ISP "strange" configuration

[giulio-leone-71]

Hi folks,
on an enterprise site I found the configuration of the scheme below.
I can't understand how an ISP can offer a single, specific, static address through each single interface of its gateway router.
The fact of knowing that you can't have the same IP address on the two interfaces of the same link made me curious. So I did some simulations on packet tracer trying to configure VLANs, ACLs
but I didn't get much out of it.
Could someone more expert give me some information?

Thanks in advance!

[Joseph W. Doherty]

MPLS/VRF being used?

[Giuseppe Larosa · Accepted solution]

Hello @giulio-leone-71 ,

look for IRB  Integrated Routing and Bridging, the gateway may implement it to have a bridge domain  ( like a VLAN a collection of L2 ports facing the enterprise equipment) with a virtual Layer 3 interface associated to the bridge domain that acts like the gateway.

See the physical ports connecting to enterprise equipment as L2 access switchports associated to the bridge domain they act only at OSI layer 2 and they don't have an IP address.

In classic IOS IRB used a logical interface named BVI that requires an explicit command to be linked to a bridge domain .

In IOS XE the logical interface is named BDI and it is associated to a bridge domain . Phyisical interfaces have ethernet service instances configured on them and these service instances ( see them as L2 subinterfaces ) are associated to the bridge domain.

The idea is the same have a router acts as a multilayer switch but now configuration use concepts taken from metro ethernet forums that define several L2 VPN services that can be point to point between two devices or they can be a collection of sites in the same  VPLS l2 forwarding instance.

On a single device IRB is still the name used and it is used not only by Cisco but also by other vendors like Juniper.

Hope to help

Giuseppe

[giulio-leone-71]

Thanks for the tip Giuseppe.
I also imagined that it is a Bridging type, but I do not have access to the equipment (among other things from the manufacturer Hua@@i) to read the type of configuration created.
In recent years I have become Cisco-Addicted  so out of curiosity I wanted to "imitate" this configuration and try to understand how the ISP-side connections to end customers work.

Have a nice day!

[balaji.bandi]

It all depends on the model of the Router provider offering the Gateway. This could be a Layer 2 port that other routers connect to. (again these all my speculation).