BGP 路径属性
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1. 概述
2. Weight
- 2.1. 选路实验(#1)
3. Local-Preference
- 3.1. 选路实验(#2)
4. Next Hop
- 4.1. 选路实验(#3, 优选本地产生的路由)
5. AS-Path
- 5.1. 选路实验(#4, 优选 as-path 较少的路径)
6. Origin
- 6.1. 观察不同 Origin
  - 6.1.1. 设备配置
  - 6.1.2. 结果分析
- 6.2. 选路实验 (#5, i>e>?)
  - 6.2.1. 实验配置
  - 6.2.2. 结果分析
7. MED
- 7.1. 实验之 MED 传递
  - 7.1.1. 设备配置
  - 7.1.2. 结果分析
8. Community
- 8.1. set community 参数
- 8.2. community-list

1 概述

Well-Known Mandatory	Well-Known Discretionay	Optional Transitive	Optional Non-Transitive
Origin	Local-Preference	Community	MED
AS-Path	Atomic Aggregate	Aggreator	Originator ID
Next Hop			Cluster List
			Weight

Well-Known Mandatory

BGP 必须都能识别 ，且在更新消息 必须包含
Well-Known Discretionay

BGP 必须都能识别 ，更新消息 可不包含
Optional Transitive

可以不支持 该属性，但即使不支持也应当接受包含该属性的路由并 传递给其他邻居
Optional Non-Transitive

可以不支持 该属性，BGP 可以忽略包含这个属性的更新消息并且 不传递 给其他邻居

2 Weight

传播范围	取值范围	默认值	备注
本地	0-65535	32768	值越大越优先

路由器本地始发的路径(network 通告/重发布直连/重发布静态/本地汇总)默认 weight 值为 32768
从其他 BGP 邻居学习到的路由，其 weight 值为 0

2.1 选路实验(#1)¹

Figure 1: 实验环境(在 R2 上部署策略)

初始配置

----------------- R1 ----------------------
en
conf t
 hostname R1
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.12.1 255.255.255.0
  no sh
 exit
 router bgp 130
  bgp router-id 1.1.1.1
  neighbor 10.1.12.2 remote-as 200
  network 8.8.8.8 mask 255.255.255.255
 exit
end

----------------- R2 ----------------------
en
conf t
 hostname R2
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int e0/0
  ip address 10.1.12.2 255.255.255.0
  no sh
 exit
 int e0/1
  ip address 10.1.23.2 255.255.255.0
  no sh
 exit
 router bgp 200
  bgp router-id 2.2.2.2
  neighbor 10.1.12.1 remote-as 130
  neighbor 10.1.23.3 remote-as 130
 exit
end

----------------- R3 ----------------------
en
conf t
 hostname R3
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.23.3 255.255.255.0
  no sh
 exit
 router bgp 130
  bgp router-id 3.3.3.3
  neighbor 10.1.23.2 remote-as 200
  network 8.8.8.8 mask 255.255.255.255
 exit
end

初始 R2 选路信息

R2#sh ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *    8.8.8.8/32       10.1.23.3                0             0 130 i
 *>                    10.1.12.1                0             0 130 i

R2 上执行策略调整 weight

因为 weight 只是本地有效，因此只能在 R2 上部署策略。

ip prefix-list P seq 10 permit 8.8.8.8/32
!
route-map R permit 10
 match ip address prefix-list p
 set weight 333
 exit
route-map R permit 20 ! permit any other
!
router bgp 200
 neighbor 10.1.23.3 route-map R in
exit

再次查看选路结果

R2#sh ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *    8.8.8.8/32       10.1.12.1                0             0 130 i
 *>                    10.1.23.3                0           333 130 i

3 Local-Preference

传播范围	默认值	备注
AS 内的 iBGP 路由器之间	100	值越大越优先

当一个 AS 收到一个去往同一目的地的，但经过两个 AS 的路由，则根据两条路由的 LP 值来决定选择哪条作为最优路径(优选 LP 值大的路由)，即 LP 影响离开 AS 的业务量。

策略一般做在 AS 边界，告诉 AS 内部从本路由走的 LP 值。

只能在 IBGP Peer 之间传递（除非做了策略否则 LP 值在 AS 内的 IBGP 邻居间传递不会丢失）
不能在 EBGP Peer 之间传递，如果在 EBGP Peer 之间收到的路由的路径属性中携带了 LP ，则会触发 Notifacation 报文，造成会话中断(但是可以再 AS 边界路由器上使用 IN 方向的策略)
BGP 路由器在向其 EBGP 邻居发送路由更新时，不能携带 LP 属性，但是邻居会在本地为这条路由设置一个默认值，也就是 100 ，然后再传递给自己的 IBGP
本地 network 通告及重发布的路由，LP 值默认为 100 ，并能在 AS 内向其他 IBGP 邻居传输，传输过程中除非部署策略，否则值保持不变

