Engineering Deathmatch

S1E3 I Heart Routers

The engineers are asked by I Heart Routers, a high end woodworking tool and cloud services company, to troubleshoot routing issues in their network.  A true cross-continental battle with Daniel Dib from Sweden and Steve Occhiogrosso from the US.  This episode was sponsored by Skyline ATS.

Scenario and Solution

The challenge details can be found at:  or for challenge details and starting router configurations, go to:

For this challenge, the OSPF network type is important because CAT1 is set to “switchport protected” on the ports connecting to router 2 and 3.  This means frames will not forward directly between these ports, so we need router 1 to act as a hub and proxy the traffic between the two routers.  By setting the network type to point-to-multipoint, router 2 will use router 1 as the next-hop to reach router 3 (and vice versa).  This brings iBGP peering up and resolves the first break in the topology.

Part 2 involves fixing BGP with the backbone.  Router 2 and 3 are advertising routes out to the backbone, but due to how BGP is designed, they will not receive routes with their own AS in path.  To bypass this behavior, you must set “allowas-in” option for backbone neighbors.  Note, that you can specify a number after allowas-in, which is an acceptable solution.

Commands that can be input to resolve this scenario:

[[Part 1]]


int fa0/0
ip ospf network point-to-multipoint


int fa0/0.100
ip ospf network point-to-multipoint


int fa0/0.100
ip ospf network point-to-multipoint

[Part 2 – Use allow-as in facing Backbone]

[[R2 Config]]

router bgp 64123
address-family ipv4 vrf PRIVATE
neighbor allowas-in

[[R3 Config]]

router bgp 64123
address-family ipv4
neighbor allowas-in



  1. Excellent. I would like to see a topology drawing with the lab documentation.

    • I’ll see what I can whip up, but simply put, the layer 3 switch has the R1 and R2 devices as well as the ISP router hooked up directly to it.