Jumbo Frames Beyond the Broadcast Domain
I’ve run into many circumstances where jumbo frames are enabled, most notably in the data center. After all, allowing for a maximum tranmission unit of greater than 1500 bytes allows us to transmit more data per frame As I explained in Part 2 of my QoS Series, MTU can be a touch subject. Do it wrong, and you encounter one of two big network problems. One potential issue when configuring jumbo frames at L2 is that stuff just doesn’t work.Intenet Access with Dot1q Tunnel
Is there a way to provide internet service over a dot1q tunnel using VLAN tunneling? Yes, there is a way, it is not the most intuitive method but works nicely. Basically it has to do with what does the switch do with untagged frames when they arrive on a tunnel port. In this configuration, the untagged frames (native VLAN 200) are not tunneled but go to the routed interface for processing. As long as the provider’s switches has a routed interface for the customer ID VLAN and a default route, traffic should reach the Internet.
To explain this, I’ll use a basic topology with 4 switches and one router. SW1 and SW2 are service provider switches, with their interfaces Fa0/21 configured as dot1q-tunnels using access VLAN 100. SW3 and SW4 are customer switches and R1 is acting as the service-providers internet edge router.
Service provider’s switches SW1 and SW2 relevant configuration:
interface FastEthernet0/21 switchport access vlan 100 switchport trunk encapsulation dot1q switchport mode dot1q-tunnel no cdp enable
Customer ID is VLAN 100 and the port fa0/21 is set to mode dot1q-tunnel.
Customer Site A’s SW3 relevant configuration:
interface FastEthernet0/21 switchport trunk encapsulation dot1q switchport trunk native vlan 200 switchport Continue reading
Cisco ASA Packet Captures for Fun and Profit
As many of you know my background isn’t in enterprise, but I currently fill that role in my $job. In order to succeed I’ve had to develop many new skills including learning Cisco Wireless, UCS, a little Fibre Channel, and of course Cisco ASA. While I have been using firewalls for many years, I’ve never used the ASA for anything more than a user firewall, or for supporting a small branch. So yes, my skills are lacking in the ASA market compared to other technologies, and when you get deep into the grind with any product you’re going to need some new tricks to aid in your troubleshooting. This is where ASA paacket captures come into place.
Define Interesting Traffic
As with any packet capture, or even log viewing the amount of noise involved generally dwarfs the data you actually want to find. In order to ease your pain Cisco has allowed us filter out packet capture using an ACL.
FW# access-list FOO line 1 extended permit ip any host 10.2.1.5 FW# access-list FOO line 2 extended permit ip host 10.2.1.5 any
Once you have your traffic defined, you need to setup your capture. Continue reading
IPv6 – SLAAC Stateless Address Autoconfiguration
How does the internet work - We know what is networking
As you will see here there are two kinds of IPv6 address autoconfiguration. One of them is the old well know way to automatically configure IP address from IPv4 world, DHCP. The other way to make the autoconfiguration in IPv6 world in new and really interesting as it leaves the host the ability to make the autoconfiguration by […]
Packet Design to exhibit at Cisco Live! 2013
Packet Design will be exhibiting at Cisco Live 2013, June 23-27 in Orlando, Florida. We'll be showcasing Route-Flow Fusion℠ from booth #1612 near the Cisco Live Lounge where you can pick up one of our new 'Now I know everything' T-shirts.
Plus, wear the shirt at the conference to enter a raffle to win a Beats-by-Dre Wireless Speaker.
Register to attend the event here:
www.ciscolive.com/us/registration-packages
Show 139 – Making Your Way Down The Path To Nirvana
Regular hosts Greg Ferro & Ethan Banks are joined by Brandon Carroll, Josh O’Brien, and Tom Hollingsworth in Packet Pushers Weekly Show 139. We translate all the SDN hype into a more practical conversation about what network engineers should be doing to update their skills. This is a mostly raw podcast with little editing – just […]
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The post Show 139 – Making Your Way Down The Path To Nirvana appeared first on Packet Pushers Podcast and was written by Ethan Banks.