3.1 选路实验(#2)

Figure 3: 实验环境

初始配置

----------------- R1 ----------------------
en
conf t
 hostname R1
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int e0/0
  ip address 10.1.12.1 255.255.255.0
  no sh
 exit
 int e0/1
  ip address 10.1.14.1 255.255.255.0
  no sh
 exit
 router bgp 123
  bgp router-id 1.1.1.1
  neighbor 10.1.12.2 remote-as 123
  neighbor 10.1.14.4 remote-as 400
 exit
end

----------------- R2 ----------------------
en
conf t
 hostname R2
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int e0/0
  ip address 10.1.12.2 255.255.255.0
  no sh
 exit
 int e0/1
  ip address 10.1.23.2 255.255.255.0
  no sh
 exit
 ip route 10.1.34.0 255.255.255.0 10.1.23.3
 ip route 10.1.14.0 255.255.255.0 10.1.12.1
 router bgp 123
  bgp router-id 2.2.2.2
  neighbor 10.1.12.1 remote-as 123
  neighbor 10.1.23.3 remote-as 123
 exit
end

----------------- R3 ----------------------
en
conf t
 hostname R3
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int e0/0
  ip address 10.1.23.3 255.255.255.0
  no sh
 exit
 int e0/1
  ip address 10.1.34.3 255.255.255.0
  no sh
 exit
 router bgp 123
  bgp router-id 3.3.3.3
  neighbor 10.1.23.2 remote-as 123
  neighbor 10.1.34.4 remote-as 400
 exit
end

----------------- R4 ----------------------
en
conf t
 hostname R4
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.14.4 255.255.255.0
  no sh
 exit
 int e0/1
  ip address 10.1.34.4 255.255.255.0
  no sh
 exit
 router bgp 400
  bgp router-id 4.4.4.4
  neighbor 10.1.14.1 remote-as 123
  neighbor 10.1.34.3 remote-as 123
  network 8.8.8.8 mask 255.255.255.255
 exit
end

初始 R2 的选路信息

R2#sh ip bgp
BGP table version is 3, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 * i  8.8.8.8/32       10.1.34.4                0    100      0 400 i
 *>i                   10.1.14.4                0    100      0 400 i

在 R3 上做 lp 策略

ip prefix-list P seq 10 permit 8.8.8.8/32
!
route-map R permit 10
 match ip address prefix-list p
 set local-preference 200
 exit
route-map R permit 20 ! permit any other
!
router bgp 123
 neighbor 10.1.23.2 route-map R out
exit

再次查看选路结果

R2#sh ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>i  8.8.8.8/32       10.1.34.4                0    200      0 400 i
 * i                   10.1.14.4                0    100      0 400 i

4 Next Hop

Figure 4: 路由传递自 EBGP

Figure 5: 路由传递自 IBGP

优选本地起源的路由，本地起源的方式有：network/redistribute/aggregate ，且优先级为：

network > redistribute > aggregate > 学习到的路由

4.1 选路实验(#3, 优选本地产生的路由)

Figure 6: 实验环境

配置清单

----------------- R1 ----------------------
en
conf t
 hostname R1
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.12.1 255.255.255.0
  no sh
 exit
 router bgp 100
  bgp router-id 1.1.1.1
  neighbor 10.1.12.2 remote-as 200
  network 8.8.8.8 mask 255.255.255.255
 exit
end

----------------- R2 ----------------------
en
conf t
 hostname R2
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.12.2 255.255.255.0
  no sh
 exit
 router bgp 200
  bgp router-id 2.2.2.2
  neighbor 10.1.12.1 remote-as 100
  network 8.8.8.8 mask 255.255.255.255
 exit
end

观察选路信息

R2#sh ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *    8.8.8.8/32       10.1.12.1                0             0 100 i
 *>                    0.0.0.0                  0         32768 i

5 AS-Path

是前往目标网络的路由经过的自制系统号列表，通告该路由的自治系统号位于列表末尾
用于确保无环，通告给 EBGP 时会加上自己的 AS 号，通告给 IBGP 时不修改 AS-Path

5.1 选路实验(#4, 优选 as-path 较少的路径)

Figure 7: 实验环境

初始配置清单

----------------- R1 ----------------------
en
conf t
 hostname R1
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.12.1 255.255.255.0
  no sh
 exit
 router bgp 100
  bgp router-id 1.1.1.1
  neighbor 10.1.12.2 remote-as 200
  network 8.8.8.8 mask 255.255.255.255
 exit
end

