A few weeks ago I described why EBGP TCP packets have TTL set to one (unless you configured EBGP multihop). Although some people claim that (like NAT) it could be a security feature, it’s not a good one. Generalized TTL Security Mechanism (GTSM, described in RFC 5082) is much better.

Most BGP implementations set TTL field in outgoing EBGP packets to one. That prevents a remote intruder that manages to hijack a host route to an adjacent EBGP peer from forming a BGP session as the TCP replies get lost the moment they hit the first router in the path.

Chris Parker wrote a wonderful blog post going deep into the weeds on how EBGP sessions use IP TTL and why we need multihop EBGP sessions between adjacent devices. However, he couldn’t find a source explaining why early BGP implementations decided to use IP TTL set to one on EBGP sessions:

If there’s a source on the internet that explains when it was decided that EBGP should use a TTL of 1, I can’t find it. I can’t even find it in any RFC. I looked in the RFC for BGP v4, and went all the way back to BGP v1. None of these documents contain the text “TTL or “time to live” or “time-to-live.” It’s not even in the RFC for EGP, back in 1984.

Two weeks ago I explained why you might want to run IBGP between CE-routers on a multihomed site. One of the blog readers didn’t like my ideas:

In such a small deployment I assume that both ISPs offer transit, so that both CEs would get a default route from their upstream.

In this case I would not iBGP the CEs together but have HSRP running on the two CEs and track the uplink (interface and/of BGP session) to determine the active gateway.

Let’s see what could possibly go wrong with that design.

One of my readers sent me a question along these lines:

Do I have to have an IBGP session between Customer Edge (CE) routers in a multihomed site if they run EBGP with the upstream provider(s)?

Let’s start with a simple diagram and a refactoring of the question:

I like using Cisco IOS for my routing protocol virtual labs¹. It uses a trivial amount of memory² and boots relatively fast. There was just one thing that kept annoying me: Cisco IOS release 15.x takes forever to install local routes in the BGP table and even longer to select the best routes and propagate them³.

I finally found the culprit: bgp update-delay nerd knob. Here’s what the documentation has to say about it:

The ipSpace.net Design Clinic has been running for a bit over than a year. We covered tons of interesting technologies and design challenges, resulting in over 13 hours of content (so far), including several BGP-related discussions:

• BGP route servers
• Redundant BGP-Based Internet Access
• Secure BGP Configuration on Customer Routers
• Enterprise WAN Routing Design

All the Design Clinic discussions are available with Standard or Expert ipSpace.net Subscription, and anyone can submit new design/discussion challenges.

Every time I mention unnumbered BGP sessions in a webinar, someone inevitably asks “and how exactly does that work?” I always replied “gee, that’s a blog post I should write one of these days,” and although some readers might find it long overdue, here it is ;)

We’ll work with a simple two-router lab with two parallel unnumbered links between them. Both devices will be running Cumulus VX 4.4.0 (FRR 8.4.0 container generates almost identical printouts).

Bela Varkonyi left two intriguing comments on my Leave BGP Next Hops Unchanged on Reflected Routes blog post. Let’s start with:

The original RR design has a lot of limitations. For usual enterprise networks I always suggested to follow the topology with RRs (every interim node is an RR), since this would become the most robust configuration where a link failure would have the less impact.

He’s talking about the extreme case of hierarchical route reflectors, a concept I first encountered when designing a large service provider network. Here’s a simplified conceptual diagram (lines between boxes are physical links as well as IBGP sessions between loopback interfaces):

Here’s the last question I’ll answer from that long list Daniel Dib posted weeks ago (answer to Q1, answer to Q2).

I am trying to understand what made the BGP designers decide that RR should not change the BGP Next Hop for IBGP-learned routes.

I’m always in a bit of a bind when I get an invitation to speak at a security conference (after all, I know just enough about security to make a fool of myself), but when the organizers of the DEEP Conference invited me to talk about Internet routing security I simply couldn’t resist – the topic is dear and near to my heart, and I planned to do a related webinar for a very long time.

Even better, that conference would have been my first on-site presentation since the COVID-19 craze started, and I love going to Dalmatia (where the conference is taking place). Alas, it was not meant to be – I came down with high fever just days before the conference and had to cancel the talk.

Here’s another question from the excellent list posted by Daniel Dib on Twitter:

BGP Split Horizon rule says “Don’t advertise IBGP-learned routes to another IBGP peer.” The purpose is to avoid loops because it’s assumed that all of IBGP peers will be on full mesh connectivity. What is the reason the BGP protocol designers made this assumption?

Time for another history lesson. BGP was designed in late 1980s (RFC 1105 was published in 1989) as a replacement for the original Exterior Gateway Protocol (EGP). In those days, the original hub-and-spoke Internet topology with NSFNET core was gradually replaced with a mesh of interconnections, and EGP couldn’t cope with that.

Most BGP implementations I’ve worked with split the neighbor BGP configuration into two parts:

• Global configuration that creates the transport session
• Address family configuration that activates the address family across a configured transport session, and changes the parameters that affect BGP updates

AS numbers, source interfaces, peer IPv4/IPv6 addresses, and passwords clearly belong to the global neighbor configuration.

Nicola Modena created an interesting presentation describing IBGP designs using BGP Additional Paths and Optimal Route Reflection functionality

Hope you’ll enjoy the presentation as much as I did… and make sure you understand potential circular dependencies you might be introducing when running a route reflector as a virtual machine.

Imagine a suboptimal design in which:

• A BGP route reflector also servers as an AS edge (PE) router¹;
• You want to use next-hop-self on AS edge routers.

Being exposed to Cisco IOS for decades, I considered that to be a no-brainer. After all, section 10 of RFC 4456 is pretty specific:

In addition, when a RR reflects a route, it SHOULD NOT modify the following path attributes: NEXT_HOP, AS_PATH, LOCAL_PREF, and MED.

Arista EOS is different – a route reflector happily modifies NEXT_HOP on reflected routes (but then, did you notice the “SHOULD NOT” wording?²)

Jeff Tantsura left me tantalizing hint after reading the BGP Labeled Unicast on Cisco IOS blog post:

Read carefully “Relationship between SAFI-4 and SAFI-1 Routes” section in RFC 8277

The start of that section doesn’t look promising (and it gets worse):

It is possible that a BGP speaker will receive both a SAFI-1¹ route for prefix P and a SAFI-4² route for prefix P. Different implementations treat this situation in different ways.

Now for the details: