L3VPN Loop Prevention Features Notes
L3VPN Loop Prevention Featurees
OSPF
– Uses DOWN-BIT
-> 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
BGP
– 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 14.0.0.4 soo 100:100
R3(config)# router bgp 100
address-family ipv4 vrf c1b2
neighbor 35.0.0.5 soo 200:200
SoO value should be the same on PE routers connected to the same branch.
EIGRP
– 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
Sitemap
– If SoO is not set, then set
– If SoO is set, then drop
