Module 2 - Understanding Cisco Switching Paths Flashcards
What are the 3 Types of Switching Paths?
Process Switching - Inspects all on the cpu
Fast Switching - Uses a cache table, inspects the first frame and then caches it. Faster than process switching.
Cisco Express Forwarding (CEF) - This is the default mode for switching. Uses both the Adjacency table and the FIB (forwarding information base) table. The FIB is populated by default and built up on existing connected interfaces. Hardware switching so it is much quicker and less intensive than the other two methods.
Cisco Express Forwarding
It’s the fastest forwarding process by default, not always efficient where load balancing is involved.
useful cmds
show ip cef
show ip adjacency
show ip route
MAC and TCAMs
Switches ask where should I forward the frame? should i even forward the frame? how should i forward the frame?
The CAM table is used for L2 mac address decisions.
The TCAM table is used for L3 and higher decisions such as ACL’s, QOS etc.
Layer 2 Switching
The switch makes decisions about forwarding frames that are based on the destination Media Access Control (MAC) address that is found within the frame.
Each port is it’s own ‘collision domain’ and because of this allows for full-duplex versus a hub which operates at half duplex.
If a frame is not in the forwarding table aka CAM table then the frame is flooded… this is known as ‘unicast flooding’
CAM and TCAM lookups are performed in parrallel. They do not suffer any degredation by enabling QOS and ACL processing.
Q.
Which table is used by a switch to figure out where a frame must be sent?
The MAC address table ise used to figure out where a frame must be sent to.
Control and Data Plane
In L3 devices the control and the data plane are relatively independent.
Control plane in a L3 devices exchanges such things as routing protocol information (this is where we learn about routing info). The control plane is very quiet and slow compared to the data plane. It populates the routing table (RIB).
Data Plane - Allows packets to be forwarded from one interface from another.
Point to note - there is no control plane in a Layer 2 device. Also remember that with Switches the 3 x F’s - forwarding, flooding and filtering.
Process Switching
The slowest switching method for L3, it sends everything that comes into the data plane up to the control plane, which is then processed by the CPU. Each packet has a unique decision made.
Fast Switching aka Route Cache Switching
Similar to process switching in that the first packet that comes in is sent to the CPU. However a cache table is formed, every packet after that initial packet then uses the route cache table.
CEF - Cisco Express Forwarding - Continued
The FIB table is pre-built and the adjacency table is built when possible from the CPU. Makes use of RIB and ARP tables
CEF has limitations with load balancing or packets that require more processing. Default mode.
The control plane is responsible for building the FIB table and adjacency tables in software.
The data plane is responsible for forwarding IP unicast traffic using hardware.
What is Administrative Distance used for?
The administrative distance is used by routers to build the global RIB. If there are two routes to the same destination then the AD with the lowest value is used to populate the RIB. Each protocol EIGRP, OSPF, BGP has it’s own associated AD.
Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.
The FIB table
This is derived from the RIB. The FIB is based on L3 prefix based lookups, the longest match prefix is deemed the most trustworthy. Matches that of the TCAM table.
The Adjacency Table
The adjacency table is built from the ARP table, a node is adjacent if it is one hop away. The Adj table contains L2 MAC address header rewrite information.
It is populated as new adjacencies are formed/discovered.
IP CEF Exceptions
Not all packets can be processed in the hardware. When traffic cannot be processed in the hardware, it must be received by software processing of the Layer 3 engine. This traffic does not receive the benefit of expedited hardware-based forwarding. Several different packet types may force the Layer 3 engine to process them.
- They use IP header options.
- They have an expiring IP TTL counter.
- They are forwarded to a tunnel interface.
- They arrive with unsupported encapsulation types.
- They are routed to an interface with unsupported encapsulation types.
- They exceed the Maximum Transmission Unit (MTU) of an output interface and must be fragmented.
Q.
Which of the following Cisco Switching Mechanisms is the default on Cisco routers?
CEF is the default mode.
Summary Review Questions
Which of the following commands will allow an administrator to view which MAC address are mapped to which ports on a Catalyst switch?
show mac address-table
