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gstubbe
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Posts: 17
Joined: Fri Feb 22, 2019 3:06 pm

AS-Override on BGP Peer does not change anything to the AS Path

Thu Apr 29, 2021 4:12 pm

Hello,

I created a small lab setup after a weird issue in our DC. We would like to change the AS path we advertise to our peers. Looking into the options on mikrotik we have the option to "AS-Override" under /routing bgp peer.

Trying that and looking to the received routers on the next router does not remove the extra as numbers from my other peers. I will share a screenshot with my lab setup and their configs.

Setup:
Image

Config - 01
# apr/29/2021 13:08:52 by RouterOS 6.48
# software id = 
#
#
#
/interface bridge
add name=bridge1
/interface ethernet
set [ find default-name=ether1 ] disable-running-check=no
set [ find default-name=ether2 ] disable-running-check=no
/routing bgp instance
set default as=65531 redistribute-connected=yes redistribute-static=yes
/ip address
add address=10.1.1.1/30 interface=ether2 network=10.1.1.0
add address=10.10.1.0/24 interface=bridge1 network=10.10.1.0
/ip dhcp-client
add disabled=no interface=ether1
/routing bgp peer
add name=peer2 remote-address=10.1.1.2 remote-as=65532
/system identity
set name=01
wrong as path on 01:
Image

Config - 02
# apr/29/2021 13:10:29 by RouterOS 6.48
# software id = 
#
#
#
/interface bridge
add name=bridge1
/interface ethernet
set [ find default-name=ether1 ] disable-running-check=no
set [ find default-name=ether2 ] disable-running-check=no
set [ find default-name=ether3 ] disable-running-check=no
/routing bgp instance
set default as=65532 redistribute-connected=yes redistribute-static=yes
/ip address
add address=10.1.1.2/30 interface=ether2 network=10.1.1.0
add address=10.1.1.5/30 interface=ether3 network=10.1.1.4
add address=10.10.2.0/24 interface=bridge1 network=10.10.2.0
/ip dhcp-client
add disabled=no interface=ether1
/routing bgp peer
add as-override=yes name=peer1 remote-address=10.1.1.1 remote-as=65531
add as-override=yes name=peer3 remote-address=10.1.1.6 remote-as=65533
/system identity
set name=02
Config - 03
# apr/29/2021 13:11:41 by RouterOS 6.48
# software id = 
#
#
#
/interface bridge
add name=bridge1
/interface ethernet
set [ find default-name=ether1 ] disable-running-check=no
set [ find default-name=ether2 ] disable-running-check=no
/routing bgp instance
set default as=65533 redistribute-connected=yes redistribute-static=yes
/ip address
add address=10.10.3.0/24 interface=bridge1 network=10.10.3.0
add address=10.1.1.6/30 interface=ether2 network=10.1.1.4
/ip dhcp-client
add disabled=no interface=ether1
/routing bgp peer
add name=peer2 remote-address=10.1.1.5 remote-as=65532
/system identity
set name=03
wrong AS path on 03:
Image

Expected behavior would be that all routes on 01 and 02 should be advertised from 65532 and should not include the other as numbers

EDIT: I also test this with public as numbers between 01 and 02 and private as numbers between 02 and 03 with the same result on both 01 and 03

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