manager applet UP_SHUT
event track 1 state down
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "router bgp 65535"
action 4.0 cli command "no neighbor 192.168.0.21 remote-as 65534"
action 5.0 cli command "end"
event manager applet UP_ON
event track 1 state up
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "router bgp 65535"
action 4.0 cli command "neighbor 192.168.0.21 remote-as 65534"
action 5.0 cli command "end"
event manager applet DOWN_ON
event track 2 state up
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "router bgp 65535"
action 4.0 cli command "neighbor 192.168.0.25 remote-as 65534"
action 5.0 cli command "end"
!
end

[rais]

Simplest option is Policy Based Routing on R6.

HTH

[gpfilgueira]

I tried implementing PBR a few different ways on R6 and it either did have no effect or blocked the BGP neighbors. Could you specify how i can the Policy Based Routing while keeping the BGP configuration, ensuring the vlans prefer a path and they still can go through the other on if the one they prefer is not available?

[rais]

We can tweak BGP to take the top [or bottom] route for all traffic.

On R6, create an SLA to monitor availability of R1 via g2/0 interface. Also a PBR for sub-interfaces vlan4-6 a rule that says source: 10.4/15 to dest: 192.168.10/24 takes g2/0 as its next hop provided SL/tracking is good.

On R1, BGP policy would prefer vlan4-5 to exit via g2/0 or R3.

HTH.

[paul driver]

Hello @gpfilgueira 
you can utilise bgp condictional route advertisement
At present i don’t proper access to CSC as im using my phone but please refer to here is should show how to achieve what you wish to accomplish