One of the principles for BGP
Reachable only own the best route to its neighbors 
what is best BGP route
<R2>display bgp routing-table
BGP Local router ID is 2.2.2.2
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 6
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.10.10.10/32 1.1.1.1 0 100 0 ?
*> 22.22.22.22/32 0.0.0.0 0 0 i
*> 44.44.44.44/32 24.1.1.4 0 0 200i
* i 3.3.3.3 0 100 0 200i
*>i 100.100.100.100/32 1.1.1.1 0 100 0 i
*>i 101.101.101.101/32 1.1.1.1 0 100 0 ? Precautions:
* sign indicates valid (effective)
> number represents the best (optimal)
think about: Why RTD segment was to reach 100 next hop is 10.1.12.1?
BGP AS in full but does not change the next hop BGP next hop will occur at the time changes between the AS and the AS
modified as follows:
peer 4.4.4.4 next-hop-local Tell 4.4.4.4 When you want to reach a certain route you to the next hop point to my own OK the
BGP routing information processing
when when received from the BGP neighbor to Update packet, the router will perform the path selection algorithm, to serve determining a best path for each prefix;
best path is derived BGP routing table stored locally (Local_RIB), and is then submitted to the local IP routing table (IP_RIB), to be considered as installation;
the elected effective optimal routing will be encapsulated Update message to send to the neighbor BGP peer.
BGP announcement of the two principles
Publish the best route up to EBGP obtained by all BGP neighbors (including IBGP and EBGP)
Three principles of BGP announcements
We will not publish the best route up to IBGP obtained by other IBGP neighbors (IBGP level of segmentation
) aimed at preventing the loop
Notice the principles of the four BGP
BGP and IGP synchronization (Huawei off by default, can not open)
undo synchronizationAn IBGP neighbor Routes learned from before publishing to a BGP neighbor, by IGP must know the route, that is, BGP and IGP synchronization 
on Huawei routers, the default is to BGP and IGP synchronization check off, because in order to achieve IBGP routes the normal notice. There will be a black hole routing problem
What is routing black hole?
描述如下：
R2 R3 R4属于AS 200的设备 并且各自的loopback接口都发布到OSPF里面 R2的2.2.2.2可以访问4.4.4.4
R1和R2之间是EBGP关系 用的是物理接口建立
R4和R5之间是EBGP关系 用的是物理接口建立
R2和R4之间是IBGP关系 用的是loopback接口建立
R3设备没有和任何设备建立BGP关系
基础配置检查如下
在R3上面查看OSPF邻居状态是否OK
<R3>display ospf peer brief
OSPF Process 1 with Router ID 3.3.3.3
Peer Statistic Information
----------------------------------------------------------------------------
Area Id Interface Neighbor id State
0.0.0.0 GigabitEthernet0/0/0 4.4.4.4 Full
0.0.0.0 GigabitEthernet0/0/1 2.2.2.2 Full
----------------------------------------------------------------------------
<R3>在R2 R3 R4上查看路由表
<R2>display ip routing-table protocol ospf
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 3 Routes : 3
OSPF routing table status : <Active>
Destinations : 3 Routes : 3
Destination/Mask Proto Pre Cost Flags NextHop Interface
3.3.3.3/32 OSPF 10 1 D 192.168.23.3 GigabitEthernet0/0/1
4.4.4.4/32 OSPF 10 2 D 192.168.23.3 GigabitEthernet0/0/1
192.168.34.0/24 OSPF 10 2 D 192.168.23.3 GigabitEthernet0/0/1
OSPF routing table status : <Inactive>
Destinations : 0 Routes : 0
<R2><R3>display ip routing-table protocol ospf
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 2 Routes : 2
OSPF routing table status : <Active>
Destinations : 2 Routes : 2
Destination/Mask Proto Pre Cost Flags NextHop Interface
2.2.2.2/32 OSPF 10 1 D 192.168.23.2 GigabitEthernet0/0/1
4.4.4.4/32 OSPF 10 1 D 192.168.34.4 GigabitEthernet0/0/0
OSPF routing table status : <Inactive>
Destinations : 0 Routes : 0<R4>display ip routing-table protocol ospf
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 3 Routes : 3
OSPF routing table status : <Active>
Destinations : 3 Routes : 3
