Chapter 2 Flashcards
What is found in the CAM table of a switch?
Found in Cisco switches
MAC address-to-port mappings
Layer 2 forwarding decisions
What is found in the TCAM table of a switch?
Found in multilayer switches and routers
ACL, QoS, and other information for upper-layer processing
Switches can have multiple TCAMs to boost performance
What is a CAM table?
The CAM table is the primary table that is used to make Layer 2 forwarding decisions. The table is built by recording the source MAC address and inbound port of all incoming frames. When a frame arrives at the switch with a destination MAC address of an entry in the CAM table, the frame is forwarded out through only the port that is associated with that specific MAC address. If no exact match is found, the switch floods the packet out of all ports in the VLAN.
What is a TCAM table?
The TCAM table stores ACL, QoS, and other information that is generally associated with upper-layer processing. Most switches have multiple TCAMs, such as one for inbound ACLs, one for outbound ACLs, one for QoS, and so on. Multiple TCAMs allow switches to perform different checks in parallel, thus shortening the packet-processing time. Cisco switches perform CAM and TCAM lookups in parallel. This behavior is why Cisco switches do not suffer any performance degradation by enabling QoS or ACL processing.
What is Process switching?
Slowest method, every packet examined by CPU and all forwarding decisions made in software
What is Fast switching (aka route caching)
Faster method, first packet in each flow examined by CPU and forwarding decision cached in hardware for subsequent packets in flow
What is Cisco Express Forwarding (aka topology-based switching)
- Fastest method, hardware forwarding table created regardless of traffic flows and all packets switched using hardware
- Fast, but does have limitations
- Cisco default method
- Considered true Layer 3 switching (uses routing table entries)
NOTE: Because this hardware FIB exists regardless of traffic flow, assuming that a destination address has a route in the routing table, all packets that are part of a flow, even the first packet, will be forwarded by the hardware.
What is the FIB?
Forwarding Information Base
- Used in Topology-Based switching.
- Built by Cisco Express Forwarding
- Control plane information (Route table, MAC table, ARP cache, etc)
What is usually referred to as the Switching Fabric
On larger chassis switches this is the physical backplane that connects multiple blades together.
What is Layer 3 Centralized Switching?
Switching decisions are made on the route processor by a central forwarding table, typically controlled by an ASIC.
What is Layer 3 Distributed Switching
Switching decisions can be made on a port or line-card level, rather than on a central route processor. Cached tables are distributed and synchronized to various hardware components so that processing can be distributed throughout the switch chassis.
What does SDM stand for in SDM template?
show sdm prefer
Switch Database Management.
SDM templated can be used to configure system resources (CAM and TCAM) in the switch to optimize support for specific features, depending on how the switch is used in the network.
What are examples of some of the different types of SDM templates?
Default: When you unbox a brand new switch, this template will be the enabled template. It provides a mix of unicast, connected, and host routes.
Routing: As one example, you would enable this template if the device is performing routing in the distribution or core of the network. The device is able to carry numerous routes, but only for IPv4.
Access: The access template maximizes system resources for access control lists (ACLs) to accommodate many ACLs.
VLAN: When you enable this template, you allocate most of the table space to Layer 2 unicasts. You would use this template when you have large subnets with many MAC addresses.
Dual IPv4 and IPv6: You would enable this template if you want to turn on the IPv6 capabilities of the device. When enabling this template, you have to choose between default, routing, and VLAN.
Default: More space is reserved for IPv6 routing and security. There is less reserved space for Layer 2 unicast.
Routing: More space is reserved for IPv6 routing than IPv4 routing.
VLAN: Suitable for when you are running a dual-stack environment with lots of VLANs.
Why might an ipv6 command not be available on a Catalyst switch?
It could be running the wrong SDM template.
Change the SDM template to a dual-stack mode by using:
(config)# sdm prefer dual-ipv4-and-ipv6 default
What command would you use to see if a SDM template change might max out the resources of a switch?
# show platform tcam utilization (3650)# show platform tcam utilization asic all
