DTP: How do I know the functionality of a switch port?

Dynamic Trunking Protocol

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Switch#show interfaces switchport

How Encryption Works

http://computer.howstuffworks.com/encryption3.htm

Public Key Encryption

One of the weaknesses some point out about symmetric key encryption is that two users attempting to communicate with each other need a secure way to do so; otherwise, an attacker can easily pluck the necessary data from the stream. In November 1976, a paper published in the journal IEEE Transactions on Information Theory, titled "New Directions in Cryptography," addressed this problem and offered up a solution: public-key encryption.

Also known as asymmetric-key encryption, public-key encryption uses two different keys at once -- a combination of a private key and a public key. The private key is known only to your computer, while the public key is given by your computer to any computer that wants to communicate securely with it. To decode an encrypted message, a computer must use the public key, provided by the originating computer, and its own private key. Although a message sent from one computer to another won't be secure since the public key used for encryption is published and available to anyone, anyone who picks it up can't read it without the private key. The key pair is based on prime numbers (numbers that only have divisors of itself and one, such as 2, 3, 5, 7, 11 and so on) of long length. This makes the system extremely secure, because there is essentially an infinite number of prime numbers available, meaning there are nearly infinite possibilities for keys. One very popular public-key encryption program is Pretty Good Privacy (PGP), which allows you to encrypt almost anything.

http://static.howstuffworks.com/flash/encryption-animation.swf

The sending computer encrypts the document with a symmetric key, then encrypts the symmetric key with the public key of the receiving computer. The receiving computer uses its private key to decode the symmetric key. It then uses the symmetric key to decode the document.

To implement public-key encryption on a large scale, such as a secure Web server might need, requires a different approach. This is where digital certificates come in. A digital certificate is basically a unique piece of code or a large number that says that the Web server is trusted by an independent source known as acertificate authority. The certificate authority acts as a middleman that both computers trust. It confirms that each computer is in fact who it says it is, and then provides the public keys of each computer to the other.

Let’s check out an IPv6 header

Let’s check out an IPv6 header.

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I touched on the IPv6 addressing scheme a few weeks ago before and I wanted to continue the trend into a few more IPv6 related posts but that last IPS post spiked my interest, so I had to publish that one. Now we know the addressing scheme is different in IPv6 but what about the packet format? Obviously the packet headers will be larger because the source and destination addresses within that header are now 128 bits but let’s see what else we have in the IPv6 header:

 

 

Now that doesn’t look too intimidating right? I think that looks a little simpler compared to the IPv4 packet header. Now let’s see what we got going on here:

• Version: This field is in an IPv4 packet and simply tells us what version of IP we are running. Since this is an IPv6 packet it’s going to have a value of 6
• Traffic Class: This is the equivalent of the DiffServ/DSCP portion of the IPv4 packet which carries the QoS markings of the packet. Just like in IPv4 the first 6 bits are designated for the DSCP value, and the next 2 bits are for ECN (Explicit Congestion Notifications) capable devices.

• Flow Label: This field is 20 bits long and is defined in RFC 6437, I’ll admit finding information about the flow label is tough, but the RFC state this field could be used as a ‘hash’ for the routing devices look at and make forwarding decisions based on the field’s value. Its intention is for stateless ECMP (Equal Cost Multi-Path) or LAG mechanisms, but we will have to see how different vendors implement this feature. I’d take guess that IPv6 CEF will use the flow label, but I’ll have to wait and see.

• Payload Length: Specifies the size of the data payload following the IPv6 header. 
• Next Header: This field is 8-bits and specifies the layer 4 transport protocol which follows the IP header. These values are hex format as well, you’ll notice ICMPv6 has a value of 0x3a, IPv6 protocol numbers use the same numbers that were used in IPv4. IANA’s list of protocol numbers can be found here.
• Hop Limit: This is also an 8-bit field and replaces the TTL field that was in the IPv4 header. Each hop decrements the hop limit value by 1 and when the hop limit reaches Zero the packet is discarded.
• Source/Destination: This should go without saying but it tells you the source IPv6 address of the packet and the destination IPv6 address this packet is destined to. As you would expect both of these field are 128-bits each.

So there is a snappy run down of the IPv6 IP Packet header, I think it is actually simpler than the IPv4 IP packet headers but don’t tell that to a Cisco router. Remember these packet headers are considerably larger than their IPv4 counterparts so it takes more processor power to process IPv6 packets which is not a problem for the ISR G2’s we have todays but it is something you might want to keep in mind when run IPv6 on older hardware.
Now back to CCIE: R/S Labbing I go!

