Visio Icons

I'm sick of trying to find the Cisco Visio icons. The generic ones are at:
http://www.cisco.com/web/about/ac50/ac47/2.html

Re-focussing study

I was recently made redundant (very unexpected given that I only started the job 6 weeks ago), so I've spent a little time contemplating study and work. I've decided the current choices focus more on IP routing and switching rather than MPLS. So I'm refocussing on my CCIE written. Even though it is not a stand alone certification it does show you are serious.

So it's back to the books looking at snippets of information like - unicast flooding being caused by asymmetric routing or flapping links and STP TCNs.

Baby Giants and Jumbo Frames

Someone asked me about Jumbo frames so I thought I would take a bit of a look (partly because I have to look at Baby Giants anyway). Here is what I have found so far:

• Baby Giants have an MTU of up to 1600 bytes. Jumbo frames can be over 9000 bytes (the exact size depends on the switch platform and possibly IOS).
• Some Cisco switches and routers don't support baby giants or jumbo frames, usually because of ASIC limitations.
• If you just want baby giants so that you can put a single MPLS tag on a frame you can try changing the interface to use 802.1q trunking and then place your MPLS traffic in the native VLAN. This changes the MTU to 1504 (so it fits one tag) and is supported on most switches.
• The interface counters will count the baby giant or jumbo frame as being over-sized even if the interface can support it - this is cosmetic.
• Some devices allow you to set a separate MTU for 10/100 and Gigabit interfaces. One of these is the 3750. For the 10/100 interfaces you use the system mtu number command. For the Gig interfaces you use the system mtu jumbo number command. If you set the system mtu but not the system mtu jumbo then the Gig interfaces use the system mtu. The reason for the two commands is that the 10/100 interfaces only support a lower MTU than the Gig interfaces. Other than trying the commands or looking up on the Web there does not seem to be a way to find out what the maximum acceptable sizes are.

CCIP MPLS and BGP combo exam

I've decided I am going to do the CCIP combo MPLS and BGP exam. There are a couple of reasons - the biggest one is time. I really want to get the CCIP out of the way before the end of September and the CCIE written done by the end of October. Realistically the CCIE written exam will probably take more than a month to study for (even though September will be routing month). Even though I am going to do the BSCI in September (my BSCI exam expired about two or three years back) I still think I'll need a good four weeks for the CCIE written - between switching, multicast, frame-realy and QoS. It should all just be revision as I did the written a few years back and have done some hard core study for it a few months back.

I'm half thinking i should book the combo exam for two weeks time to put a little pressure on myself (I'm easily distracted at the moment). Looking at the BGP exam outline I keep thinking how hard could it really be... and then I remember how hard it really can be.

Back to the books.

BGP Prefix-based outbound route filtering

Every now and then you find a command that makes you think now someone is really thinking about this stuff.

BGP prefix-based outbound route filtering is one of those. If you only want to keep some of the routes from your neighbor this capability allows you to advertise the prefix list you are using to that neighbor. The neighbor then uses the prefix-list to filter the routes BEFORE it advertises them to you. This can significantly reduce the bandwidth and time taken to synchronise routes between two neighbors. The critical command is:
neighbor address capability prefixlist-orf [send | receive | both]

Once you have enterred that command (in the receive or both directions) if you enable a prefix-list to filter incoming routes your router will send the prefix list to your neighbor so that it can filter the routes for you.

The command has been around for a while (I stumbled on it when I was looking for something else and it was available in 12.2T). Some documentation is available in BGP Prefix-Based Outbound Route Filtering 12.2T

VTP (again)

It may seem obvious but VTP only works between devices that are using VTP. What this means is that if you have a trunk port pointing to a router which isn't doing VTP then VTP pruning is not going to work on that link. What's more if a switch has one interface that isn't doing VTP pruning then it means that switch will want ALL VLANs to come over its trunks towards the rest of the network.

The moral of the story - either manually configure which VLANs to forward on the trunk towards your routers OR enable VTP on the router, use VLAN interfaces and put the physical interface in switching mode.