allowas-in v/s as-override

# allowas-in config needs to be done on CE
ON CE
router bgp 65001
network 10.7.7.0 mask 255.255.255.0
network 10.7.77.0 mask 255.255.255.0
network 10.7.27.0 mask 255.255.255.0

neighbor 10.7.27.2 remote-as 1234
neighbor 10.7.27.2 allowas-in 2

ON PE
router bgp 1234
address-family ipv4 vrf 65001
neighbor 10.7.27.7 remote-as 65001

Rack1R7#sh ip bgp
BGP table version is 10, local router ID is 10.7.7.7
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path
*> 10.7.7.0/24 0.0.0.0 0 32768 i
*> 10.7.8.0/24 10.7.27.2 0 1234 65001 i
*> 10.7.27.0/24 0.0.0.0 0 32768 i
*> 10.7.48.0/24 10.7.27.2 0 1234 65001 i
*> 10.7.77.0/24 0.0.0.0 0 32768 i
Rack1R7#


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as-override
# as-override needs to be done on PE under the address-family ipv4 vrf
ON CE
router bgp 65001
network 10.7.7.0 mask 255.255.255.0
network 10.7.77.0 mask 255.255.255.0
network 10.7.27.0 mask 255.255.255.0

neighbor 10.7.27.2 remote-as 1234
--> Remove this --->neighbor 10.7.27.2 allowas-in 2-->Unless u have a NON MPLS backdoor

ON PE
router bgp 1234
address-family ipv4 vrf 65001
neighbor 10.7.27.7 remote-as 65001
neighbor 10.7.27.7 as-override

Rack1R7#sh ip bgp
BGP table version is 14, local router ID is 10.7.7.7
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path
*> 10.7.7.0/24 0.0.0.0 0 32768 i
*> 10.7.8.0/24 10.7.27.2 0 1234 1234 i
*> 10.7.27.0/24 0.0.0.0 0 32768 i
*> 10.7.48.0/24 10.7.27.2 0 1234 1234 i
*> 10.7.77.0/24 0.0.0.0 0 32768 i
Rack1R7#

Conclusion
as-override means PE replaces all occurrences of CE ASN with its ASN