Category: OSPF

Why Is OSPF (and BGP) More Complex than STP?

I got this question from one of my readers:

Why are OSPF and BGP are more complex than STP from a designer or administrator point of view? I tried everything to come to a conclusion but I couldn’t find a concluded answer, ChatGPT gave a circular loop answer.

There are numerous reasons why a protocol, a technology or a solution might be more complex than another seemingly similar one (or as Russ White would have said, “if you haven’t found the tradeoffs, you haven’t looked hard enough”):

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Building a Small Network with ChatGPT

I must be a good prompt engineer – every time I ask ChatGPT something really simple it spews out nonsense. This time I asked it to build a small network with four routers:

I have a network with four Cisco routers (A,B,C,D). They are connected as follow: A-B, B-C, A-D, D-C. Each router has a loopback interface. Create router configurations that will result in A being able to reach loopback interfaces of all other routers.

Here’s what I got back1:

Here’s an example configuration for the four routers that should allow Router A to reach the loopback interfaces of all other routers:

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OSPF External Routes (Type-5 LSA) Mysteries

Daniel Dib posted a number of excellent questions on Twitter, including:

While forwarding a received Type-5 LSA to other areas, why does the ABR not change the Advertising Router ID to it’s own IP address? If ABR were able to change the Advertising Router ID in the Type-5 LSA, then there would be no need for Type-4 LSA which meant less OSPF overhead on the network.

TL&DR: The current implementation of external routes in OSPF minimizes topology database size (memory utilization)

Before going to the details, try to imagine the environment in which OSPF was designed, and the problems it was solving.

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Running OSPF over Unnumbered Ethernet Interfaces

Remember the unnumbered IP interfaces saga? Let’s conclude it with the final challenge: can we run link-state routing protocols (OSPF or IS-IS) over unnumbered interfaces?

Quick answer: Sure, just use IPv6.

Cheater! IPv6 doesn’t count. There are no unnumbered interfaces in IPv6 – every interface has at least a link-local address (LLA). Even more, routing protocols are designed to run over LLA addresses, including some EBGP implementations, allowing you to build an LLA-only network (see RFC 7404 for details).

OK, what about IPv4?

TL&DR: It works, but…

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LSA/LSP Flooding in OSPF and IS-IS

Peter Paluch loves blogging in microchunks on Twitter ;) This time, he described the differences between OSPF and IS-IS, and gracefully allowed me to repost the explanation in a more traditional format.

My friends, I happen to have a different opinion. It will take a while to explain it and I will have to seemingly go off on a tangent. Please have patience. As a teaser, though: The 2Way state between DRothers does not improve flooding efficiency – in fact, it worsens it.

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Why Do We Need BGP-LS?

One of my readers sent me this interesting question:

I understand that an SDN controller needs network topology information to build traffic engineering paths with PCE/PCEP… but why would we use BGP-LS to extract the network topology information? Why can’t we run OSPF with controller by simulating a software based OSPF instance in every area to get topology view?

There are several reasons to use BGP-LS:

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Unexpected Interactions Between OSPF and BGP

It started with an interesting question tweeted by @pilgrimdave81

I’ve seen on Cisco NX-OS that it’s preferring a (ospf->bgp) locally redistributed route over a learned EBGP route, until/unless you clear the route, then it correctly prefers the learned BGP one. Seems to be just ooo but don’t remember this being an issue?

Ignoring the “why would you get the same route over OSPF and EBGP, and why would you redistribute an alternate copy of a route you’re getting over EBGP into BGP” aspect, Peter Palúch wrote a detailed explanation of what’s going on and allowed me to copy into a blog post to make it more permanent:

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OSPF Inter-Process Route Selection

The traditional wisdom claimed that a Cisco IOS router cannot compare routes between different OSPF routing processes. The only parameter to consider when comparing routes coming from different routing processes is the admin distance, and unless you change the default admin distance for one of the processes, the results will be random.

Following Vladislav’s comment to a decade-old blog post, I decided to do a quick test, and found out that code changes tend to invalidate traditional wisdom. OSPF inter-process route selection is no exception. That’s why it’s so stupid to rely on undefined behavior in your network design, memorize such trivia, test the memorization capabilities in certification labs, or read decades-old blog posts describing arcane behavior.

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Single-Metric Unequal-Cost Multipathing Is Hard

A while ago we discussed whether unequal-cost multipathing (UCMP) makes sense (TL&DR: rarely), and whether we could implement it in link-state routing protocols (TL&DR: yes). Even though we could modify OSPF or IS-IS to support UCMP, and Cisco IOS XR even implemented those changes (they are not exactly widely used), the results are… suboptimal.

Imagine a simple network with four nodes, three equal-bandwidth links, and a link that has half the bandwidth of the other three:

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Packet Forwarding and Routing over Unnumbered Interfaces

In the previous blog posts in this series, we explored whether we need addresses on point-to-point links (TL&DR: no), whether it’s better to have interface or node addresses (TL&DR: it depends), and why we got unnumbered IPv4 interfaces. Now let’s see how IP routing works over unnumbered interfaces.

The Challenge

A cursory look at an IP routing table (or at CCNA-level materials) tells you that the IP routing table contains prefixes and next hops, and that the next hops are IP addresses. How should that work over unnumbered interfaces, and what should we use for the next-hop IP address in that case?

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Unequal-Cost Multipath in Link State Protocols

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.

For a more formal discussion of loop-free alternates and downstream paths, please read RFC 5714 and RFC 5286.
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