【实验要求】
在R3上配置BGP weight 值，确保R3上由155.1.37.0/24 网络到155.1.5.0 网络的流量走R1。
【实验拓扑】
【配置步骤】
- 按照上图配置网络
- 在 R1, R3, R4, and R5 上配置bgp邻居
- R1 应该 peer with R3 and R4
- R5 应该 peer with R3 and R5
- 通告 R3’s 155.1.37.0/24 网络到 BGP
- 通告 R5’s 155.1.5.0？24 网络到 BGP
- 在R3上配置BGP weight 值，确保R3上由155.1.37.0/24 网络到155.1.5.0 网络的流量走R1。
【具体配置】
R1:
interface FastEthernet0/0
ip address 155.1.146.1 255.255.255.0
!
interface Serial0/0
ip address 155.1.13.1 255.255.255.0
!
router bgp 2
neighbor 155.1.13.3 remote-as 2
neighbor 155.1.13.3 next-hop-self
neighbor 155.1.146.4 remote-as 1
R3:
interface Serial0/1
ip address 155.1.13.3 255.255.255.0
clockrate 64000
!
interface Serial0/0
ip address 155.1.0.3 255.255.255.0
encapsulation frame-relay
clock rate 64000
frame-relay map ip 155.1.0.5 305 broadcast
no frame-relay inverse-arp
!
interface Loopback0
ip add 155.1.37.3 255.255.255.0
!
router bgp 2
network 155.1.37.0 mask 255.255.255.0
neighbor 155.1.0.5 remote-as 1
neighbor 155.1.13.1 remote-as 2
neighbor 155.1.13.1 next-hop-self
R4:
interface FastEthernet0/0
ip address 155.1.146.4 255.255.255.0
!
interface Serial0/0
ip address 155.1.45.4 255.255.255.0
!
router bgp 1
neighbor 155.1.45.5 remote-as 1
neighbor 155.1.45.5 next-hop-self
neighbor 155.1.146.1 remote-as 2
R5:
interface Serial0/0
ip address 155.1.0.5 255.255.255.0
encapsulation frame-relay
frame-relay map ip 155.1.0.3 503 broadcast
no frame-relay inverse-arp
!
interface Loopback0
ip address 155.1.5.5 255.255.255.0
!
interface Serial0/1
ip address 155.1.45.5 255.255.255.0
clockrate 64000
!
router bgp 1
network 155.1.5.0 mask 255.255.255.0
neighbor 155.1.0.3 remote-as 2
neighbor 155.1.45.4 remote-as 1
neighbor 155.1.45.4 next-hop-self
【实验验证】
R3#show ip bgp
BGP table version is 3, local router ID is 155.1.37.3
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
*> 155.1.5.0/24 155.1.0.5 0 0 1 i
* i 155.1.13.1 0 100 0 1 i
*> 155.1.37.0/24 0.0.0.0 0 32768 i
在未配置weight时R3的bgp路由表，我们发现到达155.1.5.0 的网络下一跳为155.1.0.5，为什么呢？我们回顾一下BGP选路原则, 是不是比较到7步就比较出来了
1.权重：Prefer highest weight (local to router).
2.优先级：Prefer highest local preference (global within AS).
3.起源于本地：Prefer route originated by the local router (next hop = 0.0.0.0).
4.最短AS路径：Prefer shortest AS path.
5.最低的起源属性：Prefer lowest origin code (IGP < EGP < incomplete).
6.最低的MED：Prefer lowest MED.
7.EBGP优于IBGP：Prefer EBGP path over IBGP path.
8.最近的IGP邻居：Prefer the path through the closest IGP neighbor.
9.生存最长的路由：Prefer oldest route for EBGP paths.
10.最低的router-id：Prefer the path with the lowest neighbor BGP router ID.
11.最低的IP地址：Prefer the path with the lowest neighbor IP address.
【weight配置方法】
下一步配置weight值让流量走R1
配置方法：
R3：
router bgp 2
neighbor 155.1.13.1 weight 200
或者是通过route-map来配置
R3:
route-map WEIGHT permit 10
set weight 100
!
router bgp 2
neighbor 155.1.13.1 route-map WEIGHT in
此处为什么要选择in而不是out？
!
验证配置：如下所示，有限选择weight较高的路由。
R3#show ip bgp
BGP table version is 3, local router ID is 155.1.37.3
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
* 155.1.5.0/24 155.1.0.5 0 0 1 i
*>i 155.1.13.1 0 100
200
1 i
*> 155.1.37.0/24 0.0.0.0 0 32768 i
R3#
show ip bgp 155.1.5.0
BGP routing table entry for 155.1.5.0/24, version 2
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
2
1
155.1.0.5 from 155.1.0.5 (155.1.5.5)
Origin IGP, metric 0, localpref 100, valid, external
1
155.1.13.1 from 155.1.13.1 (155.1.146.1)
Origin IGP, metric 0, localpref 100,
weight 200
, valid, internal, best
本文转自zcm8483 51CTO博客，原文链接:http://blog.51cto.com/haolun/992844
