BGP Prefix Origination — My ASN appears as origin together with customer ASN

RouterOS Forwarding Protocols
1

Hi all,

I’m facing a BGP configuration issue on RouterOS v7.19.1 stable.

I’m a transit provider, receiving prefixes from a downstream customer over eBGP IPv6 session.

BGP Session Setup:

  • My side:
    /routing/bgp/connection
    add name=“Client-IPv6”
    remote.address=fdxx:xx:xx::2/128 remote.as=AS-CUSTOMER
    local.default-address=fdxx:xx:xx::1 local.role=provider
    routing-table=main router-id=xx.xx.xx.xx as=AS-MY
    output.filter-chain=downstreams-clients default-originate=always

  • Customer side (assumed):
    /routing/bgp/connection
    add name=“Upstream”
    remote.address=fdxx:xx:xx::1 remote.as=AS-MY
    local.role=customer

Issue:

Even though the customer advertises their own prefix (2a0x:xxxx::/48) via BGP, the prefix shows up in global routing tables with both their ASN and mine appearing as origin ASNs. The prefix looks like it is originated from my ASN, even though it should only be originated by the customer ASN.

Behavior Observed:

  • The route is received from customer properly.
  • However, my ASN still originates the prefix.
  • Route shows up externally like:
    Origin AS: AS-MY
    AS-PATH: AS-MY AS-CUSTOMER

My Assumptions:

  • Roles (provider / customer) are correctly set on both sides.
  • Customer is correctly announcing the prefix over BGP.

Possible root cause (so far):

  • default-originate=always is enabled on my side.
  • The output.filter-chain=downstreams-clients explicitly allows the customer prefix.
  • redistribute-connected / redistribute-static might be enabled on my BGP instance (still verifying).
  • Prefix exists locally in routing table (possibly via static or connected route).
  • As a result, RouterOS originates the prefix under my ASN, even though it is already received via BGP.

What I’m looking for:

  • Clear explanation why RouterOS still originates this prefix even if I already receive it via BGP.
  • Whether default-originate=always combined with output filters can cause this unintended origination.
  • Correct configuration best practice for eBGP customer/provider sessions on RouterOS 7.x to avoid local origination.

Notes:

  • I do not want to originate customer prefixes under my ASN.
  • Only customer ASN should appear as origin.
  • I want purely transit behavior.

Thank you for any input.