TTR Networks

Intermediate · BGP on loopbacks vs interface addresses · section 2 of 8

The lab

✓ Lab-tested 17 October 2026 on VyOS 2026.10.16-0024-rolling

Two VyOS routers (rolling 2026.10.16) run as virtual machines, with two Docker networks between them as link A and link B. Every output in this lesson is copied from that lab.

R1R2
Loopback198.51.100.1/32198.51.100.2/32
Link A (eth0)192.0.2.0/31192.0.2.1/31
Link B (eth1)192.0.2.2/31192.0.2.3/31
Announcesnothing203.0.113.0/24 (on a dummy interface)

To "break" link A, R2 disables its eth0 (set interfaces ethernet eth0 disable). R2 knows its interface is down; R1 sees nothing change on its own eth0. That's the hard case, and it's the common one.

R1: interfaces and loopback
set system host-name 'R1'
set interfaces loopback lo address '198.51.100.1/32'
set interfaces ethernet eth0 address '192.0.2.0/31'
set interfaces ethernet eth0 description 'LINK-A-TO-R2'
set interfaces ethernet eth1 address '192.0.2.2/31'
set interfaces ethernet eth1 description 'LINK-B-TO-R2'
  • Line 2: The loopback /32: R1's stable address.
  • Lines 3–6: Link A and link B, one /31 each.
R2: interfaces, loopback and a prefix to announce
set system host-name 'R2'
set interfaces loopback lo address '198.51.100.2/32'
set interfaces ethernet eth0 address '192.0.2.1/31'
set interfaces ethernet eth0 description 'LINK-A-TO-R1'
set interfaces ethernet eth1 address '192.0.2.3/31'
set interfaces ethernet eth1 description 'LINK-B-TO-R1'
set interfaces dummy dum0 address '203.0.113.1/24'
set interfaces dummy dum0 description 'CUSTOMER-PREFIX'
  • Line 2: R2's loopback.
  • Lines 7–8: A dummy interface holds 203.0.113.0/24, standing in for a customer or LAN prefix.