The beauty of SR-MPLS is that it’s a drop-in replacement for the traditional (LDP- or RSVP-based) MPLS control plane. For example, you could replace LDP with SR-MPLS in a network using MPLS to implement a BGP-free transport core, and it just keeps working.

This scenario was the first “fun” scenario in the ITNOG10 Segment Routing workshop. The core network uses the same topology as in the previous examples; I added two hosts and BGP routing.

Lab Topology

I had to make significant changes to the lab topology to keep it structured.

I used groups to apply the same set of attributes to similar nodes (hosts, PE routers, the core router). The _auto_create flag tells netlab to create the group members without them being listed in the nodes dictionary.

groups:
  _auto_create: True
  core:
    members: [ p1 ]
    module: [ isis, sr ]
  edge:
    members: [ pe1, pe2 ]
    module: [ isis, bgp, sr ]
  hosts:
    members: [ ha, hb ]
    device: linux

• The core router (P1) runs IS-IS and SR-MPLS
• The edge routers (PE1, PE1) run IS-IS, BGP, and SR-MPLS
• The hosts are Linux containers

Similarly, I used link groups to apply consistent attributes to multiple links:

- group: core
  members: [ pe1-p1, p1-pe2 ]
- group: edge
  isis: False
  bgp.advertise: True
  members: [ ha-pe1, hb-pe2 ]

• netlab automatically enables IS-IS on links between routers in the same BGP AS.
• It would also enable IS-IS on the edge (host-to-PE) links, so we have to disable IS-IS on those links (we want them advertised only in BGP).
• The edge links have to be advertised in BGP, so we’re using the bgp.advertise flag to ensure they will be.

Finally, we have to specify the BGP AS we want to use with the bgp.as topology attribute. The full lab topology file is here.

Exploring BGP-Free Core

After setting up netlab, changing into the 2-fun/1-bgp-free directory, and executing netlab up, you’ll have BGP running across an SR-MPLS network. You can do the same checks we did for the original IS-IS lab and should get the same results. The fun starts when we look at the BGP routing table entries. These entries should have an MPLS label derived from SR-MPLS:

pe1#show ip route bgp | begin 172.16 B I      172.16.1.0/24 [200/0]           via 10.0.0.3/32, IS-IS SR tunnel index 1              via 10.1.0.1, Ethernet1, label 900003

Not surprisingly, once the PE routers have MPLS labels attached to the BGP routes, we get end-to-end connectivity even though the P router does not have the BGP routes in its routing table:

$ netlab connect haConnecting to container clab-bgpfree-ha, starting bashha:/# ping -c 3 hbPING hb (172.16.1.5): 56 data bytes64 bytes from 172.16.1.5: seq=0 ttl=61 time=2.872 ms64 bytes from 172.16.1.5: seq=1 ttl=61 time=2.480 ms64 bytes from 172.16.1.5: seq=2 ttl=61 time=2.777 ms--- hb ping statistics ---3 packets transmitted, 3 packets received, 0% packet lossround-trip min/avg/max = 2.480/2.709/2.872 ms

p1#show ip route | begin GatewayGateway of last resort is not set C        10.0.0.1/32           directly connected, Loopback0 I L2     10.0.0.2/32 [115/20]           via 10.1.0.2, Ethernet1 I L2     10.0.0.3/32 [115/20]           via 10.1.0.6, Ethernet2 C        10.1.0.0/30           directly connected, Ethernet1 C        10.1.0.4/30           directly connected, Ethernet2

Try It Out

The workshop GitHub repository includes the installation guidelines; you might want to read them first. After that, you can:

• Start a GitHub Codespace
• Import an Arista cEOS container into it (alternate step-by-step instructions)
• Change directory to 2-fun/1-bgp-free
• Execute netlab up
• Have fun