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 ? －－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－ 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} －－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－－ 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？^_^ 文章录入：csh    责任编辑：csh