BGPのBest-Pathについて、Cisco CMLで演習してみようと思います
① NW構成通りにコンフィグを作成
② AS-Path Prependにより
・設定内容確認
・Pathを追加してbest-pathがどう変わるか？
③ bgp bestpath as-path ignoreでPathが無視されるのか？
BGP Best path Selection (Cisco)
| BGP Best path Selection |
| Weight (Highest) |
| Local_Pref(Highest) |
| Prefer locally originated |
| AS_Path (Shortest) |
| Origin Type (Lowest) |
| MED (Lowest) |
| Prefer eBGP Over iBGP |
| IGP metric to the BGP Next hop (Lowest) |
| Oldest Path |
| Router ID source (Lowest) |
| Minimum Cluster list length |
| Neighbor address (Lowest) |
NW構成 (Cisco Modeling Labs利用)
R2: loopback0: 2.2.2.2
R3: loopback0: 3.3.3.3
R4: loopback0: 4.4.4.4
R5: loopback0: 5.5.5.5
R6: loopback0: 6.6.6.6
eBGP Peer : R4 ⇔ R5、 R4 ⇔ R6、 R5 ⇔ R6
config
R1-config
hostname R1
!
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.12.1.1 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
ip address 10.13.1.1 255.255.255.0
duplex auto
speed auto
media-type rj45
!
router bgp 100
bgp log-neighbor-changes
redistribute connected
neighbor 10.12.1.2 remote-as 200
neighbor 10.13.1.3 remote-as 300
!
control-plane
!
line con 0
line aux 0
line vty 0 4
login
transport input none
!
no scheduler allocate
!
end
R1#
R2-config
hostname R2
!
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.12.1.2 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
ip address 10.24.1.2 255.255.255.0
duplex auto
speed auto
media-type rj45
!
router bgp 200
bgp log-neighbor-changes
redistribute connected
neighbor 10.12.1.1 remote-as 100
neighbor 10.24.1.4 remote-as 400
!
control-plane
!
line con 0
line aux 0
line vty 0 4
login
transport input none
!
no scheduler allocate
!
end
R2#
R3-config
hostname R3
!
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.13.1.3 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
ip address 10.35.1.3 255.255.255.0
duplex auto
speed auto
media-type rj45
!
router bgp 300
bgp log-neighbor-changes
redistribute connected
neighbor 10.13.1.1 remote-as 100
neighbor 10.35.1.5 remote-as 400
!
control-plane
!
line con 0
line aux 0
line vty 0 4
login
transport input none
!
no scheduler allocate
!
end
R3#
R4-config
hostname R4
!
interface Loopback0
ip address 4.4.4.4 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.24.1.4 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
ip address 10.45.1.4 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/2
ip address 10.46.1.4 255.255.255.0
duplex auto
speed auto
media-type rj45
!
router ospf 1
passive-interface GigabitEthernet0/0
network 0.0.0.0 255.255.255.255 area 0
!
router bgp 400
bgp log-neighbor-changes
redistribute connected
neighbor 5.5.5.5 remote-as 400
neighbor 5.5.5.5 update-source Loopback0
neighbor 6.6.6.6 remote-as 400
neighbor 6.6.6.6 update-source Loopback0
neighbor 10.24.1.2 remote-as 200
!
control-plane
line con 0
line aux 0
line vty 0 4
login
transport input none
!
no scheduler allocate
!
end
R4#
R5-config
hostname R5
!
interface Loopback0
ip address 5.5.5.5 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.35.1.5 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
ip address 10.45.1.5 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/2
ip address 10.56.1.5 255.255.255.0
duplex auto
speed auto
media-type rj45
!
router ospf 1
passive-interface GigabitEthernet0/0
network 0.0.0.0 255.255.255.255 area 0
!
router bgp 400
bgp log-neighbor-changes
redistribute connected
neighbor 4.4.4.4 remote-as 400
neighbor 4.4.4.4 update-source Loopback0
neighbor 6.6.6.6 remote-as 400
neighbor 6.6.6.6 update-source Loopback0
neighbor 10.35.1.3 remote-as 300
!
control-plane
line con 0
line aux 0
line vty 0 4
login
transport input none
!
no scheduler allocate
!
end
R5#
R6-config
hostname R6
!
interface Loopback0
ip address 6.6.6.6 255.255.255.255
!
