Archive for October 2012
Well, with the exception of a console server I got all my routers racked, stacked, and cabled!
It’s all cabled and ready to go! I got the following:
4x Cisco 1841’s
2x Cisco 2621xm’s
1x 3745 -This is going to get a NM-32a modules for the console server.
Aside from the lab equipment I got:
1x SonicWall TZ210
2x 2960 switches
And, I still got 2U left in the rack not sure what to fill that space up with yet. I was thinking of some old 2950G’s or maybe a Cisco ASA. I also picked up the INE workbooks last week so I here goes nothing!
In my last post about unicast Reverse Path Forwarding, I made quite a few references to CEF. CEF (known as Cisco Express Forwarding) is a feature built into every Cisco router and is enabled by default it’s really the feature that makes Cisco well Cisco. I figured I would take a little time and discuss CEF in a little detail.
From a 500 foot view CEF is a method the router uses to forward packets, without utilizing any CPU cycles. CEF builds 2 cached tables in memory, these 2 cached tables are built from every entry in the routing table. So when the next packet arrives the router simply looks up the CEF entry found within memory to forward the packet, compared to sending the packet to the packet to the CPU for a route lookup and then forwarding the packet on.
CEF works by creating two tables:
- The Adjacency table maintains the layer 2 forwarding information for each FIB entry eliminating the need for the router to send out ARP requests.
- The FIB (Forwarding Information Base, also known as the CEF table) contains information from the routing tables and tracks the next-hop for all routes. So where the adjancency table manages layer 2 information the CEF table manages the layer 3 forwarding information
Now that we know the components of CEF, lets talk a little about how CEF interacts with packets as they enter the router.
- Once the packets arrives at the router the layer 2 information is stripped off. (This is normal and happens whenever a frame is accepted to a layer 3 device)
- Next the router looks up the destination using the CEF table.
- Then the router finds the corresponding adjacency table entry.
- The router then adds the corresponding layer 2 information (found in the adjacency table) back to the packet and forwards the packet on. All from memory.
You can view the CEF table by issuing the command sh ip cef:
Now remember the CEF table is layer 3 information so we have destination prefixes and what the next hop address is along with the outgoing interface.
You can view layer 2 adjacencies by issuing the sh adjacency %Interface% detail:
Since the adjancency table pertains to layer 2 information we’ll see some MAC addresses here, one thing I want to point is that long hexidecimal number in the fifth line that line consists of the destination MAC address of 10.1.1.2, the source MAC address of this routers fa0/0 interface and the ethertype of 0800 (Meaning it’s an ethernet interface)
Now this was only the tip of the iceberg for CEF, and this post was only supposed to bring it to light on a real high level so more CEF related posts will mostly surface as time goes on.
I’ve been talking about getting my own domain for a while now, and trust me I’ve been thinking about a good website name for a while now. I figured CCIE-or-Null.net was appropriate, since I am going to be tackling the CCIE: R&S pretty heavily soon enough. I’ll be completing my home lab in the next few weeks then I’ll grab the INE workbooks and off I go so by this time next month I will be in full swing studying for the lab.
What about the written though? Well I’ve been reading and labbing for the last 4-5 months, so I’ll take a stab at that soon. The only reason I haven’t taken my written yet is because I want to jump right into my lab prep after I pass the written. I know how quickly time can fly by and 18 months is not as long as you think it is.