多宿主MPLSVPN中-应用AS-override注意的问题.docx

多宿主MPLS/VPN中，应用AS-override注意的问题由 admin 于 星期二, 04/01/2008 - 16:25 发表目的研究PE与CE间使用BGP时，可能出现的次优路径及其环路拓扑图在下面当PE与CE之间使用BGP时，在多宿主网络中，使用AS-override 有可能形成次优路径或环路，如上图所示。配置如下R1 ip vrf vpn-ard 1:10route-target export 1:10route-target import 1:10!ip cef!interface Loopback0ip address 1.1.1.1 255.255.255.255!interface FastEthernet0/0no ip addressinterface FastEthernet0/0.12encapsulation dot1Q 12ip address 12.1.1.1 255.255.255.0tag-switching ip!interface FastEthernet0/0.13encapsulation dot1Q 13ip address 13.1.1.1 255.255.255.0tag-switching ip!interface FastEthernet0/0.14encapsulation dot1Q 14ip vrf forwarding vpn-aip address 14.1.1.1 255.255.255.0router ospf 10log-adjacency-changesnetwork 0.0.0.0 255.255.255.255 area 0!router bgp 1no synchronizationbgp log-neighbor-changesneighbor 2.2.2.2 remote-as 1neighbor 2.2.2.2 update-source Loopback0neighbor 3.3.3.3 remote-as 1neighbor 3.3.3.3 update-source Loopback0no auto-summary!address-family vpnv4neighbor 2.2.2.2 activateneighbor 2.2.2.2 send-community extendedneighbor 3.3.3.3 activateneighbor 3.3.3.3 send-community extendedexit-address-family!address-family ipv4 vrf vpn-aneighbor 14.1.1.4 remote-as 2neighbor 14.1.1.4 activateneighbor 14.1.1.4 as-overrideno auto-summaryno synchronizationexit-address-familyR2ip vrf vpn-ard 1:10route-target export 1:10route-target import 1:10!ip cef!interface Loopback0ip address 2.2.2.2 255.255.255.255!interface FastEthernet0/0no ip address!interface FastEthernet0/0.12encapsulation dot1Q 12ip address 12.1.1.2 255.255.255.0tag-switching ip!interface FastEthernet0/0.23encapsulation dot1Q 23ip address 23.1.1.2 255.255.255.0tag-switching ip!interface FastEthernet0/0.25encapsulation dot1Q 25ip vrf forwarding vpn-aip address 25.1.1.2 255.255.255.0router ospf 10log-adjacency-changesnetwork 0.0.0.0 255.255.255.255 area 0!router bgp 1no synchronizationbgp log-neighbor-changesneighbor 1.1.1.1 remote-as 1neighbor 1.1.1.1 update-source Loopback0neighbor 3.3.3.3 remote-as 1neighbor 3.3.3.3 update-source Loopback0no auto-summary!address-family vpnv4neighbor 1.1.1.1 activateneighbor 1.1.1.1 send-community extendedneighbor 3.3.3.3 activateneighbor 3.3.3.3 send-community extendedexit-address-family!address-family ipv4 vrf vpn-aneighbor 25.1.1.5 remote-as 2neighbor 25.1.1.5 activateneighbor 25.1.1.5 as-overrideno auto-summaryno synchronizationexit-address-familyR3ip vrf vpn-ard 1:10route-target export 1:10route-target import 1:10!ip cefinterface Loopback0ip address 3.3.3.3 255.255.255.255!interface FastEthernet0/0no ip addressduplex half!interface FastEthernet0/0.13encapsulation dot1Q 13ip address 13.1.1.3 255.255.255.0tag-switching ip!interface FastEthernet0/0.23encapsulation dot1Q 23ip address 23.1.1.3 255.255.255.0tag-switching ip!interface FastEthernet0/0.36encapsulation dot1Q 36ip vrf forwarding vpn-aip address 36.1.1.3 255.255.255.0router ospf 10log-adjacency-changesnetwork 0.0.0.0 255.255.255.255 area 0!router bgp 1no synchronizationbgp