4-4-1. Cisco dual BGP with Community string (Redundancy)

Cisco dual BGP with Community string (Redundancy)

ISPs : 2 WAN links : 2 CPE : 1 PE : 2 Device : Cisco router with LAN/WAN ports. WAN1 / Serial IP : 10.20.1.0/30 (mask 255.255.255.252) WAN2 / Serial IP : 10.30.1.0/30 (mask 255.255.255.252) LAN / Ethernet IP : 100.100.100.0/24 Receiving Route : Default route only from ISP Memory : Full route(recommend 512M), partial route (recommend 128-256M) LAN : ISP_A (AS20), ISP_B(AS30), Customer (AS10) and Other ISP(AS50)

[Traffic flow]

Outbound traffic on CPE

Both links are receiving default route thru BGP, but on Shadow link CPE put local-pref 80 to make less prefer. So outbound traffic will go out to ISP_"A" link all the time.  

version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname CPE
!
ip cef
no ip domain lookup
!
interface FastEthernet0/0
 no ip address
 duplex auto
 speed auto
!
interface FastEthernet0/1
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface Serial1/0
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/1
 ip address 10.20.1.1 255.255.255.252
 serial restart-delay 0
!
interface Serial1/2
 ip address 10.30.1.1 255.255.255.252
 serial restart-delay 0
!
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
interface FastEthernet2/0
 ip address 100.100.100.1 255.255.255.0
 duplex auto
 speed auto
!
router bgp 10
 no synchronization
 bgp log-neighbor-changes
 network 100.100.100.0 mask 255.255.255.0
 redistribute connected
 neighbor 10.20.1.2 remote-as 20 <—————— BGP neighbor to ISP_"A"
 neighbor 10.20.1.2 send-community <————– Must configure to send community string
 neighbor 10.20.1.2 prefix-list to-ISP out <——— Allow only IP block need to be advertised to IS
 neighbor 10.30.1.2 remote-as 30 <—————— BGP neighbor to ISP_"B"
 neighbor 10.30.1.2 send-community <————– Must configure to send community string
 neighbor 10.30.1.2 prefix-list to-ISP out <——— Allow only IP block need to be advertised to IS
 neighbor 10.30.1.2 route-map shadow-in in <——— To control outbound traffic
 neighbor 10.30.1.2 route-map shadow-out out <—- To control inbound traffic
 no auto-summary
!
ip http server
no ip http secure-server
ip forward-protocol nd
!
ip bgp-community new-format <————– For new BGP community format
!
ip prefix-list to-ISP seq 5 permit 100.100.100.0/24
!
route-map shadow-in permit 10
 set local-preference 80
!
route-map shadow-out permit 10
 set community 30:3
!
control-plane
!
line con 0
 exec-timeout 0 0
 logging synchronous
line aux 0
line vty 0 4
 login
!
end

version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ISP_B
!
ip cef
no ip domain lookup
!
interface FastEthernet0/0
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface FastEthernet0/1
 no ip address
 shutdown
 duplex auto
 speed auto
!
interface Serial1/0
 ip address 10.30.1.2 255.255.255.252
 serial restart-delay 0
!
interface Serial1/1
 ip address 30.30.1.1 255.255.255.252
 serial restart-delay 0
!
interface Serial1/2
 no ip address
 shutdown
 serial restart-delay 0
!
interface Serial1/3
 no ip address
 shutdown
 serial restart-delay 0
!
router bgp 30
 no synchronization
 bgp log-neighbor-changes
 redistribute connected
 neighbor 10.30.1.1 remote-as 10 <—————— BGP neighbor to Customer
 neighbor 10.30.1.1 default-originate <————– Sending default route thru BGP
 neighbor 10.30.1.1 route-map customer-in in <——- Applying BGP policy for incoming traffic
 neighbor 10.30.1.1 route-map no-routes out <——- No BGP routes will be sent to Customer
 neighbor 30.30.1.2 remote-as 30
 no auto-summary
!
ip http server
no ip http secure-server
ip forward-protocol nd
!
ip bgp-community new-format <————– For new BGP community format
ip community-list 1 permit 30:1
ip community-list 2 permit 30:2
ip community-list 3 permit 30:3
ip community-list 4 permit 30:80
ip community-list 5 permit 30:120
!
route-map no-routes deny 10
!
route-map customer-in permit 10
 match community 1
 set as-path prepend 30
!
route-map customer-in permit 20
 match community 2
 set as-path prepend 30 30
!
route-map customer-in permit 30
 match community 3
 set as-path prepend 30 30 30
!
route-map customer-in permit 40
 match community 4
 set local-preference 80
!
route-map customer-in permit 50
 match community 5
 set local-preference 120 <—————- Tagging local-pref 120 to route
!
route-map customer-in permit 60
!
control-plane
!
line con 0
 exec-timeout 0 0
 logging synchronous
line aux 0
line vty 0 4
 login
!
end