Cisco 2600: How many IPv4 prefixes can fit in BGP table
Today I wanted to try and see how many IPv4 prefixes can a Cisco 2600 accommodate in BGP table both global and in VRF table. I have lying around a Cisco 2621XM with 64MB of RAM, so I said to stress it a bit like in the old days when it was productive. I did […]Quick Tip: Improvised File Transfer
The Python module "SimpleHTTPServer" is traditionally a quick and dirty way to test web code without the overhead of installing a full webserver. You can also use it as a quick way to transfer files between systems, if you have Python available:jswan@ubuntu:~$ python -m SimpleHTTPServer 8080
Serving HTTP on 0.0.0.0 port 8080 ...
This causes Python to make all the files in the current directory available over HTTP on port 8080. From the client system, you can then use a browser, curl, or wget to transfer the file.
No, it's not secure, and yes, it may violate data exfiltration policies (and as an aside, Bro detects this by default). But I've used it fairly often to move files between Windows and Mac or Linux in situations where SCP isn't available and I don't feel like setting up a fileshare.
I've also used this in conjunction with the "time" command to test the effects of latency on various network protocols, and as an improvised way to test WAN optimization software.
Rambling on about IP fragmentation
Fragmentation! Squarely on-topic for this blog, I guess.An issue on a customer's network had me thinking about IP fragmentation recently, and now I find myself pounding some things that I find interesting about fragmentation into my keyboard.
Where should an oversized datagram be sliced?
RFC791 suggests a scheme by which an IP datagram is sliced up so that the resulting fragments just fit out the constraining interface. This seems sensible, but there are some gotchas:
- If we fragment a 1500 byte packet to fit into a PPPoE link, we might wind up with 1492 bytes in the first datagram (20 bytes header, 1472 bytes payload) and 28 bytes in the second packet (20 bytes header, 8 bytes payload). This works great until that first fragment tries to transit a GRE tunnel (MTU 1476) further along its path. If the PPPoE router had chopped the datagram in half, both fragments would fit through the GRE tunnel without any problem.
- Depending on the MTU, we might not be able to make precisely MTU-sized fragments. This is because the fragment offset value in the IP header is expressed in terms of 8-byte chunks. Every IP fragment must have an offset that's a multiple Continue reading
Network Field Day 5
Yesterday kicked off the 5th iteration of Network Field Day. For those that haven’t heard of Tech Field Day, you need to check it out - there’s something for everyone, and it’s a great event that gets the technical details from vendors on their solutions. The delegates that are invited are what I consider thought leaders in each field. I’ve had the privileged of blogging, podcasting, and even meeting with them in person over the past few years, and they’re just the right kind of folks to help keep these vendors honest.Network Field Day 5
Yesterday kicked off the 5th iteration of Network Field Day. For those that haven’t heard of Tech Field Day, you need to check it out - there’s something for everyone, and it’s a great event that gets the technical details from vendors on their solutions. The delegates that are invited are what I consider thought leaders in each field. I’ve had the privileged of blogging, podcasting, and even meeting with them in person over the past few years, and they’re just the right kind of folks to help keep these vendors honest.Extracting The Most Value From Network Vendor Presentations
Vendors love nothing more than getting in front of their customers and talking about their products. You’ll always learn something from a presentation, but mostly they are an exercise in death-by-powerpoint. In this post, I’ll provide some some tips on getting the most from your time in these presentations. Vendor presentations can be really informative […]
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The post Extracting The Most Value From Network Vendor Presentations appeared first on Packet Pushers Podcast and was written by John Harrington.
How to ‘Sell’ IPv6 to Executive Management – Guidance for Engineers
(aka ‘IPv6 for Managers for Network Engineers’) “Genius!”, the CEO says as he enters your office (ok more likely your cubicle or desk-space or whatever corner of the office you prefer to go hide out in to concentrate), “I’ve been … Continue reading
Why Would A Vendor Care About Network Field Day Events?
I’m in San Jose, California as a member of the Network Field Day 5 delegation this week. NFD is under the Tech Field Day umbrella of events, and is not a Packet Pushers event as such – although we’ve been a part of them, and Greg in particular has helped to organize some of them. […]
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The post Why Would A Vendor Care About Network Field Day Events? appeared first on Packet Pushers Podcast and was written by Ethan Banks.
What did West Virginia officials buy exactly?