----------------- R2 ----------------------
en
conf t
 hostname R2
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int e0/0
  ip address 10.1.12.2 255.255.255.0
  no sh
 exit
 int e0/1
  ip address 10.1.23.2 255.255.255.0
  no sh
 exit
 router bgp 200
  bgp router-id 2.2.2.2
  neighbor 10.1.12.1 remote-as 100
  neighbor 10.1.23.3 remote-as 300
 exit
end

----------------- R3 ----------------------
en
conf t
 hostname R3
 no ip do lo
 line con 0
  exec-timeout 0 0
  logging synchronous
 exit
 int lo0
  ip address 8.8.8.8 255.255.255.255
 exit
 int e0/0
  ip address 10.1.23.3 255.255.255.0
  no sh
 exit
 router bgp 300
  bgp router-id 3.3.3.3
  neighbor 10.1.23.2 remote-as 200
  network 8.8.8.8 mask 255.255.255.255
 exit
end

最初选路

R2#sh ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *    8.8.8.8/32       10.1.23.3                0             0 300 i
 *>                    10.1.12.1                0             0 100 i

R1 上调整 AS-PATH

ip prefix-list P seq 10 permit 8.8.8.8/32
!
route-map R permit 10
 match ip address prefix-list p
 set as-path prepend 100 100
 exit
route-map R permit 20 ! permit any other
!
router bgp 100
 neighbor 10.1.12.2 route-map R out
exit

再次查看选路结果

R2#sh ip bgp
BGP table version is 3, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   8.8.8.8/32       10.1.23.3                0             0 300 i
 *                     10.1.12.1                0             0 100 100 100 i

6 Origin

标识路由的起源：

i

表示 IGP ，此处并非指的是 OSPF/EIGRP 之类的，而是表示由 BGP(包括eBGP和iBGP) 传递过来的路由 (由 network 通告进入 BGP)
e

由 EGP 这种早期的协议重发布而来
?

标识 Incomplete ，是从其他渠道学习到的，路由来源的信息不完整，通常是 重发布 的路由

6.1 观察不同 Origin

6.1.1 设备配置

router bgp 100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 neighbor 10.1.12.2 remote-as 200

ip prefix-list pfx22 seq 5 permit 22.22.22.22/32
 match ip address prefix-list pfx22
route-map rtmap22 permit 10
 match ip address prefix-list pfx22
router bgp 200
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 network 2.2.2.2 mask 255.255.255.255
 redistribute connected metric 20 route-map rtmap22
 neighbor 10.1.12.1 remote-as 100

6.1.2 结果分析

R1#show ip bgp
BGP table version is 3, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   2.2.2.2/32       10.1.12.2                0             0 200 i
 *>   22.22.22.22/32   10.1.12.2               20             0 200 ?

R2#show ip bgp
BGP table version is 3, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   2.2.2.2/32       0.0.0.0                  0         32768 i
 *>   22.22.22.22/32   0.0.0.0                 20         32768 ?

6.2 选路实验 (#5, i>e>?)

Figure 9: \(i > e > ?\)

6.2.1 实验配置

ip prefix-list PRX_SUBNET_192 seq 5 permit 192.168.12.0/24
route-map RTMAP_NETWORK_SUBNET_192 permit 10
 match ip address prefix-list PRX_SUBNET_192
 set metric 2
router bgp 120
 bgp router-id 1.1.1.1
 network 192.168.12.0 route-map RTMAP_NETWORK_SUBNET_192 # network 方式通告，Origin 为 i
 neighbor 10.1.13.3 remote-as 300

ip prefix-list subnet192 seq 5 permit 192.168.12.0/24
route-map BGP_REDIS_CONNECTED permit 10
 match ip address prefix-list subnet192
router bgp 120
 bgp router-id 2.2.2.2
 redistribute connected metric 2 route-map BGP_REDIS_CONNECTED # 重发布直连，Origin 为 ?
 neighbor 10.1.23.3 remote-as 300

router bgp 300
 bgp router-id 3.3.3.3
 neighbor 10.1.13.1 remote-as 120
 neighbor 10.1.23.2 remote-as 120

6.2.2 结果分析

R3#show ip bgp summary
BGP router identifier 3.3.3.3, local AS number 300
BGP table version is 5, main routing table version 5
1 network entries using 144 bytes of memory
2 path entries using 168 bytes of memory
2/1 BGP path/bestpath attribute entries using 320 bytes of memory
1 BGP AS-PATH entries using 24 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 656 total bytes of memory
BGP activity 1/0 prefixes, 3/1 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.1.13.1       4          120      16      19        5    0    0 00:10:04        1
10.1.23.2       4          120      24      26        5    0    0 00:17:52        1
# 分别从 R1 和 R2 收到 1 条路由信息
R3#show ip bgp
BGP table version is 5, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   192.168.12.0     10.1.13.1                2             0 120 i # R1 传递过来的路由优先级更高
 *                     10.1.23.2                2             0 120 ?

