多宿主MPLSVPN中-应用AS-override注意的问题

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

ipvrfvpn-a

rd1:10

route-targetexport1:10

route-targetimport1:10

!

ipcef

!

interfaceLoopback0

ipaddress55

!

interfaceFastEthernet0/0

noipaddress

interfaceFastEthernet0/0.12

encapsulationdot1Q12

ipaddress

tag-switchingip

!

interfaceFastEthernet0/0.13

encapsulationdot1Q13

ipaddress

tag-switchingip

!

interfaceFastEthernet0/0.14

encapsulationdot1Q14

ipvrfforwardingvpn-a

ipaddress

routerospf10

log-adjacency-changes

network55area0

!

routerbgp1

nosynchronization

bgplog-neighbor-changes

neighborremote-as1

neighborupdate-sourceLoopback0

neighborremote-as1

neighborupdate-sourceLoopback0

noauto-summary

!

address-familyvpnv4

neighboractivate

neighborsend-communityextended

neighboractivate

neighborsend-communityextended

exit-address-family

!

address-familyipv4vrfvpn-a

neighborremote-as2

neighboractivate

neighboras-override

noauto-summary

nosynchronization

exit-address-family

R2

ipvrfvpn-a

rd1:10

route-targetexport1:10

route-targetimport1:10

!

ipcef

!

interfaceLoopback0

ipaddress55

!

interfaceFastEthernet0/0

noipaddress

!

interfaceFastEthernet0/0.12

encapsulationdot1Q12

ipaddress

routerbgp2

nosynchronization

bgplog-neighbor-changes

networkmask55

redistributeconnected

neighborremote-as1

noauto-summary

R6

interfaceLoopback0

ipaddress55

!

interfaceFastEthernet0/0

noipaddress

interfaceFastEthernet0/0.36

encapsulationdot1Q36

ipaddress

interfaceFastEthernet0/0.56

encapsulationdot1Q56

ipaddress

routerospf100

router-id

log-adjacency-changes

network55area0

routerospf10

log-adjacency-changes

routerbgp2

nosynchronization

bgplog-neighbor-changes

redistributeconnected

neighborremote-as1

noauto-summary

!

在R2上看，BGPVPN表

r2#shipbgvpall

BGPtableversionis18,localrouterIDis

Statuscodes:ssuppressed,ddamped,hhistory,*valid,>best,i-internal,

rRIB-failure,SStale

Origincodes:i-IGP,e-EGP,?-incomplete

NetworkNextHopMetricLocPrfWeightPath

RouteDistinguisher:1:10(defaultforvrfvpn-a)

*>i/32010002?

*>/32002i

*>i/32010002?

*>i/24010002?

r>/24002?

*>i/24010002?

*i/24010002?

*>002?

在R5上，

r5#shiprou

/24issubnetted,1subnets

B[20/0]via,00:27:04

/32issubnetted,1subnets

B[20/0]via,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

Typeescapesequencetoabort.

Tracingtherouteto

156msec112msec140msec

2[AS1]312msec104msec320msec

3[AS1]316msec212msec*//运营商中路由

前缀/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11，故形成了环路。

解决方案：

AS-override同SOO（siteoforigin）一起使用,在多宿主MPLSVPN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点被发送回同一站点。SOO检测环路是自动的，需做在in的方向上。

R2上配置

address-familyipv4vrfvpn-a

neighborroute-mapdogin

route-mapdogpermit10

setextcommunitysoo2:2

//当前缀从PE2通告给PE3时，会被附上

SOO值，当PE3检测到相同的SOO时，

该前缀便不发给站点2中CE3

R3上配置

address-familyipv4vrfvpn-a

neighborroute-mapboyin

route-mapboypermit10

setextcommunitysoo2:2

//SOO值要相同

在R5，

r5#shiprou

/24issubnetted,1subnets

O[110/2]via,00:05:23,FastEthernet0/0.56

/32issubnetted,1subnets