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HS08
HS08
Visitor III
January 26, 2026
Question

BGP AS Path

  • January 26, 2026
  • 2 replies
  • 338 views

I have topology like below picture. Why azure see prefix 10.100.0.0/16 from AS65100-65103?

In the hub i have prefix list only subnet 10.103.0.0/16 will be advertised to azure and in the spoke will advertise subnet 10.100.0.0/16

image.png

 

Screenshot 2026-01-26 100242.png

2 replies

hpenmetsa
Staff
hpenmetsa
Staff
January 26, 2026

Hi, HUB might be advertising the learned spoke prefix to Azure. Could you check the advertised routes from the HUB to Azure?  Please run the below command on the HUB to check the advertised routes.

get router info bgp neighbors <neighbor IP> advertised-routes

Thanks

HS08
HS08Author
Visitor III
January 26, 2026

i have prefix list to only advertise subnet in the hub (10.103.0.0/16 and 172.30.30.0/24) to azure

 

FW01 (Internet) # get router info bgp neighbors 10.201.0.4 advertised-routes
VRF 0 BGP table version is 43404, local router ID is 10.103.103.103
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight RouteTag Path
*> 10.103.0.0/16 10.210.0.103 100 32768 0 i <-/->
*> 172.30.30.0/24 10.210.0.103 100 32768 0 i <-/->

Total number of prefixes 2

 

FW01 (Internet) # get router info bgp neighbors 10.201.0.5 advertised-routes
VRF 0 BGP table version is 43404, local router ID is 10.103.103.103
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight RouteTag Path
*> 10.103.0.0/16 10.210.0.203 100 32768 0 i <-/->
*> 172.30.30.0/24 10.210.0.203 100 32768 0 i <-/->

Total number of prefixes 2

Toshi_Esumi
SuperUser
Toshi_Esumi
SuperUser
January 26, 2026

Because you have ADVPN setup and the HUB is acting as a route reflector. If I remember correctly you asked the same question a while ago and somebody from FTNT answered you with an option for the hub not to act as an rout reflector at that time. Do you remember?

 

Toshi

itozz
itozz
New Member
January 26, 2026

Think about how BGP uses the AS-PATH and its length a little bit.

When a router receives a route from peer A and announces it to peer B what happens to the AS-PATH?

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