bgp decicion algorithm
as_path attribute
當有更新資訊通過as 時，bgp將在前面加上自己的as號。
origin attribute 路由的origin屬性有以下三種：
igp---路由起源於同一as域內，用show ip bgp時由i代表
egp---路由通過exterior gateway protocol學得，用show ip bgp時由e代表。
incomplete---路由起源未知或通過其他方式學得，用？表示
1. external bgp peering 例:
router a in as100
inte***ce ethernet 5/0
ip address 222.222.10.2 255.255.255.240
router bgp 100
network 220.220.8.0 mask 255.255.255.0
neighbor 222.222.10.1 remote-as 101
neighbor 222.222.10.1 distribute-list 180 in
neighbor 222.222.10.1 distribute-list 181 out
router b in as101
inte***ce ethernet 1/0/0
ip address 222.222.10.1 255.255.255.240
router bgp 101
network 220.220.16.0 mask 255.255.240.0
neighbor 222.222.10.2 remote-as 100
neighbor 222.222.10.2 distribute-list 150 in
neighbor 222.222.10.2 distribute-list 151 out
2. stable ibgp peering(peering to loopback address)
.peer with loopback address
.ibgp session is not dependent on a single inte***ce
.loopback inte***ce does not go down ever 例:
router a:
inte***ce loopback 0
ip address 215.10.7.1 255.255.255.255
router bgp 100
network 220.220.1.0
neighbor 215.10.7.2 remote-as 100
neighbor 215.10.7.2 update-source loopback 0
neighbor 215.10.7.3 remote-as 100
neighbor 215.10.7.3 update-source loopback 0
router b:
inte***ce loopback 0
ip address 215.10.7.2 255.255.255.255
router bgp 100
network 220.220.5.0
neighbor 215.10.7.1 remote-as 100
neighbor 215.10.7.1 update-source loopback 0
neighbor 215.10.7.3 remote-as 100
neighbor 215.10.7.3 update-source loopback 0
3. local preference
. default: 100
選用local preference 高的路由
.local to as
. used to influence bgp path selection
.path with highest local preference selection 例
: router bgp 400
neighbor 220.5.1.1 remote-as 300
neighbor 220.5.1.1 route-map local-pref in
route-map local-pref permit 10
match ip address 1
set local preference 800
access-list 1 permit 160.10.0.0 0.0.255.255
access-list 1 deny any
example:
autonomous-system 200
定義as號
router bgp 100
network 200.200.4.0 mask 255.255.252.0
neighbor 200.200.7.224 remote-as 100
neighbor 200.200.7.224 update-source loopback 0
sh ip bgp summary
sh ip bgp
no router bgp 100
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