Blog Posts in July 2011
I got an interesting question after writing the Asymmetric MPLS MTU Problem post:
Why does PHP happen only on directly-connected interfaces but not on other non-MPLS routes?
Russell Heilling made a highly interesting observation in a comment to my MPLS MTU challenges post: you could get asymmetric MTUs in MPLS networks due to penultimate hop popping.
I was sort of upset that my vacations were making me miss the VMware vSphere 5.0 launch event (on the other hand, being limited to half hour Internet access served with early morning cappuccino is not necessarily a bad thing), but after I managed to get home, I realized I hadn’t really missed much. Let me rephrase that – VMware launched a major release of vSphere and the networking features are barely worth mentioning (or maybe they’ll launch them when the vTax brouhaha subsides).
After the bumpy start of our holidays, we thoroughly enjoyed the crystal-clear waters, hot sunny weather and the hospitality of inhabitants of Croatian island Brač ... until my daughter came to me quietly asking “hey, I don’t want to raise panic, but my friend saw a weird cloud ... would you mind checking if it’s a forest fire” A short walk to a vantage point confirmed the initial observation – we were facing what turned out to be the worst forest fire in more than a decade. Obviously I was bound to receive another hefty dose of disaster recovery lessons.
My recent vacation included a few perfect lessons in disaster recovery. Fortunately the disasters were handled by total pros that managed them perfectly. It all started when we were already packed and driving – my travel agent called me to tell me someone mixed up the dates and shifted them by two months; we were expected to arrive in late August. Not good when you have small kids all excited about going to the seaside sitting in the car.
Lots of interesting articles accumulated in my Inbox while I tried to figure out what one could possibly do when being stranded in an easy chair next to the sea with no Internet access. By far the best article that I stumbled upon in my Twitter feed is a 10-year-old IS-IS versus OSPF presentation by the legendary Dave Katz (thank you @yelfathi).
@MCL_Nicolas sent me the following tweet:
Finished @packetpushers Podcast show 7 with @ioshints ... I Want to learn more about Mpls+Mtu problem
You probably know I have to mention that a great MPLS/VPN book and a fantastic webinar describe numerous MPLS/VPN-related challenges and solutions (including MTU issues), but if MTU-related problems are the only thing standing between you and an awesome MPLS/VPN network, here are the details.
A comment left on my dense-mode FCoE post is a perfect example of the dangers of using vague, marketing-driven and ill-defined word like “switching”. The author wrote: “FC-SW is by no means routing ... Fibre Channel is switching.” As I explained in one of my previous posts, switching can mean anything, from circuit-based activities to bridging, routing and even load balancing (I am positive some vendors claim their load balancers ... oops, application delivery controllers ... are L4-L7 switches), so let’s see whether Fibre Channel “switching” is closer to bridging or routing.
Yandy sent me an interesting question:
Is it just me or do you also see the Nexus 2000 series not having any type of distributed forwarding as a major design flaw? Cisco keeps throwing in the “it's a line-card” line, but any dumb modular switch nowadays has distributed forwarding in all its line cards.
I’m at least as annoyed as Yandy is by the lack of distributed switching in the Nexus port (oops, fabric) extender product range, but let’s focus on a different question: does it matter?
Michael modified one of my EEM applets to monitor CRC errors on WAN interfaces and notify the operator (via e-mail) when an interface has more than two errors per minute. He wanted to monitor multiple interfaces and asked me whether it’s possible to modify the SNMP event detector somehow. I only had to point him to the event correlation feature of EEM version 2.4 and he sent me the following (tested) applet a few days later.
Chris Marget sent me the following interesting observation:
One of the things we learned back at the beginning of Ethernet is no longer true: hardware filtering of incoming Ethernet frames by the NICs in Ethernet hosts is gone. VMware runs its NICs in promiscuous mode. The fact that this Networking 101 level detail is no longer true kind of blows my mind.
So what exactly is going on and does it matter?
Matthew sent me the following remarkable fact (and he just might have saved some of you a few interesting troubleshooting moments):
I was bringing up an OSPF adjacency between a Catalyst 6500 and an ASR 9006 and kept getting an MTU mismatch error. The MTU was set exactly the same on both sides. So I reset them both back to default (1500 on the 6500 and 1514 on the ASR 9006) and the adjacency came back up, even though now the MTU is off by 14 bytes. So I attempted to bump the MTU up again, this time setting the MTU on 6500 to 1540 and the MTU on the ASR 9006 to 1554. Adjacency came right up. Is there something I am missing?
The 14 byte difference is the crucial point – that’s exactly the L2 header size (12 bytes for two 6-byte MAC addresses and 2 bytes for ethertype). When you specify MTU size on the IOS classic (either with the ip mtu command or with the mtu command), you specify the maximum size of the layer-3 payload without the layer-2 header. Obviously IOS XR works differently – there you have to specify the maximum size of a layer-2 frame, not of its layer-3 payload (comments describing how other platforms behave are most welcome!).
My web site statistics are (yet again) confirming the inevitable truth: the holiday season has started in the northern hemisphere. I hope you’ll be busy doing things that are more fun than reading my blog, so I’ll publish only two or three articles per week to prevent information overload, returning to the regular daily schedule in late August.