The state of West Virginia has landed in the headlines a couple of times recently due to their purchase of wildly inappropriate Cisco routers for over 1000 locations around the state. There have been no shortage of commenters laying the blame at Cisco's feet, but the auditor's report disagrees. It's back in the headlines now because of the recent release of the report, and because of Cisco's pledge to buy the gear back and help the state buy more appropriate equipment.The auditor's report is an interesting read. Here are some of the highlights from my reading of it:
3945 routers were selected because of two key requirements communicated by the state:
- Dual internal power supplies, which narrows the selection to 3925 and 3945 models
- Use of two service module slots on day one, and a desire to have the option to grow beyond these two at some point in the future.
Unfortunately, these two requirements seem to have been communicated in a meeting, because no emails were available to corroborate them, but members of the team who purchased the gear acknowledge that the requirement did originate with the state. So, that's how they wound up in a 3945 chassis.
Continue reading
Why do we need specialized switches in Data Centers?
Whats the big deal about Data centers and why do they need special routers and switches anyway? Why cant they use the existing switches that folks use in their back offices or service providers in their networks. What’s so special, really, about a bunch of servers that need Internet connectivity, huh?
Working in the metro Ethernet space all my life I wasn’t sure if I really understood the hype and the reason why Data centers required specialized HW.
It’s only once I started reading about Data centers and how they work and what they’re supposed to do that I was able to appreciate their need for specialized HW – and why the existing products may not be cut for them.
In the world of Wall Street, milliseconds can mean billions of dollars. Really, am not kidding here. Packets carrying Wall Street transactions get delivered to the switch and are then forwarded to the server in the Data Center. There they ride up the protocol stack to the application that executes the trade. The commit message then has to go back down the stack and then be sent over the wire to the switch. The switch does a lookup in its Continue reading
An Operational Reason for Knowing Trivia
I've been largely out of touch with the IT certification scene lately, but I'm sure that people are still complaining incessantly about the fact that they need to memorize "trivia" in order to pass certification tests. Back when I was teaching Cisco classes full-time, my certification-oriented students were particularly bitter about this. Of course, this is a legitimate debate and the definition of "trivia" varies from person to person.When I saw this article about CloudFlare's world-wide router meltdown, however, I immediately felt a bit smug about all those hours spent learning and teaching about packet-level trivia. If you don't want to read the article, here's the tl;dr:
- their automated DDoS detection tool detected an attack against a customer using packets sized in the 99,000 byte range.
- their ops staff pushed rules to their routers to drop those packets
- their routers crashed and burned
In order for this meltdown to happen, they had to have a compounded series of errors:
- the attack detection tool was coded to Continue reading
MAC Cloning on Linksys router for Internet connectivity
When you start an Xfinity (Comcast) Internet service, the activation process involves plugging in your service provider provided modem's cable port to a cable outlet, and the connecting the modem's ethernet port to your laptop. You see the green blinking lights for RX/TX and a steady green light for Internet.
However, if you connect the modem's ethernet port to a wireless router (Linksys E2000 in my case), assuming you have correctly configured all LAN settings, wifi settings, you may still not be able to connect to the Internet.
Log in to the router's admin page, typically http://192.168.1.1, with username admin and password admin (again, unless you have changed these default settings). Go to the status page and notice the IP address. What you'll see is a '0.0.0.0' IP Address for the router's Internet connection.
I tried everything to get a public IP (from Comcast's range) here. Then I read Comcast's documentation which says connect a laptop to the ethernet port on the modem. Well, what's so special about a laptop which a router can't do?
Not always, but some times Comcast's modem expects a laptop on the other end of the ethernet Continue reading
Show 138 – HP’s Software-Defined Networking (SDN) Strategy and Solution
Show 138 – HP’s Software-Defined Networking (SDN) Strategy and Solution [Written by HP.] There has been a lot of interest in the market place recently around software-defined Networking (SDN). HP has been a leader in SDN technologies from the very beginning. HP has played an instrumental role in the development of OpenFlow and continues to […]
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The post Show 138 – HP’s Software-Defined Networking (SDN) Strategy and Solution appeared first on Packet Pushers Podcast and was written by Ethan Banks.