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L3VPN Loop Prevention Features Notes

L3VPN Loop Prevention Featurees



     -> When a PE router receives an update on a VRF interface, it performs a DOWN-BIT operation

          -> If the down-bit is not set in the update packet, it is set by the PE router

          -> If the down-bit is set, the PE router drops the packet

          -> Down-bit checking is only done on type 3 LSAs (Inter-Area)

          -> Doesn’t work for type 1 LSAs used by sham-links

          -> No configuration required


– Can use extended community called “site-of-origin”

     -> Can be set on incoming VRF interface

     -> SoO = 64 bit value represented at X:Y

          -> Must be different on each PE router

R1(config)# router bgp 100

address-family ipv4 vrf c1b1

  neighbor soo 100:100

R3(config)# router bgp 100

address-family ipv4 vrf c1b2

  neighbor soo 200:200

SoO value should be the same on PE routers connected to the same branch.


 – Can use “site-of-origin” to prevent loops

R1(config)# router-map SOO

 set ext-community soo 100:100

int s0/0

 ip vrf sitemap SOO


 – If SoO is not set, then set

 – If SoO is set, then drop

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