7 MED

传播范围	默认值	备注
AS 间的 eBGP ，仅能传递一个 AS	0	值越小越优先

度量值越小路径越优
用于向外部邻居告知进入本 AS 的首选路径，即当入口有多个时，自治系统可以使用 MED 动态的影响其他 AS 如何选择去往本 AS 的路径
MED 是在 AS 之间交换，发送给 EBGP 对等体，并可以在 AS 内传播，但 不传递给下一个 AS
仅当路径 来自同一个 AS 中的不同 EBGP 邻居 时，路由器才会比较他们的 MED 属性
可以这样理解：MED 是为了 向外部告知 进入本 AS 的路由优先级，而 LP 是 在内部传播 去往外部 AS 的路由优先级

7.1 实验之 MED 传递

7.1.1 设备配置

router bgp 100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 redistribute connected route-map RTMAP_BGP_REDIS
 neighbor 10.1.12.2 remote-as 230
!
ip prefix-list PFX_LOOPBACK0 seq 5 permit 1.1.1.1/32
!
route-map RTMAP_BGP_REDIS permit 10
 match ip address prefix-list PFX_LOOPBACK0
 set metric 33

router bgp 230
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 neighbor 10.1.12.1 remote-as 100
 neighbor 10.1.23.3 remote-as 230
 neighbor 10.1.23.3 next-hop-self

router bgp 230
 bgp router-id 3.3.3.3
 bgp log-neighbor-changes
 neighbor 10.1.23.2 remote-as 230
 neighbor 10.1.34.4 remote-as 400

router bgp 400
 bgp router-id 4.4.4.4
 bgp log-neighbor-changes
 neighbor 10.1.34.3 remote-as 230

7.1.2 结果分析

R1#show ip bgp
BGP table version is 5, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
*>   1.1.1.1/32       0.0.0.0                 33         32768 ?

R2#show ip bgp
BGP table version is 2, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
*>   1.1.1.1/32       10.1.12.1               33             0 100 ?

R3#show ip bgp
BGP table version is 2, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
*>i  1.1.1.1/32       10.1.23.2               33    100      0 100 ?
# IBGP 间可以传递

R4#show ip bgp
BGP table version is 2, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   1.1.1.1/32       10.1.34.3                              0 230 100 ? # EBGP 间不传递

8 Community

传播范围	备注
只对邻居有效	须先执行 neighbor x.x.x.x send-community

该属性的作用可以理解为 打TAG ，对路由进行标记，以简化策略的定义和执行。即可以将某些路由分配一个特定的 COMMUNITY 属性，之后可以基于 COMMUNITY 值而不是每条路由进行 BGP 属性的设置。

8.1 `set community` 参数

Figure 13: 实验拓扑

8.1.1 无参

8.1.1.1 设备配置

router bgp 100
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 redistribute connected metric 3
 neighbor 10.1.12.2 remote-as 230
 neighbor 10.1.12.2 send-community # 必须手动指定发送
 neighbor 10.1.12.2 route-map RTMAP_COMMUNITY_1111 out
!
ip bgp-community new-format
!
ip prefix-list PFX_LOOPBACK0 seq 5 permit 1.1.1.1/32
!
route-map RTMAP_COMMUNITY_1111 permit 10
 match ip address prefix-list PFX_LOOPBACK0
 set community 100:1111
!
route-map RTMAP_COMMUNITY_1111 permit 20

router bgp 230
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 neighbor 10.1.12.1 remote-as 100
 neighbor 10.1.23.3 remote-as 230
 neighbor 10.1.23.3 send-community
 neighbor 10.1.24.4 remote-as 400
 neighbor 10.1.24.4 send-community

8.1.1.2 实验现象

IBGP 和 EBGP 都能收到 COMMUNITY ，以 R4 为例：

R4#show ip bgp
BGP table version is 6, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   1.1.1.1/32       10.1.24.2                              0 230 100 ?
 *>   10.1.12.0/24     10.1.24.2                              0 230 100 ?
R4#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 6
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 4
  230 100
    10.1.24.2 from 10.1.24.2 (2.2.2.2)
      Origin incomplete, localpref 100, valid, external, best
      Community: 100:1111
      rx pathid: 0, tx pathid: 0x0
R4#show ip bgp 10.1.12.0
BGP routing table entry for 10.1.12.0/24, version 5
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 4
  # 这条路由信息并没有打上标签
  230 100
    10.1.24.2 from 10.1.24.2 (2.2.2.2)
      Origin incomplete, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

