Packet Forwarding

The number of MAC addresses that a switch may need to store in comparison to the number of routes it needs to hold depends on where it is deployed in the network. The memory used for TCAM tables is limited and allocated during the boot-up process on the switch. When a section of hardware resources…

Routers designed for high availability (HA) include hardware redundancy such as dual power supplies (PSU) and router processors (RP). The RP is responsible for for learning the network topology and building the route table (RIB). If the RP fails it can trigger routing protocol adjacencies to reset resulting in network instability. During a RP failure…

ASICs in hardware platforms are expensive to design, produce and troubleshoot and limited in their functionality yet they allow for very high packet rates due to only being programmed for specific tasks. Routers can also be equipped with network processing units (NPUs). NPUs have an advantage over ASICs as they are programmable meaning their programming…

Software CEF, more commonly known as software Forwarding Information Base (FIB) consists of two parts: Forwarding Information Base The forwarding information base is built from the routing table on the local device and contains the next hop IP for each destination that the device knows on the network. When a routing or topology change occurs…

Distributed forwarding is when there are line cards that can make decisions without the input of a route processor. When a packet is received on the ingress line card, it is transmitted to the local forwarding engine. The forwarding engine performs a packet lookup and determines if the outbound interface is local. If the outbound…

With the low cost of general purpose central processing units, the price of software based routers is become more affordable at the expensive of total packet throughput. When a route processor (RP) is equipped with a forwarding engine so it can make all of the packet switching decisions, this is known as a centralised forwarding…

A platforms TCAM allows for the matching and evaluation of a packet on more than one field. TCAM is an extension of the content addressable memory (CAM) architecutre but is enhanced to allow for upper layer procerssing such as identifying the Layer 2 and 3 source and destination addresses, protocol, QoS markings and more. TCAM…

A Cisco proprietary switching mechanism, Cisco Express Forwarding has been the default switching mechanism on most Cisco platforms that do all their packet switching and forwarding using the general purpose CPU since the 1990s, and is the default switching mechanism used by all Cisco platforms that use specialised ASICs and Network Processing Units (NPUs) for…

Process switching can also be referred to as software switching or slow path switching. Process switching is when the general central processing unit (rather than any dedicated ASIC chip) takes charge of packet switching on the hardware device. Within Cisco IOS the process ip_input runs on the general CPU for incoming network packets that can…

In the beginning Cisco routers on recieving a packet would remove the Layer 2 information and verify that the route existed for the destination address on that packet. If a matching route could not be found on the router, the packet would be dropped. Otherwise if a matching route was found, new Layer 2 information…