[Verifing output]

CPE#sh ip bgp 0.0.0.0
BGP routing table entry for 0.0.0.0/0, version 2
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  20
    10.20.1.2 from 10.20.1.2 (20.20.1.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
  30
    10.30.1.2 from 10.30.1.2 (30.30.1.1)
      Origin IGP, metric 0, localpref 80, valid, external
CPE#

 

ISP_A#sh ip bgp nei 10.20.1.1 ro
BGP table version is 22, local router ID is 20.20.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
              r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 100.100.100.0/24 10.20.1.1                0             0 10 i

Total number of prefixes 1

ISP_A#sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 22
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1
  10
    10.20.1.1 from 10.20.1.1 (100.100.100.1)
      Origin IGP, metric 0, localpref 100, valid, external, best
ISP_A#

ISP_B#sh ip bgp nei 10.30.1.1 ro
BGP table version is 22, local router ID is 30.30.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
              r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*  100.100.100.0/24 10.30.1.1                0             0 30 30 30 10 i

Total number of prefixes 1

ISP_B#sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 22
Paths: (2 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  20 10
    20.30.1.1 from 30.30.1.2 (30.50.1.1)
      Origin IGP, metric 0, localpref 100, valid, internal, best
  30 30 30 10
    10.30.1.1 from 10.30.1.1 (100.100.100.1)
      Origin IGP, metric 0, localpref 100, valid, external
      Community: 30:3
ISP_B#

TR_A#sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 21
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1
  10
    10.20.1.1 from 20.20.1.1 (20.20.1.1)
      Origin IGP, metric 0, localpref 100, valid, internal, best
TR_A#

TR_A#tr 100.100.100.2
Type escape sequence to abort.
Tracing the route to 100.100.100.2

  1 20.20.1.1 48 msec 56 msec 32 msec
  2 10.20.1.1 16 msec 40 msec 36 msec
  3 100.100.100.2 [AS 10] 20 msec 64 msec *
TR_A#

TR_B#sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 20
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1          2
  50 20 10
    30.50.1.2 from 30.50.1.2 (30.50.1.2)
      Origin IGP, localpref 100, valid, external
  20 10
    20.30.1.1 from 20.30.1.1 (20.50.1.1)
      Origin IGP, localpref 100, valid, external, best
TR_B#
 

 

TR_B#tr 100.100.100.2
Type escape sequence to abort.
Tracing the route to 100.100.100.2

  1 20.30.1.1 24 msec 64 msec 16 msec
  2 20.20.1.1 [AS 20] 20 msec 36 msec 40 msec
  3 10.20.1.1 [AS 20] 20 msec 52 msec 72 msec
  4 100.100.100.2 [AS 10] 24 msec 64 msec *
TR_B#

Internet#sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 21
Paths: (2 available, best #2, table Default-IP-Routing-Table)
  Advertised to update-groups:
     1
  30 20 10
    30.50.1.1 from 30.50.1.1 (30.50.1.1)
      Origin IGP, localpref 100, valid, external
  20 10
    20.50.1.1 from 20.50.1.1 (20.50.1.1)
      Origin IGP, localpref 100, valid, external, best
Internet#

Internet#tr 100.100.100.2
Type escape sequence to abort.
Tracing the route to 100.100.100.2

  1 20.50.1.1 52 msec 40 msec 52 msec
  2 20.20.1.1 [AS 20] 20 msec 40 msec 40 msec
  3 10.20.1.1 [AS 20] 24 msec 44 msec 48 msec
  4 100.100.100.2 [AS 10] 48 msec 112 msec *
Internet#

[Dynamips testing]

Download and test it yourself with below Dynamips configuration files.

  Dynamips NET file

  Router configuration files

 

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