MPLS经典问题利用MPLS解决和避免bgp路由黑洞问题

R1－－－(R2－－－R3－－－R4)－－R5
|__EBGP__|_____IBGP______|__EBGP_|
 12.0/24          23.0/24      34.0/24   45.0/24

R2-R4是MPLS域
在R2和R4上重分布直连面向EBGP邻居的接口
使用R1和R5上的loopback1做为internet路由

r1#sh ip b
BGP table version is 7, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Next Hop            Metric LocPrf Weight Path
*> 11.11.11.11/32   0.0.0.0                  0         32768 i
*> 55.55.55.55/32   192.168.12.2                           0 1 3 i
*> 192.168.12.0     192.168.12.2             0             0 1 ?
*> 192.168.45.0     192.168.12.2                           0 1 ?

r5#sh ip b
BGP table version is 7, local router ID is 5.5.5.5
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Next Hop            Metric LocPrf Weight Path
*> 11.11.11.11/32   192.168.45.4                           0 1 2 i
*> 55.55.55.55/32   0.0.0.0                  0         32768 i
*> 192.168.12.0     192.168.45.4                           0 1 ?
*> 192.168.45.0     192.168.45.4             0             0 1 ?
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r2#  sh ip b
BGP table version is 5, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure
Origin codes: i - IGP, e - EGP, ? - incomplete

Network          Next Hop            Metric LocPrf Weight Path
*> 11.11.11.11/32   192.168.12.1             0             0 2 i
*>i55.55.55.55/32   192.168.45.5             0    100      0 3 i
*> 192.168.12.0     0.0.0.0                  0         32768 ?
*>i192.168.45.0    4.4.4.4                  0    100      0 ?

r2#sh ip route
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/129] via 192.168.23.3, 01:26:32, Serial2/0

r2查找路由表，发现i55.55.55.55/32是一条bgp路由，而mpls不会为从bgp收到的路由分配标签，它再次查找，发现 i55.55.55.55/32的下一跳地址是192.168.45.5，同理，再次查找路由表，终于发现4.4.4.4是一条IGP路由，正好R3给4.4.4.4分配了一个301的标签给R2，这时候，最无耻的事情发生了，R2将这个标签copy到CEF表里面关于到55.55.55.55和192.168.45.0/24,的压入标签里面，也就是所有从192.168.45.5收到的bgp路由都有这种行为，如下所示：

r2#show ip cef detail

4.4.4.4/32, version 10, epoch 0, cached adjacency to Serial2/0
0 packets, 0 bytes
  tag informationset, shared
    local tag: 201
    fast tag rewrite with Se2/0, point2point, tags imposed: {301}
  via 192.168.23.3, Serial2/0, 1 dependency
    next hop 192.168.23.3, Serial2/0
    valid cached adjacency
    tag rewrite with Se2/0, point2point, tags imposed: {301}

55.55.55.55/32, version 19, epoch 0, cached adjacency to Serial2/0
0 packets, 0 bytes
  tag information from 4.4.4.4/32, shared
    local tag: 201
    fast tag rewrite with Se2/0, point2point, tags imposed: {301}
  via 192.168.45.5, 0 dependencies, recursive
    next hop 192.168.23.3, Serial2/0 via 192.168.45.0/24
    valid cached adjacency
    tag rewrite with Se2/0, point2point, tags imposed: {301}

192.168.45.0/24, version 18, epoch 0, cached adjacency to Serial2/0
0 packets, 0 bytes
  tag information from 4.4.4.4/32, shared
    local tag: 201
    fast tag rewrite with Se2/0, point2point, tags imposed: {301}
  via 4.4.4.4, 1 dependency, recursive
    next hop 192.168.23.3, Serial2/0 via 4.4.4.4/32
    valid cached adjacency
    tag rewrite with Se2/0, point2point, tags imposed: {301}

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路由器不会对从bgp收到的路由分配标签，所以在这里看不到55.55.55.55/32的标签绑定
r2#sho tag tdp bind     
  tib entry: 2.2.2.2/32, rev 4
        local binding:  tag: imp-null
        remote binding: tsr: 3.3.3.3:0, tag: 300
  tib entry: 3.3.3.3/32, rev 6
        local binding:  tag: 200
        remote binding: tsr: 3.3.3.3:0, tag: imp-null
  tib entry: 4.4.4.4/32, rev 8
        local binding:  tag: 201
        remote binding: tsr: 3.3.3.3:0, tag: 301
  tib entry: 192.168.12.0/24, rev 2
        local binding:  tag: imp-null
  tib entry: 192.168.23.0/24, rev 10
        local binding:  tag: imp-null
        remote binding: tsr: 3.3.3.3:0, tag: imp-null
  tib entry: 192.168.34.0/24, rev 12
        local binding:  tag: 202
        remote binding: tsr: 3.3.3.3:0, tag: imp-null

当R3收到带有301标签的mpls packet的时候，根据自己的标签转发表，进行次末跳弹出，下一跳是4.4.4.4/32，而4.4.4.4/32已经具有internet路由，所以它可以正常转发

r3#show tag-switching forwarding-table
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop   
tag    tag or VC   or Tunnel Id      switched   interface             
300    Pop tag     2.2.2.2/32        4061       Se2/0      point2point 
301    Pop tag     4.4.4.4/32        4457       Se1/0      point2point

而这时候，在R3上不存在55.55.55.55/32路由
r3#sh ip rou
Codes: C - connected, S - static, I - IGRP, 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, E - EGP
       i - IS-IS, 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

Gateway of last resort is not set

2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/65] via 192.168.23.2, 00:28:23, Serial2/0
     3.0.0.0/32 is subnetted, 1 subnets
C       3.3.3.3 is directly connected, Loopback0
     4.0.0.0/32 is subnetted, 1 subnets
O       4.4.4.4 [110/65] via 192.168.34.4, 00:28:23, Serial1/0
C    192.168.23.0/24 is directly connected, Serial2/0
C    192.168.34.0/24 is directly connected, Serial1/0

这就是利用mpls来避免bgp的路由黑洞问题，应该是说清楚了吧？^_^

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