The second scenario Brad Hedlund described in the Clos Fabrics Explained webinar is a large leaf-and-spine fabric using 10GE uplinks and QSFP+ breakout cables between leaf and spine switches (thus increasing the number of spine switches to 16).

Ronald sent me an interesting question: What's the point of having a secondary path set up for a certain LSP, when this LSP also has fast-reroute enabled (for example, with the Junos fast-reroute command)?

The idea of having a pre-established secondary LSP backing up a traffic engineering tunnel was commonly discussed before FRR was widely adopted, but should have quietly faded away by now.

A while ago I wrote a blog post about remote ND attacks, which included the idea of having /120 prefixes on server LANs. As it turns out, it was a bad idea, and as nosx pointed out in his comment: “there is quite a long list of caveats in all vendor camps regarding hardware in the last 6-8 years that has some potentially painful hardware issues regarding prefix length. Classic issues include ACL construction and TCAM specificity.”

One would hope that the newly-release data center switches fare better. Fat chance!

Mark Berly, the guest star of my VXLAN Technical Deep Dive webinar focused on VXLAN gateways. Here’s the first part of his presentation, explaining what VXLAN gateways are and where you’d need them.

Imagine you have a typical 2-tier data center network (because 3-tier is so last millennium): layer-2 top-of-rack switches redundantly connected to a pair of core switches running MLAG (to get around spanning tree limitations) and IP forwarding between VLANs.

Next thing you know, a rep from your favorite vendor comes along and says: “did you know you could connect all ToR switches into a virtual fabric and manage them as a single entity?” Is that a good idea?

When an IPv4/IPv6 host wants to send a packet to another host, it has to answer the following simple questions:

• Can I reach the destination IP address directly (is the destination on the same LAN/subnet)?
• If not, who will help me forward the packet (who is the first-hop router)?

In IPv4 world, the host can get all the information it needs through DHCP. In IPv6 world, things are way more complex (but also way more correct if you’re a theoretician).

Hamid sent me the following question:

I have already memorized (bad idea, BTW) that a loop can occur if FD < RD. Could you please tell me how a loop could occur assuming FD < RD and we ignore the feasibility condition.

I’ll use a simple three-router network (see the following diagram) to illustrate why EIGRP cannot figure out whether an alternate more expensive path could lead to a loop or not.

In the introductory part of the IPv6 security webinar, Eric Vyncke explained how the huge IPv6 subnet sizes won’t stop a determined attacker, but will make the task of network or security engineers trying to take host inventory much harder.

I’m constantly getting questions about the intricate interworking of various flags present in IPv6 Router Advertisement messages. Here’s a (hopefully comprehensive) summary taken primarily from RFC 4861.

In a comment to the Firewalls in a Small Private Cloud blog post I wrote “VXLAN is NOT a viable inter-DC solution” and Jason wasn’t exactly happy with my blanket response. I hope Jason got a detailed answer in the VXLAN Technical Deep Dive webinar, here’s a somewhat shorter explanation.

In the Clos Fabrics Explained webinar I focused on the Clos fabrics principles of operation and design options, and Brad Hedlund who graciously agreed to be my guest explained how you can use Dell Force10 switches to build them. In this video he’s describing a simple leaf-and-spine topology with 40GE uplinks.

An incredible amount of IPv6 deployment documents has been published as IETF drafts recently, amongst them:

• Operational security considerations for IPv6 networks
• Design guidelines for IPv6 networks
• Stateless IP/ICMP Translation in IPv6 Data Centre Environments (aka IPv6-only data centers)
• Enterprise IPv6 Deployment Guidelines

Enjoy ... and don’t forget to join the v6ops mailing list ;)

Kaage added a great comment to my Virtual Firewall Taxonomy post:

And many of physical firewalls can be virtualized. One physical firewall can have multiple virtual firewalls inside. They all have their own routing table, rule base and management interface.

He’s absolutely right, but there’s a huge difference between security contexts (to use the ASA terminology) and firewalls running in VMs.

I’m exposed to an incredible variety of topics in my ExpertExpress engagements, but there are always a few recurring themes, one of them being “we’re experiencing long convergence times and high packet loss after our primary Internet link fails.” Almost always the root cause turns out to be full Internet routing table being received on inadequate hardware.

The murky details of IPv6 implementations never crop up till you start deploying it (or, as Randy Bush recently wrote: “it is cheering to see that the ipv6 ivory tower still stands despite years of attack by reality”).

Here’s another one: in theory the prefixes delegated through DHCPv6 should be static and permanently assigned to the customers for long periods of time.