title: BGP state: OPEN author: dkluffy labels: -不完整/随手记, *network comments: 0 assignees: projects: milestone: number: 3

BGP

version 0.1 2019/09/04

WHY BGP

BGP要解决的问题

当IGP不能提供必须的工具来执行路由策略，或者当归奶无法控制路由表规模
当区域里使用IGP时，BGP是十分有用的，比起试图在所有IGP内重分发，BGP更简单有效
一个用户的路由区域必须由一个自治系统号来标识

BGP不是必须的

加入额外的路由协议使网络变得复杂，即使是网络被分解成多个路由域，也不一定要BGP
即使是自治系统之间互联

BGP Highlights

自治系统号

私用保留 64512 ~ 65535

入业务和出业务必须分开考虑

TCP 179 ，点对点连接

“路径”向量协议 BGP是距离向量协议，因此每个节点依靠下游邻居来将他的路由表传送下去节点在公布路由的基础上进行路由计算并将结果传给上行邻居

BGP使用AS号列表，数据包必须通过这些AS才能到达目的地。 这不同于的距离向量协议

route-views.wide.routeviews.org> show ip bgp
BGP table version is 0, local router ID is 202.249.2.166
Status codes: s suppressed, d damped, h history, * valid, > best, = multipath,
              i internal, r RIB-failure, S Stale, R Removed
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop                          Metric LocPrf Weight Path
*> 1.0.0.0/24       202.249.2.169                          0 2497 13335 i
*                   202.249.2.86                           0 7500 2516 13335 i
*> 1.0.4.0/22       202.249.2.169                          0 2497 4826 38803 56203 i
*                   202.249.2.169                          0 7500 2497 4826 38803 56203 i

r1#sh ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route, + - replicated route

Gateway of last resort is not set

      1.0.0.0/32 is subnetted, 1 subnets
C        1.1.1.1 is directly connected, Loopback0
      12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        12.0.0.0/24 is directly connected, Ethernet0/0
L        12.0.0.1/32 is directly connected, Ethernet0/0
      13.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        13.0.0.0/24 is directly connected, Ethernet0/1
L        13.0.0.1/32 is directly connected, Ethernet0/1

BGP 消息类型

Open
Keepalive
Update
Notification

BGP cap


Border Gateway Protocol - UPDATE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 64
    Type: UPDATE Message (2)
    Withdrawn Routes Length: 0
    Total Path Attribute Length: 39
    Path attributes
        Path Attribute - ORIGIN: EGP
        Path Attribute - AS_PATH: empty
        Path Attribute - NEXT_HOP: 192.168.0.33 
        Path Attribute - MULTI_EXIT_DISC: 0
        Path Attribute - LOCAL_PREF: 100
        Path Attribute - COMMUNITIES: 65033:500 65033:600 
    Network Layer Reachability Information (NLRI)
        10.0.0.0/8
            NLRI prefix length: 8
            NLRI prefix: 10.0.0.0

Border Gateway Protocol - UPDATE Message
    Marker: ffffffffffffffffffffffffffffffff
    Length: 98
    Type: UPDATE Message (2)
    Withdrawn Routes Length: 0
    Total Path Attribute Length: 72
    Path attributes
        Path Attribute - ORIGIN: INCOMPLETE
        Path Attribute - AS_PATH: {500, 500} 65211 
        Path Attribute - NEXT_HOP: 192.168.0.15 
        Path Attribute - LOCAL_PREF: 100
        Path Attribute - ATOMIC_AGGREGATE
        Path Attribute - AGGREGATOR: AS: 65210 origin: 192.168.0.10
        Path Attribute - COMMUNITIES: 65215:1 790:4 340:250 
        Path Attribute - ORIGINATOR_ID: 192.168.0.15 
        Path Attribute - CLUSTER_LIST: 192.168.0.250
    Network Layer Reachability Information (NLRI)
        172.16.0.0/16
            NLRI prefix length: 16
            NLRI prefix: 172.16.0.0

BGP 决策过程

路由信息数据库 RIB

Adj-RIBs-In 未经处理的路由消息
Loc-RIB 使用本地策略选择的路由
Adj-RIBs-Out 向对等公布的路由

RIB更新过程

为每一条可用路由计算首选等级
从到特定目的地址的所有可用路由中选取最好的路由，并放入Loc-RIB。
将合适的路由放入Adj-RIBs-Out

最好路由的选择准则

最高管理权值（有cisco提出，Cisco特有）
最高 LOCAL_PREF
最短AS_PATH
最低ORIGIN CODE (IGP<EGP<Incomplete)
最低MUTIL_EXIT_DISC （MED）：限相同AS
EBGP>联盟EBGP,联盟EBGP>IBGP
最低IGP度量路由
如果路由相同，它们来自相同的相邻AS并且通过 MAXIMUM-PATHS 命令使BGP多条路径可用，那么将所有开销相同的路由写入Loc-RIB
如果多条路径不可用，首选最低BGP路由器ID的路由

Cisco 路由器规定的路由协议缺省管理距离:

eBGP| 2
iBGP| 200