CCIE Pursuit Blog

June 10, 2009

Core Knowledge Question of the Day: 10 June 2009

Which Cisco-proprietary STP feature detects indirect failures in the core of the backbone?

Highlight for answer: BackboneFast.

June 9, 2009

Core Knowledge Question of the Day: 09 June 2009

The Dynamic ARP Inspection and IP Source Guard features both require which additional feature to be configured?

Highlight for answer:  DHCP Snooping must be enabled.  Both Dynamic ARP Inspection and IP Source Guard rely on the DHCP Snooping database.

June 8, 2009

Core Knowledge Question of the Day: 08 June 2009

Given the output below, how long will it take for a MAC address to age out?

Rack1SW1(config-if)#do sh port-security int f0/4
Port Security              : Enabled
Port Status                : Secure-down
Violation Mode             : Shutdown
Aging Time                 : 0 mins
Aging Type                 : Absolute
SecureStatic Address Aging : Disabled
Maximum MAC Addresses      : 1
Total MAC Addresses        : 0
Configured MAC Addresses   : 0
Sticky MAC Addresses       : 0
Last Source Address:Vlan   : aaaa.bbbb.cccc:1
Security Violation Count   : 0

Highlight for answer: By setting ‘switchport port-security aging’ to 0(the default), aging is disabled and the MAC address will never age out.

June 1, 2009

Memory Tricks To Remember IEEE STP Designations

The INE(yes, I’m adapting the new abreviation of INE over IE for Internetwork Expert) blog has a nice post up about the different flavors of Spanning Tree.  What I really like about the posting is that memory tricks you can use to remember the letter designation of the IEEE specification.

802.1d(“Classic” Spanning Tree) – It’s dog-gone slow
802.1w(Rapid Spanning Tree) – Imagine Elmer Fudd saying ‘rapid’ as “wapid”
802.1s(Multiple Spanning Tree) – You add the letter ‘s‘ to nouns to make them plural(multiple)

I’ve always had trouble with tasks that refer to the flavor of STP by the IEEE designation*.  These simple memory tricks make it much easier to quickly decipher tasks that refer to the IEEE designation.

* This is a pet peeve of mine as I’ve never had someone in the “real world” say something like, “Go ahead and configure that switch to use 802.1w.”  If that ever happened I would just smile as I slowly twisted their nipples off.

January 20, 2009

Enabling Portfast On Trunks

I was working through one of the IE volume I labs and ran across a very easy task.  It simply asked you to configure portfast on all ports connected to routers.  Easy enough, just slap ‘spanning-tree portfast’ on the interfaces connected to the routers.  Easy points.  Next task.

Unfortunately I overlooked the fact that one or the ports connected to a router was trunking (part of a router-on-a-stick configuration).  I blew by the BIG FREAKING CLUE that IOS gave me:

SW2(config)#int f0/6
SW2(config-if)#spann portfast
%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators,  switches, bridges, etc… to this interface  when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION

%Portfast has been configured on FastEthernet0/6 but will only
have effect when the interface is in a non-trunking mode.

I don’t think that I’ve ever used the ‘trunk’ option with portfast before:

SW2(config-if)#spann portf ?
disable  Disable portfast for this interface
trunk    Enable portfast on the interface even in trunk mode

spanning-tree portfast (interface configuration)
trunk – (Optional) Enable the Port Fast feature on a trunking interface.

Sure enough, even though the interface configuration showed that portfast was configured, it was not working on the trunk interface:

interface FastEthernet0/6
switchport trunk encapsulation dot1q
switchport mode trunk
switchport nonegotiate
spanning-tree portfast

SW2#sh spanning-tree int f0/6 portfast
VLAN0001         disabled
VLAN0005         disabled
VLAN0007         disabled
VLAN0008         disabled
VLAN0009         disabled
VLAN0010         disabled
VLAN0022         disabled
VLAN0043         disabled
VLAN0058         disabled
VLAN0067         disabled
VLAN0079         disabled
VLAN0146         disabled

So much for those ‘easy points’.

Here’s what happens with the ‘trunk’ option enabled:

SW2(config)#int fa0/6
SW2(config-if)#spann portfast trunk
%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators,  switches, bridges, etc… to this interface  when portfast is enabled, can cause temporary bridging loops.
Use with CAUTION

SW2#sh spanning-tree int f0/6 portfast
VLAN0001         enabled
VLAN0005         enabled
VLAN0007         enabled
VLAN0008         enabled
VLAN0009         enabled
VLAN0010         enabled
VLAN0022         enabled
VLAN0043         enabled
VLAN0058         enabled
VLAN0067         enabled
VLAN0079         enabled
VLAN0146         enabled

May 1, 2008

New Cisco Campus Switching Product On The Horizon?

