Imagine you want to create a simple multi-site network with netlab:

• The lab should have two sites (A and B).
• Each site has a layer-3 switch, a single VLAN (VLAN 100), and two hosts connected to that VLAN.
• As you don’t believe in the magic powers of stretched VLANs, you have a layer-3 (IPv4) link between sites.

A simplistic attempt to model this network would define a single VLAN and use it on both sites:

nodes: [ s1, s2, h1, h2, h3, h4 ]

groups: 
  switches:
    members: [ s1, s2 ]
    module: [ vlan, ospf ]
    device: eos
  hosts:
    members: [ h1, h2, h3, h4 ]
    device: linux

vlans:
  blue:
    links: [ s1-h1, s1-h2, s2-h3, s2-h4 ]

links: [ s1-s2 ]

Notes:

• The lab has six devices (line 1).
• The devices belong to two groups. Switches are EOS devices using VLANs and OSPF (lines 4-7). Hosts are running Linux (lines 8-10).
• The lab has a single VLAN (lines 12-13) with four access links (line 14).
• The two site switches have an extra WAN link (line 16).

This approach has just a tiny little problem: a single VLAN has one IP prefix, and so netlab assigns the same IP prefix to both sites. Here’s the addressing report to prove it:

Node/Interface	IPv4 Address	IPv6 Address	Description
h1
eth1	172.16.0.3/24		h1 -> [s1,h2,h3,s2,h4]
h2
eth1	172.16.0.4/24		h2 -> [h1,s1,h3,s2,h4]
h3
eth1	172.16.0.5/24		h3 -> [h1,s1,h2,s2,h4]
h4
eth1	172.16.0.6/24		h4 -> [h1,s1,h2,h3,s2]
s1	10.0.0.1/32		Loopback
Ethernet1	10.1.0.1/30		s1 -> s2
Ethernet2			[Access VLAN blue] s1 -> h1
Ethernet3			[Access VLAN blue] s1 -> h2
Vlan1000	172.16.0.1/24		VLAN blue (1000) -> [h1,h2,h3,s2,h4]
s2	10.0.0.2/32		Loopback
Ethernet1	10.1.0.2/30		s2 -> s1
Ethernet2			[Access VLAN blue] s2 -> h3
Ethernet3			[Access VLAN blue] s2 -> h4
Vlan1000	172.16.0.2/24		VLAN blue (1000) -> [h1,s1,h2,h3,h4]

Site-Specific VLANs

Back to the drawing board. We could define two VLANs (one per site) to get two IP prefixes:

nodes: [ s1, s2, h1, h2, h3, h4 ]

groups: 
  switches:
    members: [ s1, s2 ]
    module: [ vlan, ospf ]
    device: eos
  hosts:
    members: [ h1, h2, h3, h4 ]
    device: linux

vlans:
  blue_a:
    links: [ s1-h1, s1-h2 ]
  blue_b:
    links: [ s2-h3, s2-h4 ]

links: [ s1-s2 ]

Notes:

• We defined two VLANs (Blue VLAN on site A and site B), each with two access links (lines 13-16)

This topology results in the desired addressing scheme but uses different VLAN IDs on each site (netlab automatically assigns VLAN IDs). The interface descriptions in the addressing reports clearly prove we have VLAN 1000 on one site and VLAN 1001 on the other.

Node/Interface	IPv4 Address	IPv6 Address	Description
h1
eth1	172.16.0.3/24		h1 -> [s1,h2]
h2
eth1	172.16.0.4/24		h2 -> [h1,s1]
h3
eth1	172.16.1.5/24		h3 -> [s2,h4]
h4
eth1	172.16.1.6/24		h4 -> [h3,s2]
s1	10.0.0.1/32		Loopback
Ethernet1	10.1.0.1/30		s1 -> s2
Ethernet2			[Access VLAN blue_a] s1 -> h1
Ethernet3			[Access VLAN blue_a] s1 -> h2
Vlan1000	172.16.0.1/24		VLAN blue_a (1000) -> [h1,h2]
s2	10.0.0.2/32		Loopback
Ethernet1	10.1.0.2/30		s2 -> s1
Ethernet2			[Access VLAN blue_b] s2 -> h3
Ethernet3			[Access VLAN blue_b] s2 -> h4
Vlan1001	172.16.1.2/24		VLAN blue_b (1001) -> [h3,h4]

Static VLAN IDs

Fortunately, netlab allows you to specify static VLAN IDs and does not check whether they overlap. We can thus set the same vlan.id on both VLANs:

nodes: [ s1, s2, h1, h2, h3, h4 ]

groups: 
  switches:
    members: [ s1, s2 ]
    module: [ vlan, ospf ]
    device: eos
  hosts:
    members: [ h1, h2, h3, h4 ]
    device: linux

vlans:
  blue_a:
    links: [ s1-h1, s1-h2 ]
    id: 100
  blue_b:
    links: [ s2-h3, s2-h4 ]
    id: 100

links: [ s1-s2 ]

Let’s redo the addressing report, this time using netlab report addressing.md --node s1,s2 to limit the report to the two switches:

Node/Interface	IPv4 Address	IPv6 Address	Description
s1	10.0.0.1/32		Loopback
Ethernet1	10.1.0.1/30		s1 -> s2
Ethernet2			[Access VLAN blue_a] s1 -> h1
Ethernet3			[Access VLAN blue_a] s1 -> h2
Vlan100	172.16.0.1/24		VLAN blue_a (100) -> [h1,h2]
s2	10.0.0.2/32		Loopback
Ethernet1	10.1.0.2/30		s2 -> s1
Ethernet2			[Access VLAN blue_b] s2 -> h3
Ethernet3			[Access VLAN blue_b] s2 -> h4
Vlan100	172.16.1.2/24		VLAN blue_b (100) -> [h3,h4]

As you can see, we have two VLANs (blue_a and blue_b) with independent IP prefixes (172.16.0.0/24 and 172.16.1.0/24) but with the same VLAN ID (100). Mission Accomplished ;)

Credits

This blog post is based on a discussion idea by @astrotokii.