8.1.2 no-adv

收到该 COMMUNITY 的路由器不会将 该路由 通告给任何 BGP peer 。

8.1.2.1 设备配置

在 R1 上修改 route-map 的设置：

route-map RTMAP_COMMUNITY_1111 permit 10
 match ip address prefix-list PFX_LOOPBACK0
 set community 100:1111 no-advertise
route-map RTMAP_COMMUNITY_1111 permit 20

8.1.2.2 实验现象

在 R2 上观察到：

R2#show ip bgp
BGP table version is 8, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
 *>   1.1.1.1/32       10.1.12.1                3             0 100 ? # 该路由信息传递过来了
 r>   10.1.12.0/24     10.1.12.1                3             0 100 ?
R2#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 8
Paths: (1 available, best #1, table default, not advertised to any peer)
  Not advertised to any peer
  Refresh Epoch 5
  100
    10.1.12.1 from 10.1.12.1 (1.1.1.1)
      Origin incomplete, metric 3, localpref 100, valid, external, best
      Community: 100:1111 no-advertise
      rx pathid: 0, tx pathid: 0x0

R3 和 R4 上都显示：

R3#show ip bgp
BGP table version is 1, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
              x best-external, a additional-path, c RIB-compressed,
              t secondary path,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
* i  10.1.12.0/24     10.1.12.1                3    100      0 100 ? # 1.1.1.1 的路由没有传递过来

8.1.3 no-export

收到该 COMMUNITY 的路由器不会将 该路由 通告给任何 EBGP peer (联邦 EBGP 仍会传递)。

8.1.3.1 设备配置

在 R1 上修改 route-map 的设置：

route-map RTMAP_COMMUNITY_1111 permit 10
 match ip address prefix-list PFX_LOOPBACK0
 set community 100:1111 no-export
route-map RTMAP_COMMUNITY_1111 permit 20

8.1.3.2 实验现象

R3 上仍然可以收到这条路由，但 R4 收不到了：

R3#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 0
Paths: (1 available, no best path)
  Not advertised to any peer
  Refresh Epoch 5
  100
    10.1.12.1 (inaccessible) from 10.1.23.2 (2.2.2.2)
      Origin incomplete, metric 3, localpref 100, valid, internal
      Community: 100:1111 no-export
      rx pathid: 0, tx pathid: 0

8.1.4 local-as

只能在本 AS 内部传递，与 no-export 不同的是，不会传递给联邦 EBGP peer 。

在没有联邦的 AS 内部， local-as 和 no-export 作用是一样的，不会传递出大 AS
在有联邦的 AS 内， local-as 只能在联邦 iBGP 间传递， no-export 能在联邦的 iBGP/eBGP 之间传递，但都不会传递出大 AS

8.2 community-list

ip community-list statndard MY_C_LIST permit 1000   ! 匹配 Community 属性为 1000
!
router-map R4-R5-OUT deny 10
 match community MY_C_LIST
router-map R4-R5-OUT permit 20
!
router bgp 65535
 neighbor 192.168.1.1 route-map R4-R5-OUT out
!

Footnotes:

BGP十三条选路原则

BGP 路径属性 {Back to Index} Expand All Details on Page

Table of Contents

1 概述

2 Weight

2.1 选路实验(#1)1

3 Local-Preference

3.1 选路实验(#2)

4 Next Hop

4.1 选路实验(#3, 优选本地产生的路由)

5 AS-Path

5.1 选路实验(#4, 优选 as-path 较少的路径)

6 Origin

6.1 观察不同 Origin

6.1.1 设备配置

6.1.2 结果分析

6.2 选路实验 (#5, i>e>?)

6.2.1 实验配置

6.2.2 结果分析

7 MED

7.1 实验之 MED 传递

7.1.1 设备配置

7.1.2 结果分析

8 Community

8.1 set community 参数

8.1.1 无参

8.1.1.1 设备配置

8.1.1.2 实验现象

8.1.2 no-adv

8.1.2.1 设备配置

8.1.2.2 实验现象

8.1.3 no-export

8.1.3.1 设备配置

8.1.3.2 实验现象

8.1.4 local-as

8.2 community-list

Footnotes:

BGP 路径属性
{Back to Index}

2.1 选路实验(#1)¹

8.1 `set community` 参数