IS-IS

Welcome to Heavy Networking from the Packet Pushers.

If you know OSPF pretty well and have been meaning to learn IS-IS for years because, "I know OSPF, how hard can IS-IS be?" But you still haven't gotten around to it, yeah, you found your tribe.

I'm Ethan Banks along with co-host Drew Conry-Murray.

Follow us on LinkedIn and talk to us via the Packet Pushers community Slack group, which you can join for free via packetpushers.net/community.

IP-in-IP VXLAN overlays inherit whatever the underlay's IP routing gives them.

Path selection is left to ECMP hashing over shortest path IGP routes, OSPF, IS-IS, BGP underlay.

You can influence this at the margins, weighted ECMP, some SDN-driven path steering, but you cannot natively say, "This specific flow takes this specific physical path, and this other flow takes a completely different one, regardless of IGP cost." MPLS gives you that natively via RSVP-TE signals explicit resource-reserved LSPs, label-switched paths with bandwidth guarantees computed via CSPF against real-time link utilization, not just topological shortest path.

Segment routing, SR-MPLS, encodes an explicit path as a stack of segment labels, letting a headend steer a flow through specific nodes, links without per-hop signaling state, while still keeping the lightweight MPLS forwarding plane.

When a link or node fails, the local node immediately reroutes onto the pre-signaled backup, typically in under fifty milliseconds without waiting for the control plane to reconverge globally.

IP-in-IP VXLAN overlays are at the mercy of underlay IGP convergence.

When a link fails, one, the IGP, OSPF, IS-IS detects the failure.

Two, it floods the topology change.