Filed under: Cisco,Switching — cciepursuit @ 6:09 pm
Tags: ,

It looks like there may be a replacement for the 6500 on the horizon according to NetworkWorld:

As expected, Cisco is developing a significant product launch for the enterprise campus under the codename “Big Bang.”

Marie Hattar, vice president of network systems and security solutions marketing, would not divulge details on Big Bang at the Interop 2008 conference. But she said it is a code name for a campus product launch that’s not likely to happen in calendar 2008.

“You’ll see a big bang but not a forklift,” Hattar says on the upcoming campus refresh. “It’s an evolutionary big bang.”

—Read The Rest Here—

The name ‘Big Bang’ (and its overuse by the Cisco suit) sort of reminds me of a project that we worked on a couple of years ago.  We were expanding our campus LAN by implementing a GeoMax implementation.  All of the infrastructure was priced out and the business plans written.  All that remained was to pitch the project to the VP and have her sign off on it.  She loved it except for one thing.  “Change the name now.” 

The name of the project? MAN Enhanced.  Even I (with my admittedly dirty mind) didn’t catch the connotation at first. 🙂  The Enzyte and Swedish pump jokes sponsorship jokes lasted weeks.

April 21, 2008

Beware The Man Of One Platform

Filed under: Cisco,IOS,Switching,Work — cciepursuit @ 3:26 pm
Tags: , , , ,

I vow to never be one of those guys that expects my word to be law once I am a CCIE.  This is not because I am humble (I’m not) or because the ‘Argument From Authority’ is a logical fallacy; it’s because I am wrong more often than I care to be and I will continue to be wrong more often than I care to be regardless of any digits or abbreviations after my name.  🙂

Case in point: I was troubleshooting an issue last week and was surprised to find that the VLAN interfaces (SVIs) on a 6500 series switch (an old piece of shit 6500 switch running DECNet….but I digress) each shared a single (virtual) MAC address.  I pointed this out to one of my co-workers and he said that this was normal.  I disagreed.  I jumped on a 3750 and showed him that each SVI had a unique MAC address.  I even labbed it up quickly on my 3560 to prove my point.

We noted that this might be an interesting anomaly, but it most likely was not our issue as we were troubleshooting a duplicate IP/HSRP/DECNet/STP loop issue.

Well it turns out that we were both right (and both wrong).  Depending on the platform (and IOS version?) Cisco switches may use the System MAC Address for each SVI or they may use a unique MAC Address (derived from the System MAC Address).  CCIE candidates can see this in their labs by noting the differences between the 3560s and 3550s:

3560 uses a unique MAC for each SVI:
sw1#sh ver | i IOS|emo
Cisco IOS Software, C3560 Software (C3560-IPSERVICESK9-M), Version 12.2(25)SEE2, RELEASE SOFTWARE (fc1)
cisco WS-C3560-48PS (PowerPC405) processor (revision G0) with 118784K/12280K bytes of memory.
512K bytes of flash-simulated non-volatile configuration memory.

sw1(config-if)#do sh int | i Vlan|bia
Vlan1 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c0(bia 0012.018f.d5c0)
Vlan2 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c5(bia 0012.018f.d5c5)
Vlan3 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c6(bia 0012.018f.d5c6)
Vlan4 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c7(bia 0012.018f.d5c7)

3550 uses the same MAC for each SVI:
sw3#sh ver | i IOS|emo
Cisco IOS Software, C3550 Software (C3550-IPSERVICESK9-M), Version 12.2(25)SEE2, RELEASE SOFTWARE (fc1)
Cisco WS-C3550-24(PowerPC) processor (revision D0) with 65526K/8192K bytes of memory.

Vlan1 is administratively down, line protocol is down
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)
Vlan2 is up, line protocol is up
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)
Vlan3 is up, line protocol is up
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)
Vlan4 is up, line protocol is down
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)

Scott Morris has an article on this issue.

Now you’ll notice that all of the VLAN interfaces have the same MAC address. This is the System MAC address. The reason this is OK has to do with where MAC addresses are used.

A MAC address must be unique within a Layer2 network, a broadcast domain or subnet. Each VLAN is a separate L2 network, broadcast domain and subnet. So there should be no possibility for overlap here and nothing to worry about.

If your configuration is creating some strange bridging or other cross-VLAN behavior, there may be the possibility of odd behavior, but that isn’t the normal issue at all!