Destination/Mask Proto Pre Cost Flags NextHop Interface
2.2.2.2/32 OSPF 10 2 D 192.168.34.3 GigabitEthernet0/0/0
3.3.3.3/32 OSPF 10 1 D 192.168.34.3 GigabitEthernet0/0/0
192.168.23.0/24 OSPF 10 2 D 192.168.34.3 GigabitEthernet0/0/0
OSPF routing table status : <Inactive>
Destinations : 0 Routes : 0检查BGP的邻居关系
R1和R2之间的EBGP关系
<R1>display bgp peer
BGP local router ID : 1.1.1.1
Local AS number : 100
Total number of peers : 1 Peers in established state : 1
Peer V AS MsgRcvd MsgSent OutQ Up/Down State PrefRcv
192.168.12.2 4 200 17 18 0 00:14:39 Established 1
<R1>R2的R1的EBGP关系 R2和R4跨邻居的IBGP关系
<R2>display bgp peer
BGP local router ID : 2.2.2.2
Local AS number : 200
Total number of peers : 2 Peers in established state : 2
Peer V AS MsgRcvd MsgSent OutQ Up/Down State PrefRcv
4.4.4.4 4 200 17 18 0 00:14:22 Established 1
192.168.12.1 4 100 18 18 0 00:15:02 Established 1R4和R2的IBGP关系 R4和R5的EBGP关系
<R4>display bgp peer
BGP local router ID : 4.4.4.4
Local AS number : 200
Total number of peers : 2 Peers in established state : 2
Peer V AS MsgRcvd MsgSent OutQ Up/Down State PrefRcv
2.2.2.2 4 200 17 17 0 00:14:49 Established 1
192.168.45.5 4 300 18 18 0 00:15:21 Established 1R5和R4的EBGP关系
<R5>display bgp peer
BGP local router ID : 5.5.5.5
Local AS number : 300
Total number of peers : 1 Peers in established state : 1
Peer V AS MsgRcvd MsgSent OutQ Up/Down State PrefRcv
192.168.45.4 4 200 18 19 0 00:15:34 Established 1
<R5>路由黑洞实验现象
在R1的BGP里面宣告1.1.1.1
在R5的BGP里面宣告5.5.5.5
#
bgp 100
network 1.1.1.1 255.255.255.255
##
bgp 300
network 5.5.5.5 255.255.255.255
#在R2上面查看是否学习到关于1.1.1.1和5.5.5.5的BGP路由
<R2>display bgp routing-table
BGP Local router ID is 2.2.2.2
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 1.1.1.1/32 192.168.12.1 0 0 100i
*>i 5.5.5.5/32 4.4.4.4 0 100 0 300i
<R2>在R4上面查看是否学习到关于1.1.1.1和5.5.5.5的BGP路由
<R4>display bgp routing-table
BGP Local router ID is 4.4.4.4
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 1.1.1.1/32 2.2.2.2 0 100 0 100i
*> 5.5.5.5/32 192.168.45.5 0 0 300i
<R4>R4到达1.1.1.1的下一跳是2.2.2.2 因为在R2这边敲了如下命令
#
bgp 200
peer 4.4.4.4 next-hop-local
#在R5上面查看是否学习到关于1.1.1.1的BGP路由
[R5]display bgp routing-table
BGP Local router ID is 5.5.5.5
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Total Number of Routes: 2
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 1.1.1.1/32 192.168.45.4 0 200 100i
*> 5.5.5.5/32 0.0.0.0 0 0 i
[R5]在R5设备上去访问1.1.1.1
[R5]ping -a 5.5.5.5 1.1.1.1
PING 1.1.1.1: 56 data bytes, press CTRL_C to break
Request time out
Request time out
Request time out
Request time out
Request time out
--- 1.1.1.1 ping statistics ---
5 packet(s) transmitted
0 packet(s) received
100.00% packet loss在R5设备上去访问tracert 1.1.1.1
<R5>tracert -a 5.5.5.5 1.1.1.1
traceroute to 1.1.1.1(1.1.1.1), max hops: 30 ,packet length: 40,press CTRL_C to break
1 192.168.45.4 50 ms 40 ms 20 ms //发现到R4就停止了
2 * * *
3 * *
<R5>发现数据包交给R4后 R4后面就不通了 我们看下R4到达1.1.1.1的路由表
<R4>display ip routing-table 1.1.1.1
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface
1.1.1.1/32 IBGP 255 0 RD 2.2.2.2 GigabitEthernet0/0/0
<R4>发现R4到达1.1.1.1的下一跳是2.2.2.2 R4上面到达2.2.2.2也是非直连网段 R4会递归查询到达2.2.2.2 发现到达2.2.2.2下一跳是R3 因为R4到达1.1.1.1的时候 数据包经过递归后 下一跳是R3
<R4>display ip routing-table 2.2.2.2
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Table : Public
Summary Count : 1
Destination/Mask Proto Pre Cost Flags NextHop Interface
2.2.2.2/32 OSPF 10 2 D 192.168.34.3 GigabitEthernet0/0/0
<R4>R4会把数据包交给R3 R3收到后 到达1.1.1.1该如何走呢？
<R3>display ip routing-table 1.1.1.1R3 found that there is simply no routing table entry route to 1.1.1.1 to 1.1.1.1 will visit so R5 R3 loss like a man-eating tiger machine as long as the R1 side are routed to a black hole R3, R5 want to visit this is the black hole routing to eat
analysis
Cause 1: because there is no running BGP protocol R3
reason 2: BGP neighbor across established