Category: Ip Routing

A few weeks ago we covered transparent bridging fundamentals, now it’s time to recap IP routing fundamentals… and then we’ll be ready to compare the two.

In the world of ubiquitous Ethernet and IP, it’s common to think that one needs addresses in packet headers in every layer of the protocol stack. We have MAC addresses, IP addresses, and TCP/UDP port numbers… and low-level addresses are assigned to individual interfaces, not nodes.

Turns out that’s just one option… and not exactly the best one in many scenarios. You could have interfaces with no addresses, and you could have addresses associated with nodes, not interfaces.

Last week I described the new features added to netsim-tools release 0.4, including support for unnumbered interfaces and OSPF routing. Now let’s see how I used them to build a multi-vendor lab to test which platforms could be made to interoperate when running OSPF over unnumbered Ethernet interfaces.

TL&DR: You get unequal-cost multipath for free with distance-vector routing protocols. Implementing it in link state routing protocols is an order of magnitude more CPU-consuming.

Continuing our exploration of the Unequal-Cost Multipath world, why was it implemented in EIGRP decades ago, but not in OSPF or IS-IS?

Ignoring for the moment the “does it make sense” dilemma: finding downstream paths (paths strictly shorter than the current best path) is a side effect of running distance vector algorithms.

In the previous video in this series, I described how path discovery works in source routing and virtual circuit environments. I couldn’t squeeze the discussion of hop-by-hop forwarding into the same video (it would make the video way too long); you’ll find it in the next video in the same section.

One of my readers sent me this question:

My job required me to determine if one IP address is unicast or anycast. Is it possible to get this information from the bgp dump?

TL&DR: Not with anything close to 100% reliability. An academic research paper (HT: Andrea di Donato) documents a false-positive rate of around 10%.

If you’re not familiar with IP anycast: it’s a brilliant idea of advertising the same prefix from multiple independent locations, or the same IP address from multiple servers. Works like a charm for UDP (that’s how all root DNS servers are built) and supposedly pretty well across distant-enough locations for TCP (with a long list of caveats when used within a data center).

Every now and then I’m getting questions along the lines “why doesn’t X support unequal-cost multipathing (UCMP)?” for X in [ OSPF, BGP, IS-IS ].

To set the record straight: BGP does support some rudimentary form of unequal-cost multipathing with the DMZ Bandwidth community, but it only works across multiple egress points from a single autonomous system. Follow-up nerd knobs described how to use the same community over EBGP sessions; not sure whether anyone implemented that part (comments welcome).

Now that we know what regions and availability zones are, let’s go back to Daniel Dib’s question:

As I understand it, subnets in Azure span availability zones. Do you see any drawback to this? Does subnet matter if your VMs are in different AZs?

Wait, what? A subnet is stretched across multiple failure domains? Didn’t Ivan claim that’s ridiculous?

TL&DR: What I claimed was that a single layer-2 network is a single failure domain. Things are a bit more complex in public clouds. Keep reading and you’ll find out why.

Regardless of the technology used to get packets across the network, someone has to know how to get from sender to receiver(s), and as always, you have multiple options:

• Almighty controller
• On-demand dynamic path discovery (example: probing)
• Participation in a routing protocol

For more details, watch Finding Paths Across the Network video.

This podcast introduction was written by Nick Buraglio, the host of today’s podcast.

In today’s evolving landscape of whitebox, brightbox, and software routing, a small but incredibly comprehensive routing platform called FreeRTR has quietly been evolving out of a research and education service provider network in Hungary. 

Kevin Myers of IPArchitechs brought this to my attention around March of 2019, at which point I went straight to work with it to see how far it could be pushed.