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Category: BGP

Next Hops of BGP Routes Reflected by Arista EOS

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)

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BGP Labeled Unicast Interoperability Challenges

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:

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Combining BGP and IGP in an Enterprise Network

Syed Khalid Ali left the following question on an old blog post describing the use of IBGP and EBGP in an enterprise network:

From an enterprise customer perspective, should I run iBGP or iBGP+IGP (OSPF/ISIS/EIGRP) or IGP while doing mutual redistribution on the edge routers. I was hoping if you could share some thoughtful insight on when to select one over the another?

We covered tons of relevant details in the January 2022 Design Clinic, here’s the CliffNotes version. Keep in mind that the road to hell (and broken designs) is paved with great recipes and best practices, and that I’m presenting a black-and-white picture because I don’t feel like transcribing the discussion we had into an oversized blog post. People wrote books on this topic; I’m pretty sure you can search for Russ White and find a few of them.

Finally, there’s no good substitute for understanding how things work (which brings me to another webinar ;).

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BGP Labeled Unicast on Cisco IOS

While researching the BGP RFCs for the Three Dimensions of BGP Address Family Nerd Knobs, I figured out that the BGP Labeled Unicast (BGP-LU, advertising MPLS labels together with BGP prefixes) uses a different address family. So far so good.

Now for the intricate bit: a BGP router might negotiate IPv4 and IPv4-LU address families with a neighbor. Does that mean that it’s advertising every IPv4 prefix twice, once without a label, and once with a label? Should that be the case, how are those prefixes originated and how are they stored in the BGP table?

As always, the correct answer is “it depends”, this time on the network operating system implementation. This blog post describes Cisco IOS behavior, a follow-up one will focus on Arista EOS.

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Should We Use LISP?

LISP started as yet-another ocean-boiling project focused initially on solving the “we use locators as identifiers” mess (not quite), and providing scalable IPv6 connectivity over IPv4-only transport networks by adding another layer of indirection and thus yet again proving RFC 1925 rule 6a. At least those are the diagrams I remember from the early “look at this wonderful tool” presentations explaining for example how Facebook is using LISP to deploy IPv6 (more details in this presentation).

Somehow that use case failed to gain traction and so the pivots1 started explaining how one can use LISP to solve IP mobility or IP multihoming or live VM migration, or to implement IP version of conversational learning in Cisco SD-Access. After a few years of those pivots, I started dismissing LISP with a short “cache-based forwarding never worked well” counterargument.

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Worth Reading: Performance Testing of Commercial BGP Stacks

For whatever reason, most IT vendors attach “you cannot use this for performance testing and/or publish any results” caveat to their licensing agreements, so it’s really hard to get any independent test results that are not vendor-sponsored and thus suitably biased.

Justin Pietsch managed to get a permission to publish test results of Junos container implementation (cRPD) – no surprise there, Junos outperformed all open-source implementations Justin tested in the past.

What about other commercial BGP stacks? Justin did the best he could: he published Testing Commercial BGP Stacks instructions, so you can do the measurements on your own.

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Running BGP between Virtual Machines and Data Center Fabric

Got this question from one of my readers:

When adopting the BGP on the VM model (say, a Kubernetes worker node on top of vSphere or KVM or Openstack), how do you deal with VM migration to another host (same data center, of course) for maintenance purposes? Do you keep peering with the old ToR even after the migration, or do you use some BGP trickery to allow the VM to peer with whatever ToR it’s closest to?

Short answer: you don’t.

Kubernetes was designed in a way that made worker nodes expendable. The Kubernetes cluster (and all properly designed applications) should recover automatically after a worker node restart. From the purely academic perspective, there’s no reason to migrate VMs running Kubernetes.

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Three Dimensions of BGP Address Family Nerd Knobs

Got into an interesting BGP discussion a few days ago, resulting in a wild chase through recent SRv6 and BGP drafts and RFCs. You might find the results mildly interesting ;)

BGP has three dimensions of address family configurability:

  • Transport sessions. Most vendors implement BGP over TCP over IPv4 and IPv6. I’m sure there’s someone out there running BGP over CLNS1, and there are already drafts proposing running BGP over QUIC2.
  • Address families enabled on individual transport sessions, more precisely a combination of Address Family Identifier (AFI) and Subsequent Address Family Identifier.
  • Next hops address family for enabled address families.
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Feedback: Recursive BGP Next Hop Resolution

The Recursive BGP Next Hops: an RFC 4271 Quirk blog post generated tons of feedback (thanks a million to everyone writing a comment on my blog or LinkedIn).

Starting with Robert Razsuk who managed to track down the original email that triggered the (maybe dubious) text in RFC 4271:

The text in section 5.1.3 was not really targeting to prohibit load balancing. Keep in mind that it is FIB layer which constructs actual forwarding paths.

The text has been suggested by Tom Petch in discussion about BGP advertising valid paths or even paths it actually installs in the RIB/FIB. The entire section 5.1.3 is about rules when advertising paths by BGP.

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Recursive BGP Next Hops: an RFC 4271 Quirk

All BGP implementations I’ve seen so far use recursive next hop lookup:

  • The next hop in the IP routing table is the BGP next hop advertised in the incoming update
  • That next hop is resolved into the actual next hop using one or more recursive lookups into the IP routing table.

Furthermore, all BGP implementations I’ve seen used multiple recursive next hops (if available) to implement load balancing toward the BGP next hop – that’s how we made EBGP load balancing work in Stone Age of networking.

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Highlights: Dynamic Negotiation of BGP Capabilities

The Dynamic Negotiation of BGP Capabilities blog post generated almost no comments, apart from the #facepalm realization that a certain network operating system resets IBGP sessions when the sole EBGP session goes down, but there were a few interesting comments on LinkedIn and Twitter.

While most engineers easily relate to the awkwardness of bringing down a BGP session to enable new functionality (Tearing down BGP session, as a solution reminds me rebooting a host, as a solution.), it’s not as easy as it looks. As Adam Chappell put itDynamic capability renegotiation does tend to sound a bit like changing the tyres while still moving. Very neat if you can pull it off but so much to go wrong…

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