interface GigabitEthernet0/0
ip address 10.46.1.6 255.255.255.0
duplex auto
speed auto
media-type rj45
!
interface GigabitEthernet0/1
ip address 10.56.1.6 255.255.255.0
duplex auto
speed auto
media-type rj45
!
router ospf 1
network 0.0.0.0 255.255.255.255 area 0
!
router bgp 400
bgp log-neighbor-changes
redistribute connected
neighbor 4.4.4.4 remote-as 400
neighbor 4.4.4.4 update-source Loopback0
neighbor 5.5.5.5 remote-as 400
neighbor 5.5.5.5 update-source Loopback0
!
control-plane
!
line con 0
line aux 0
line vty 0 4
login
transport input none
!
no scheduler allocate
!
end
R6#
確認コマンド
R1から見た show ip bgp 6.6.6.6
R1#show ip bgp 6.6.6.6
BGP routing table entry for 6.6.6.6/32, version 7
Paths: (2 available, best #2, table default)
Advertised to update-groups:
2
Refresh Epoch 2
300 400
10.13.1.3 from 10.13.1.3 (3.3.3.3)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Refresh Epoch 2
200 400
10.12.1.2 from 10.12.1.2 (2.2.2.2)
Origin incomplete, localpref 100, valid, external, best
//bestになる理由：Router ID source (Lowest)
rx pathid: 0, tx pathid: 0x0
R1から見た show ip bgp
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.6/32 10.13.1.3 0 300 400 ?
*> 10.12.1.2 0 200 400 ?
// 「>」が付いている方が best-path
R1から見た show ip route 6.6.6.6
R1#show ip route 6.6.6.6
Routing entry for 6.6.6.6/32
Known via “bgp 100”, distance 20, metric 0
Tag 200, type external
Last update from 10.12.1.2 03:19:49 ago
Routing Descriptor Blocks:
* 10.12.1.2, from 10.12.1.2, 03:19:49 ago
Route metric is 0, traffic share count is 1
AS Hops 2
Route tag 200
MPLS label: none
set as-path prepend でPathを追加してみる
set as-path prependのための設定内容
// 6.6.6.6の経路に対して、PREPENDします。
!
route-map BGP-PREPEND permit 10
match ip address prefix-list AS-PRE
set as-path prepend 1000 2000
// 1000 と 2000をPathを追加します。これは実際にPathを通ったわけではなく、設定上追加されただけです。また、as-path を追加することはできますが、as-path を削除することはできません。
!
route-map BGP-PREPEND permit 20router bgp 200
neighbor 10.12.1.1 route-map BGP-PREPEND out
R1から見た show ip bgp 6.6.6.6
R1#show ip bgp 6.6.6.6
BGP routing table entry for 6.6.6.6/32, version 6
Paths: (2 available, best #2, table default)
Advertised to update-groups:
3
Refresh Epoch 1
200 1000 2000 400 //1000 2000が追加されて結果
10.12.1.2 from 10.12.1.2 (2.2.2.2)
Origin incomplete, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Refresh Epoch 2
300 400
10.13.1.3 from 10.13.1.3 (3.3.3.3)
Origin incomplete, localpref 100, valid, external, best
//1000 2000が追加されて結果、Best Pathは 10.13.1.3へ移った
rx pathid: 0, tx pathid: 0x0
R1から見た show ip bgp
R1#show ip bgp
Network Next Hop Metric LocPrf Weight Path
* 6.6.6.6/32 10.12.1.2 0 200 1000 2000 400 ?
*> 10.13.1.3 0 300 400 ?
// 「>」が付いている方が best-pathで、1000 2000が追加されて結果、Best Pathは 10.13.1.3へ移った
R1から見た show ip route 6.6.6.6
R1#show ip route 6.6.6.6
Routing entry for 6.6.6.6/32
Known via “bgp 100”, distance 20, metric 0
Tag 300, type external
Last update from 10.13.1.3 00:14:30 ago
Routing Descriptor Blocks:
* 10.13.1.3, from 10.13.1.3, 00:14:30 ago
Route metric is 0, traffic share count is 1
AS Hops 2
Route tag 300
MPLS label: none
set as-path prepend でPathを追加してみる
bgp bestpath as-path ignore //IOS-XE 15.9 ではas-path ignoreコマンドは無くなっていました。
