2 AS in the same box JunOS

Network Engineering Asked by Marcos Max on December 13, 2020

Today we have an MX204 and we announced our AS using Juniper, but we also have partnership with another internet provider that has another AS. This other AS has much less client and a much smaller network traffic, and we only need a static route, we control the ads and we received full routing in the main AS.

I was curious if it is possible to have the two announcing them within the Junos, the equipment handles a lot of network traffic and we could eliminate another Mikrotik from the network.

If possible and someone has some example of configuration I will be very grateful.

Sorry for the bad English.

2 Answers

There are several ways to do this.

Logical Systems (LSYS)

Logical Systems are a means of segmenting your Junos router into multiple logical systems, which have separate configurations, routing protocols, and interfaces.

This is similar to having separate routers that happen to share a physical chassis. The neighboring routers, such as your BRAS, have no idea there is any overlap among the networks.

To connect the LSYS together look into Logical Tunnel (lt-) interfaces.


VRFs / virtual-routers / L3VPNs are another means. You could configure the smaller network as a VRF or L3VPN.

You can do this without support for L3VPN on your BRAS by treating it as a CE device. It can have ordinary routing protocol sessions for e.g. BGP or OSPF.

To share transit or peering, you can leak routes between the main routing table (called master by Junos) and the VRF/L3VPN table(s).

     |      |
     /      PE
   |         CE
   |          |
  AS1        AS2
 Custs       BRAS

In truth, though, if your goal is to consolidate the equipment of the other "ISP" into your own, what you should really do is merge them into a single AS. This will simplify your network management in the future. It's not clear why you don't want to do that, but I see you've commented that you don't.

Correct answer by Jeff Wheeler on December 13, 2020

The best way is to ask the smaller ISP to modify their configuration to accept your AS number.

Answered by Ron Trunk on December 13, 2020

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