So, in the grand scheme of things, you shouldn’t see any duplicate MAC addresses in any place that makes a difference.

February 12, 2008

Cisco: Free Multiple Spanning Tree Training

Although this training is not specific to the CCIE, it may still be a good resource for those who are learning or reviewing MST (you will need a CCO account to access the CCNP Prep site):

CCNP TV: BCMSN – Implementing Multiple Spanning Tree Protocol, February 28, 2007

Join more than 40,000 CCNP Prep Center users for an hour long monthly online TV talk show.
Sign up now: Attendance space is limited, so visit and click “Register Now” under the Prep Center TV heading to reserve your space today.

Date: Thursday, February 28, 2007
Time: 11 a.m. Eastern Time, 8 a.m. Pacific Time, and 16:00 GMT
Title: Multiple Spanning Tree Protocol
The program will focus on the following objectives and is designed to provide information that will assist in passing the Cisco Certified Network Professional exam. After the presentation, we’ll be taking live calls from the audience during the Q&A session.  You may also submit questions electronically.

During the show, Cisco Experts will discuss:

  • Motivation behind 802.1s MSTP
  • Comparison of MSTP Instances to PVST
  • MST Region, what is it?  What needs to Match?
  • MST BPDUs and MRecords
  • MST and CST interaction
  • MST configurationss
  • MST verification

To learn more, visit the page.

January 31, 2008

Cisco Unleashes The Überswitch

Filed under: Cisco,Switching — cciepursuit @ 7:54 am
Tags: , , ,

Cisco is about to unleash a new switch on the world.  It sounds like an absolute beast.:

The Nexus brings Cisco into not just a new territory for its business, but a new product category: a unified switch that spans storage and computing in data centers and has security built in. Given the stakes, superlatives are natural.

– A single Nexus chassis will be able to handle more than 15Tbps of traffic ripping through a data center, up from just 2Tbps for a current Catalyst 6500 switch.

– At that rate, the switch could run 5 million concurrent transcontinental conferencing sessions using Cisco’s TelePresence Collaboration system. It could also copy the entire searchable Internet in 7.5 minutes.

– One interface module for the Nexus 7000 chassis will come with 32 10Gbps ports, and the platform is designed to support future interfaces including 100Gbps.

– The company spent about $250 million on research and development for the new platform, and at its peak, the Nexus R&D team numbered more than 500 engineers, according to Tom Edsall, senior vice president and chief technology officer of Cisco’s Data Center Business Unit.

As with the Catalyst 6000 Series and the CRS-1, Cisco developed the Nexus with an eye to long-term needs. Where the CRS marked the debut of IOS XR, the first modular version of IOS, the Nexus will have Cisco’s first OS that can be fully virtualized, called NX-OS. The Nexus will also break new ground with its lossless switching fabric, a departure from traditional Ethernet — though backward compatible with it, Cisco said.

It looks great too:

Cisco Nexus Switch

November 19, 2007

Google To Make Its Own (10Gig) Switches?

Filed under: Cisco — cciepursuit @ 7:32 am
Tags: , , , , ,

Here’s an interesting story concerning Google deciding to make their own switches:

Google, has been known to come up with its own technologies when dissatisfied with commercial and/or open source offerings. The company had previously started making its own server hardware. And now it seems the company engineers are building high-speed switches according to its own stringent specifications. It is part of company’s efforts to ensure that its services – search, advertising and everything else – are delivered to end users with minimum delay.

Andrew Schmitt reports that company has been buying up components that are going into some sort of a 10 Gigabit Ethernet switch. The system is based on Broadcom’s silicon, he reports. I had heard similar story about three months ago but failed to find any confirmation. My hats off to Andrew for his scoop. This dovetails with some information I have received that the company has been making a run at router geeks and raiding Cisco’s pantry so to speak.

Here are some highlights about Google’s 10 GigBE Switch:

* It used Broadcom 20-port 10GE switch silicon (BCM56800) & SFP+ based interconnect.
* Google is using non-standard solutions in order to build products specific to its needs.
* Andrew estimates that Google might be using about 5,000 ports per month

I think this development merits further investigation about Force 10 Networks, which was one of the suppliers to Google, as I had previously reported. That company has raised a mountain of cash and was supposed to go public. I have heard that they recently lost out to Myricom, another 10 GigBE vendor to supply gear to Argonne Leadership Computing Facility (ALCF), a 445-teraflops Blue Gene/P super computer built by IBM and the U.S. Department of Energy’s Argonne National Laboratory.

Coming soon to a Data Center near you: the Google switch?

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