Blog Posts in May 2012

Equal-Cost Multipath in Brocade’s VCS Fabric

Understanding equal-cost multipathing in Brocade’s VCS Fabric is a bit tricky, not because it would be a complex topic, but because it’s a bit counter-intuitive (while still being perfectly logical once you understand it). Michael Schipp tried to explain how it works, Joel Knight went even deeper, and I’ll try to draw a parallel with the routed networks because most of us understand them better than the brave new fabric worlds.

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ARP reply with multicast sender MAC address is indeed illegal

A while ago I was writing about the behavior of Microsoft’s Network Load Balancing, the problems it’s causing and how Microsoft tried to hack around them using multicast MAC addresses as the hardware address of sender in ARP replies (which is illegal). A few days ago one of my readers asked me whether I know which RFC prohibits the use of multicast MAC address in ARP replies.

A quick consultation with friendly Google search engine returned this web page, which contained the answer: section 3.3.2 of RFC 1812 (Requirements for IP Version 4 Routers):

A router MUST not believe any ARP reply that claims that the Link Layer address of another host or router is a broadcast or multicast address.

Problem solved – now I know the real reason we have to configure static ARP entries on Cisco routers and switches.

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Layer-2 Network Is a Single Failure Domain

This topic has been on my to-write list for over a year and its working title was phrased as a question, but all the horror stories you’ve shared with me over the last year or so (some of them published in my blog) have persuaded me that there’s no question – it’s a fact.

If you think I’m rephrasing the same topic ad nauseam, you’re right, but every month or so I get an external trigger that pushes me back to the same discussion, this time an interesting comment thread on Massimo Re Ferre’s blog.

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Goodbye Echo, I’ll miss you!

Some of you have noticed that I’d changed the commenting system on my blog recently. Here’s the full story (with a question for you at the very end).

I was totally fed up with Blogger comments years ago and decided to look for an alternative. JS-Kit was a perfect solution and it even allowed me to import Blogger comments and synchronize new entries with Blogger (so I could turn it off at any time and retain my comments).

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HTTP-over-IPv6 on Cisco IOS

Stumbled across this marvel while updating my IPv6 presentations for a 2-day seminar in Milano and Rome (straight from 15.2M&T command reference):

With IPv6 support added in Cisco IOS Release 12.2(2)T, the ip http server command simultaneously enables and disables both IP and IPv6 access to the HTTP server. However, an access list configured with the ip http access-class command will only be applied to IPv4 traffic. IPv6 traffic filtering is not supported.

Wait ... WHAT? I cannot control who can access the HTTP(S) server running in Cisco IOS over IPv6 (apart from kludges like ingress ACLs on all interfaces or CoPP), and this stupidity has been left unfixed for nine(9) years?. Are we really in 2012, less than a month away from World IPv6 Launch or have I been transported to 1990’s?

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OpenFlow @ Google: Brilliant, but not revolutionary

Google unveiled some details of its new internal network at Open Networking Summit in April and predictably the industry press and OpenFlow pundits exploded with the “this is the end of the networking as we know it” glee. Unfortunately I haven’t seen a single serious technical analysis of what it is they’re actually doing and how different their new network is from what we have today.

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Are Fixed Switches More Efficient Than Chassis Ones?

Brad Hedlund did an excellent analysis of fixed versus chassis-based switches in his Interop presentation and concluded that fixed switches offer higher port density and lower per-port power consumption than chassis-based ones. That’s true when comparing individual products, but let’s ask a different question: how much does it take to implement a 384-port non-blocking fabric (equivalent to Arista’s 7508 switch) with fixed switches?

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Virtual Networks: the Skype Analogy

I usually use the “Nicira is Skype of virtual networking” analogy when describing the differences between Nicira’s NVP and traditional VLAN-based implementations. Cade Metz liked it so much he used it in his What Is a Virtual Network? It’s Not What You Think It Is article, so I guess a blog post is long overdue.

Before going into more details, you might want to browse through my Cloud Networking Scalability presentation (or watch its recording) – the crucial slide is this one:

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Transparent Bridging (aka L2 Switching) Scalability Issues

Stephen Hauser sent me an interesting question after the Data Center fabric webinar I did with Abner Germanow from Juniper:

A common theme in your talks is that L2 does not scale. Do you mean that Transparent (Learning) Bridging does not scale due to its flooding? Or is there something else that does not scale?

As is oft the case, I’m not precise enough in my statements, so let’s fix that first:

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NHRP Rate Limiting Can Hurt Your DMVPN Network

NHRP-based interface state control is a fantastic feature that you can use for faster convergence of very large DMVPN networks (as explained in the DMVPN Designs webinar, you can also use it to solve some interesting backup scenarios). We tested it in a network with over 1000 spokes (using ASR1K as the hub router) using very short registration timeouts, and the CPU utilization of the NHRP process rarely exceeded a few percents.

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Does Optimal L3 Forwarding Matter in Data Centers?

Every data center network has a mixture of bridging (layer-2 or MAC-based forwarding, aka switching) and routing (layer-3 or IP-based forwarding); the exact mix, the size of L2 domains, and the position of L2/L3 boundary depend heavily on the workload ... and I would really like to understand what works for you in your data center, so please leave as much feedback as you can in the comments.

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