Category:  IP routing

After I got the testing infrastructure in place (simple DHCP relay, VRF-aware DHCP relay), I was ready for the real fun: DHCP relaying in VXLAN (and later EVPN) segments.

TL&DR: It works exactly as expected. Even though I had anycast gateway configured on the VLAN, the Arista vEOS switches used their unicast IP addresses in the DHCP relaying process. The DHCP server had absolutely no problem dealing with multiple copies of the same DHCP broadcast relayed by different switches attached to the same VLAN. One could only wish things were always as easy in the networking land.

After figuring out how DHCP relaying works and testing it in a simple lab, I went a step further and tested VRF-aware DHCP relaying.

Lab Topology

I had to make just a few changes to the DHCP relaying lab topology:

• DHCP server is running on CSR 1000v. IOSv DHCP server does not support subnet selection DHCP option and thus doesn’t work with relays that do inter-VRF DHCP relaying.
• I put the link between the DHCP client and DHCP relay into a VRF.

After figuring out how DHCP relaying works, I decided to test it out in a lab. netlab has no DHCP configuration module (at the moment); the easiest way forward seemed to be custom configuration templates combined with a few extra attributes.

Lab Topology

This is how I set up the lab:

Chinar Trivedi asked an interesting question about DHCP relaying in VXLAN/EVPN world on Twitter and my first thought was “that shouldn’t be hard” but when I read the first answer that turned into “wait a minute, how exactly does DHCP relaying works?”

I’m positive there’s a tutorial out there somewhere, but I decided to go back to the sources of wisdom: the RFCs. It turned out to be a long walk down the IETF history lane.

The Routing Protocols Overview part of How Networks Really Work webinar introduced the concepts of distance-vector and link-state routing protocols. Next step: the basics of link-state routing protocols.

The simplest way to implement layer-3 forwarding in a network fabric is to offload it to an external device¹, be it a WAN edge router, a firewall, a load balancer, or any other network appliance.

In 2018 I tried to figure out whether the rush to deploy new routing protocols in leaf-and-spine fabrics is anything more than another blob of hype (RIFT, OpenFabric, BGP), considering OSPF got the job done for AWS. Those discussions probably sounded like a bunch of smart kids trying to measure outside temperature with a moist finger, so the only recommendation I could give in 2021 was “use the best tool for the job, keeping in mind you’re not Google or Microsoft”

It’s always better to measure than to have opinions, and a group of academics did just that. They developed Sybil – a tool to measure routing protocol performance in leaf-and-spine fabrics – and Dip Singh used it to compare BGP to IS-IS and OpenFabric.

It took vendors like Cisco years to start supporting routing protocols between MLAG-attached routers and a pair of switches in the MLAG cluster. That seems like a no-brainer scenario, so there must be some hidden complexities. Let’s figure out what they are.

We’ll use the familiar MLAG diagram, replacing one of the attached hosts with a router running a routing protocol with both members of the MLAG cluster (for example, R, S1, and S2 are OSPF neighbors).

A while ago, I wrote a blog post explaining why we should (mostly) disable ICMP redirects, triggering a series of comments discussing the root cause of ICMP redirects. A few of those blamed static routes, including:

Put another way, the presence or absence of ICMP Redirects is a red herring, usually pointing to architectural/design issues instead. In this example, using vPC Peer Gateway or, better yet, running a minimal IGP instead of relying on static routes eliminates ICMP Redirects from both the problem and solution spaces simultaneously.

Unfortunately, that’s not the case. You can get suboptimal routing that sometimes triggers ICMP redirects in well-designed networks running more than one routing protocol.

Imagine you built a layer-2 fabric with tons of VLANs stretched all over the place. Now the users want to exchange traffic between those VLANs, and the obvious question is: which devices should do layer-2 forwarding (bridging) and which ones should do layer-3 forwarding (routing)?

There are four typical designs you can use to solve that challenge:

• Exchange traffic between VLANs outside of the fabric (edge routing)
• Route on core switches (centralized routing)
• Route on ingress (asymmetric IRB)
• Route on ingress and egress (symmetric IRB)

This blog post is an overview of the design models; we’ll cover each design in a separate blog post.

After discussing network addressing and switching, routing, and bridging in the How Networks Really Work webinar, it was high time for a deep dive into routing protocols, starting (as always) with an overview.

Mark Seery wrote a fantastic must-read article explaining why routing will never be a solved problem.

You might want to enjoy it as a relaxing antidote after a painful exposure to SD-WAN (or SD-something-else) brainwashing.

Early bridges implemented a single bridging domain across all ports. Within a few years, we got multiple bridging domains within a single device (including bridging implementation in Cisco IOS). The capability to have multiple bridging domains stretched across several devices was still missing… until the modern-day Pandora opened the VLAN box and forever swamped us in the complexities of large-scale bridging.

Network terminology was easy in the 1980s: bridges forwarded frames between Ethernet segments based on MAC addresses, and routers forwarded network layer packets between network segments. That nirvana couldn’t last long; eventually, a big-enough customer told Cisco: “I don’t want to buy another box if I already have your too-expensive router. I want your router to be a bridge.”

Turning a router into a bridge is easier than going the other way round¹: add MAC table and dynamic MAC learning, and spend an evening implementing STP.

When I started implementing the netlab VLAN module, I encountered (at least) three different ways of configuring physical interfaces and bridging domains even though the underlying packet forwarding operations (and sometimes even the forwarding hardware) are the same. That confusopoly is guaranteed to make your head spin for years, and the only way to figure out what’s going on behind the scenes is to go back to the fundamentals.