Cisco Express Forwarding (CEF)

From Cisco Learning Network

"Process switching requires the CPU to be personally involved with every forwarding decision.

Fast switching still uses the CPU, but after a packet has been forwarded, information about how to reach the destination is stored in a fast-switching cache.  This way, when another packet going to the same destination is seen, the next hop information can be re-used from the cache, so the processor doesn’t have to look up and assemble all the information again.  If the information is not cached, (for example a first packet for a given destination network) the CPU will have a similar workload, for that packet, as if fast switching was not in use.

Cisco Express Forwarding (CEF), is the evolution of optimizing the router to make it be able to forward more packets faster.  CEF cheats a little, by building a Forwarding Information Base (FIB), and an adjacency table.  The FIB is accessed very quickly based on how they built it (it is Cisco proprietary), and contains pre-computed reverse lookups, next hop information for routes including the interface and L2 information to use.  (All the stuff a router would have to consider when forwarding a packet).

In short:

Process switching is like doing math, long hand.   

Fast switching, using the cache, is like doing a problem once long hand, and subsequent problems you remember the answer for, (from memory, or the cache).

CEF is like having programmed an excel spreadsheet, and when the numbers hit the cells, the answer is already calculated.

Best wishes,

Keith (Barker)"

From Routing bits

What is CEF?

Definition from Cisco.com :

Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. CEF optimizes network performance and scalability for networks with large and dynamic traffic patterns, such as the Internet, on networks characterized by intensive Web-based applications, or interactive sessions.

To understand this better, one has to understand why and how CEF came about.  With Cisco IOS  there are different Switching Methods, that define how packets are forwarded through a router. The first method, which happens to be the oldest and slowest is Process-Switching. Alternatively when  packets arrive, the interface processor can interrupt the central CPU and asks it to switch the packet according to a route cache or switching table. That cache or table can be built in several ways, the two of interest here are Fast-Switching and CEF.

With Process-Switching, when a packet enters the router the Layer-2 info is stripped off,   the packet is then copied to the CPU memory where a L3  lookup is performed, and any other CPU required features, like NAT, queuing, compression etc, along with any other housekeeping like lowering the TTL, recalculating the CRC are done, before the frame is rewritten with the new L2 destination address and sent to the outgoing interface. The Big, the Bad and the Ugly!

With Fast-Switching, the first packet in a flow is still copied to the CPU for the L3 lookup and housekeeping, before being rewritten with the L2 destination address. The switching of the first packet by the central CPU gives the CPU the opportunity to build a cache called the fast-switching cache, which is used to switch all subsequent packets for the same destination using the same switching path across the router. With Fast-Switching the cache is only built on demand, which can be time consuming when huge numbers of potential destinations are involved. To avoid this a pre-build cache was needed, and thus CEF was born.

With CEF (Cisco Express Forwarding), there are two main data structures:

• The Adjacency-Table : is responsible for the MAC or Layer 2 rewrite. This adjacency can be built from ATM,  Frame Relay map statements, dynamic information learned from Ethernet-ARP, inverse ARP on ATM, or Frame Relay. The Layer 2 rewrite string contains the new Layer 2 header which is used on the forwarded frame.  For Ethernet, this is the new destination and source MAC address and the Ethertype. For PPP, the Layer 2 header is the complete PPP header, including the Layer 3 protocol ID.

• FIB (Forwarding Information Base) Table : The CEF table/FIB table holds the essential information, taken from the routing table, to be able to make a forwarding decision for a received IP packet. This information includes the IP prefix, the recursively evaluated next hop, and the outgoing interface.

The CEF process flow:

1. When a packet enters the router, the router strips off the Layer 2 information.
2. The router looks up the destination IP address in the CEF table (FIB), and it makes a forwarding decision.
3. The result of this forwarding decision points to one adjacency entry in the adjacency table.
4. The information retrieved from the adjacency table is the Layer 2 rewrite string, which enables the router to put a new Layer 2 header onto the frame,
5. The packet is switched out onto the outgoing interface toward the next hop.

To enable Proccess-Switching on a interface you have to disable Fast-Switching and CEF that is enabled by default:

#no ip route-cache
#no ip cef

To enable Fast-Switching on a interface use the following command:

#ip route-cache

To display the IP Fast-Switching Route-Cache :

#show ip cache [verbose] [prefix mask]

To enable CEF globally  (Default = Enabled) :

#ip cef

To enable CEF on the interface :

#ip route-cache cef


To see the CEF Adjacency Table, the [detail] option displays the L2-Rewrite string:

#show adjacency [detail]

To see the CEF FIB Table:

#sh ip cef [prefix]