log-neighbor-changesneighbor 1.1.1.1 remote-as 1neighbor 1.1.1.1 update-source Loopback0neighbor 2.2.2.2 remote-as 1neighbor 2.2.2.2 update-source Loopback0no auto-summary!address-family vpnv4neighbor 1.1.1.1 activateneighbor 1.1.1.1 send-community extendedneighbor 2.2.2.2 activateneighbor 2.2.2.2 send-community extendedexit-address-family!address-family ipv4 vrf vpn-aneighbor 36.1.1.6 remote-as 2neighbor 36.1.1.6 activateneighbor 36.1.1.6 as-overrideno auto-summaryno synchronizationexit-address-family R4interface Loopback0ip address 4.4.4.4 255.255.255.255!interface FastEthernet0/0no ip addressinterface FastEthernet0/0.14encapsulation dot1Q 14ip address 14.1.1.4 255.255.255.0!router bgp 2no synchronizationbgp log-neighbor-changesnetwork 1.1.1.1 mask 255.255.255.255redistribute connectedneighbor 14.1.1.1 remote-as 1no auto-summaryR5interface Loopback0ip address 5.5.5.5 255.255.255.255!interface FastEthernet0/0no ip addressinterface FastEthernet0/0.25encapsulation dot1Q 25ip address 25.1.1.5 255.255.255.0!interface FastEthernet0/0.56encapsulation dot1Q 56ip address 56.1.1.5 255.255.255.0serial restart-delay 0!router ospf 100router-id 5.5.5.5log-adjacency-changesnetwork 0.0.0.0 255.255.255.255 area 0!router bgp 2no synchronizationbgp log-neighbor-changesnetwork 5.5.5.5 mask 255.255.255.255redistribute connectedneighbor 25.1.1.2 remote-as 1no auto-summaryR6interface Loopback0ip address 6.6.6.6 255.255.255.255!interface FastEthernet0/0no ip addressinterface FastEthernet0/0.36encapsulation dot1Q 36ip address 36.1.1.6 255.255.255.0interface FastEthernet0/0.56encapsulation dot1Q 56ip address 56.1.1.6 255.255.255.0router ospf 100router-id 6.6.6.6log-adjacency-changesnetwork 0.0.0.0 255.255.255.255 area 0router ospf 10log-adjacency-changesrouter bgp 2no synchronizationbgp log-neighbor-changesredistribute connectedneighbor 36.1.1.3 remote-as 1no auto-summary!在R2上看，BGP VPN表r2#sh ip bg vp allBGP table version is 18, local router ID is 2.2.2.2Status codes: s suppressed, d damped, h history, * valid, best, i - internal,r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incompleteNetwork Next Hop Metric LocPrf Weight PathRoute Distinguisher: 1:10 (default for vrf vpn-a)*i4.4.4.4/32 1.1.1.1 0 100 0 2 ?* 5.5.5.5/32 25.1.1.5 0 0 2 i*i6.6.6.6/32 3.3.3.3 0 100 0 2 ?*i14.1.1.0/24 1.1.1.1 0 100 0 2 ?r 25.1.1.0/24 25.1.1.5 0 0 2 ?*i36.1.1.0/24 3.3.3.3 0 100 0 2 ?* i56.1.1.0/24 3.3.3.3 0 100 0 2 ?* 25.1.1.5 0 0 2 ?在R5上，r5#sh ip rou36.0.0.0/24 is subnetted, 1 subnetsB 36.1.1.0 20/0 via 25.1.1.2, 00:27:046.0.0.0/32 is subnetted, 1 subnetsB 6.6.6.6 20/0 via 25.1.1.2, 00:27:04可以看到原本红色部分是从OSPF学来，现在变成从EBGP学来，下一条变为R2，r5#trace 6.6.6.6Type escape sequence to abort.Tracing the route to 6.6.6.61 25.1.1.2 56 msec 112 msec 140 msec2 36.1.1.3 AS 1 312 msec 104 msec 320 msec3 36.1.1.6 AS 1 316 msec 212 msec * /运营商中路由前缀6.6.6.6/32，在CE3上生成被发往PE3，当PE3把这条前缀通回给CE2时，由于AS-override作用，AS-PATH变成 1 1 ，故形成了环路。解决方案：AS-override同SOO （site of origin）一起使用,在多宿主MPLS VPN中，SOO用来标识客户站点，防止从某一点离开该站点的流量从另外一