
Cisco Single BGP with Origin value
Lowest Origin Type is one of important criteria for BGP best path selection.
– Prefer the path with the lowest origin type: IGP is lower than EGP, and EGP is lower than INCOMPLETE. (IGP < EGP < Incomplete)
– IGP origin is created when there is a network statement
– EGP origin is created via ‘redistribute
– Incomplete origin is created via redistribute
[CPE/Customer Cisco Router]
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
ip address 100.100.100.1 255.255.255.0
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
load-interval 30
serial restart-delay 0
!
interface Serial1/2
no ip address
load-interval 30
shutdown
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
interface FastEthernet2/0
no ip address
shutdown
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
neighbor 10.20.1.2 prefix-list to-ISP out <—— Allow only IP block need to be advertised to ISP
neighbor 10.20.1.2 route-map route-to-ISP out
no auto-summary
!
ip http server
no ip http secure-server
ip forward-protocol nd
!
ip prefix-list to-ISP seq 5 permit 100.100.100.0/24
!
route-map route-to-ISP permit 10 <———— Change origin value to ‘IGP’
set origin igp
!
control-plane
!
line con 0
exec-timeout 0 0
logging synchronous
line aux 0
line vty 0 4
login
!
end
[PE / ISP Cisco Router]
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname PE
!
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
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/1
ip address 10.20.1.2 255.255.255.252
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 20
no synchronization
bgp log-neighbor-changes
redistribute connected
neighbor 10.20.1.1 remote-as 10 <—————— BGP neighbor to ISP
neighbor 10.20.1.1 default-originate <————– Send default route thru BGP
neighbor 10.20.1.1 route-map no-routes out
no auto-summary
!
ip http server
no ip http secure-server
ip forward-protocol nd
!
route-map no-routes deny 10
!
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 nei 10.20.1.2 ad
BGP table version is 4, local router ID is 100.100.100.1
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incompleteNetwork Next Hop Metric LocPrf Weight Path
*> 100.100.100.0/24 0.0.0.0 0 32768 iTotal number of prefixes 1
CPE#sh ip bgp nei 10.20.1.2 ro
BGP table version is 4, local router ID is 100.100.100.1
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incompleteNetwork Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 10.20.1.2 0 0 20 iTotal number of prefixes 1
CPE#PE#sh ip bgp neighbors 10.20.1.1 ro
BGP table version is 3, local router ID is 10.20.1.2
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incompleteNetwork Next Hop Metric LocPrf Weight Path
*> 100.100.100.0/24 10.20.1.1 0 0 10 iTotal number of prefixes 1
PE#sh ip bgp neighbors 10.20.1.1 ad
BGP table version is 3, local router ID is 10.20.1.2
Status codes: s suppressed, d damped, h history, * valid, > best, i – internal,
r RIB-failure, S Stale
Origin codes: i – IGP, e – EGP, ? – incompleteOriginating default network 0.0.0.0
Network Next Hop Metric LocPrf Weight Path
Total number of prefixes 0
PE#sh ip bgp 100.100.100.0
BGP routing table entry for 100.100.100.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
10
10.20.1.1 from 10.20.1.1 (100.100.100.1)
Origin IGP, metric 0, localpref 100, valid, external, best
PE#
[Dynamips testing]
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