Category: IP routing

James got confused by a statement made by Hannes Gredler in his IS-IS book:

Things behave really badly if the total IGP cost over the tunnel undermines the total topologies’ cost. What happens next is that the tunnel “wraps” around itself, ultimately causing a meltdown of the entire network.

Let’s unpack that, starting with “Why would you need a tunnel?”

As a response to my LISP vs EVPN: Mobility in Campus Networks blog post, Route Abel provided interesting real-life details of a large-scale campus wireless testing using EVPN and VXLAN tunnels to a central aggregation point (slightly edited):

I was arguing for VxLAN EVPN with some of my peers, but I had no direct hands-on knowledge of how it would actually perform and very limited ability to lab it on hardware. My client was considering deploying Campus VxLAN, and they have one of the largest campuses in North America.

This LinkedIn snippet just came in from the someone is not exactly right on the Internet department:

Unlike IGP protocols, BGP is not dependent on a single type of metric to choose the best path.

EIGRP is an immediate counterexample that brought the above quote to my attention, but it’s worth exploring the topic in more detail.

In the Path Failure Detection on Multi-Homed Servers blog post, I mentioned running BGP on servers as one of the best ways to detect server-to-network failures. As always, things aren’t as simple as they look, as Cathal Mooney quickly pointed out:

One annoyance is what IP address gets used by default by the system for outbound traffic. It would be nice to have a generic OS-level way to say, “This IP on lo0 should be default for outbound IP traffic unless to the connected link subnet itself.”

That’s definitely a tough nut to crack, and Cathal described a few solutions he used in the past:

One of my readers wanted to use EIBGP to load balance outgoing traffic from a pair of WAN edge routers (hint: wrong tool for this particular job¹). He’s using a design very similar to this one with VRRP running between WAN edge routers, and the adjacent firewall cluster using a default route to the VRRP IP address.

The problem: all output traffic goes to the VRRP IP address which is active on one of the switches, and only a single uplink is used for the outgoing traffic.

After I published the Source IP Address in Multicast Packets blog post, Erik Auerswald sent me several examples of network devices sending IP packets with source IP address set to 0.0.0.0:

• Cisco wireless access points using 0.0.0.0 as the source IP address in VRRP packets.
• Extreme (formerly Avaya) switches sending IGMP queries with source IP address 0.0.0.0 on VLANs on which they have no IP address.

Security researches found another ICMP redirect SNAFU: a malicious wireless client can send redirects on behalf of the access point redirecting another client’s traffic to itself.

I’m pretty sure the same trick works on any layer-2 technology; the sad part of this particular story is that the spoofed ICMP packet traverses the access point, which could figure out what’s going on and drop the packet. Unfortunately, most of the access points the researchers tested were unable to do that due to limitations in the NPUs (a fancier word for SmartNIC) they were using.

After introducing the routing protocols and explaining the basics of link-state routing it was time for implementation considerations including:

• Collecting local endpoint reachability information
• Finding neighbors and exchanging the collected information (hint: a link-state topology database is just a distributed key-value store)
• Running the SPF algorithm (including partial SPF details) and installing the results

One of my readers sent me this (paraphrased) question:

What I have seen in my network are multicast packets with the IP source address set to 0.0.0.0 and source port set to 0. Is that considered acceptable? Could I use a multicast IP address as a source address?

TL&DR: **** NO!!!

It also seemed like a good question to test ChatGPT, and this time it did a pretty good job.

I mentioned IP source address validation (SAV) as one of the MANRS-recommended actions in the Internet Routing Security webinar but did not go into any details (as the webinar deals with routing security, not data-plane security)… but I stumbled upon a wonderful companion article published by RIPE Labs: Why Is Source Address Validation Still a Problem?.

The article goes through the basics of SAV, best practices, and (most interesting) using free testing tools to detect non-compliant networks. Definitely worth reading!

After figuring out how DHCP relaying works and testing it with VRFs and in VXLAN segments, it seems like a no-brainer to make it work with EVPN.

TL&DR: It works, at least when using Arista vEOS as the relay and Cisco CSR 1000v as the DHCP server.

Lab Topology

We’ll keep using the exact same “physical” topology we used in the VXLAN DHCP relaying lab, add EVPN and BGP to the control-plane cocktail, and put the VXLAN segment into a VRF. We’ll use CSR 1000v as the DHCP server because Cisco IOSv doesn’t support some of the DHCP option-82 sub-options we need.

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.