BGP笔记（四）

小鱼儿 2022-01-17 08:45 206阅读 0赞

**一、BGP Attributes（属性）**

[![p\_w\_picpath][p_w_picpath]][p_w_picpath_p_w_picpath]

\*的是公认比遵属性，任何厂商都必须支持。

[![p\_w\_picpath][p_w_picpath 1]][p_w_picpath_p_w_picpath 1]

[![p\_w\_picpath][p_w_picpath 2]][p_w_picpath_p_w_picpath 2]

[![p\_w\_picpath][p_w_picpath 3]][p_w_picpath_p_w_picpath 3]

上面BGP选路的一个大前提是，都关闭同步而且下一跳都是可达的，这样才有资格竞争。

**1、weight（权重）（cisco私有）：越大越好，默认本地通告的为32768，其他的为0。**

[![p\_w\_picpath][p_w_picpath 4]][p_w_picpath_p_w_picpath 4]

下面用一个实例来演示如何**修改weight值**：

在下面这个图中，R3和R4之间是用交叉线，其他都是串行线。所以没有修改weight值之前，R3去往2.2.2.0/24和9.9.9.0/24的bpg路由应该都是走R4这条线的。下面我们通过修改在R3上修改weight值，让R3去往2.2.2.0/24和9.9.9.0/24的bpg路由都走R1这条线。

[![p\_w\_picpath][p_w_picpath 5]][p_w_picpath_p_w_picpath 5]

还没修改weight值之前R3的BGP路由：

[![p\_w\_picpath][p_w_picpath 6]][p_w_picpath_p_w_picpath 6]

修改weight值：

[![p\_w\_picpath][p_w_picpath 7]][p_w_picpath_p_w_picpath 7]

**neighbor 1.1.1.1 weight 1**这条命令是把从1.1.1.1发过来的BGP路由都把它的weight值修改为1，这个方向是in方向而不是out方向。

看看效果：

先软清一下：

[![p\_w\_picpath][p_w_picpath 8]][p_w_picpath_p_w_picpath 8]

[![p\_w\_picpath][p_w_picpath 9]][p_w_picpath_p_w_picpath 9]

如果我们要让R3去往2.2.2.0/24的路由往R1走，去往9.9.9.0/24的路由往R4走。

只需要在R3上把收到来自R1的2.2.2.0/24的BGP路由的weight值设置得比0高就可以了。

[![p\_w\_picpath][p_w_picpath 10]][p_w_picpath_p_w_picpath 10]

上面的命令中，还额外配置了一条route-map W permit 20，这是一条空的route-map，是为了允许其他路由进入R3.

看看最后效果：

[![p\_w\_picpath][p_w_picpath 11]][p_w_picpath_p_w_picpath 11]

这样可以做到负载分担的作用。

**2、local preference attribute本地优先级属性（告诉IBGP邻居怎样离开AS）越大越好，默认为100.**

[![p\_w\_picpath][p_w_picpath 12]][p_w_picpath_p_w_picpath 12]

[![p\_w\_picpath][p_w_picpath 13]][p_w_picpath_p_w_picpath 13]

[![p\_w\_picpath][p_w_picpath 14]][p_w_picpath_p_w_picpath 14]

另外，单独控制：

[![p\_w\_picpath][p_w_picpath 15]][p_w_picpath_p_w_picpath 15]

[![p\_w\_picpath][p_w_picpath 16]][p_w_picpath_p_w_picpath 16]

[![p\_w\_picpath][p_w_picpath 17]][p_w_picpath_p_w_picpath 17]

**3、AS path，个数越少越好**

[![p\_w\_picpath][p_w_picpath 18]][p_w_picpath_p_w_picpath 18]

注意上图中，1 2的顺序。

下面我们要通告修改R2通告的AS path值，让R3去2.2.2.0/24的BGP路由原本往R4走变成往R1走。

[![p\_w\_picpath][p_w_picpath 19]][p_w_picpath_p_w_picpath 19][![p\_w\_picpath][p_w_picpath 20]][p_w_picpath_p_w_picpath 20][![p\_w\_picpath][p_w_picpath 21]][p_w_picpath_p_w_picpath 21]  
[![p\_w\_picpath][p_w_picpath 22]][p_w_picpath_p_w_picpath 22]

[![p\_w\_picpath][p_w_picpath 23]][p_w_picpath_p_w_picpath 23]

[![p\_w\_picpath][p_w_picpath 24]][p_w_picpath_p_w_picpath 24]

但是AS号不能像上面那样随便取，要是附近真有那样的AS号，则就真的会往哪个AS发，所以，我们一般是“加自己的AS号”像下面这样：

[![p\_w\_picpath][p_w_picpath 25]][p_w_picpath_p_w_picpath 25]

**4、MED Attribute（告诉EBGP邻居怎样进入AS）越小越好**

[![p\_w\_picpath][p_w_picpath 26]][p_w_picpath_p_w_picpath 26]

[![p\_w\_picpath][p_w_picpath 27]][p_w_picpath_p_w_picpath 27]

现在R2去往8.8.8.0网络是往R4走的，因为这条路径cost值较小。

我们现在要让其往R1走，由于metric值默认为0，所以我们选择在R1提高metric值。。

[![p\_w\_picpath][p_w_picpath 28]][p_w_picpath_p_w_picpath 28]  
[![p\_w\_picpath][p_w_picpath 29]][p_w_picpath_p_w_picpath 29]  
[![p\_w\_picpath][p_w_picpath 30]][p_w_picpath_p_w_picpath 30]  
[![p\_w\_picpath][p_w_picpath 31]][p_w_picpath_p_w_picpath 31]这是拓展ping的结果。[![p\_w\_picpath][p_w_picpath 32]][p_w_picpath_p_w_picpath 32]  
这是debug ip bgp update的结果。

因为你AS2有时候是不可控的，你只能控制本AS，所以得使用这些方法知道我们配置起了作用没有。

上面的各种属性中，我们最常用的就是**本地优先级**和metric值

转载于:https://blog.51cto.com/zjskobe/836969

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