INDEX TOC 3
Copyright © 2012
Routing-Bits.com
> Bridges introduced the technology called transparent bridging. (Refer to section below)
- Switches
> Switches are multi-port transparent bridges.
> Switches are evolved bridges with superior performance, additional features, higher port density and typically use hardware ASICs for packet forwarding.
> Switches deploy smarter switching methods, e.g. cut-through switching examines only header portions before the packet is switched out the correct port.
> Switches also utilize Transparent Bridging.
> The term switch or Layer 2 switch is synonymous with the term bridge today.
> With switches, the collision domains are isolated to each port on a switch.
- Transparent Bridging
> Is a technique used to isolate collision domains by building a forwarding database and using it to forward traffic only to the necessary segments.
> The forwarding table (a.k.a. CAM table) always starts empty after a device booted up.
> When receiving a frame the CAM table is populated, by recording the source MAC address and the port the frame was received on.
> If a received frame does not have the destination MAC address listed, it will be transparently send (flooded) out all ports except source the port.
> The act of flooding is an attempt to populate the CAM table by learning the MAC address and port of the responding destination host.
> When a frame is received and the destination MAC address is already listed in the CAM table, the frame is only forwarded out the listed port.
> Thus source MAC addresses are used to populate the table, while destination MAC addresses used to lookup the port the hosts are connected to.
> Note, that the MAC learning process only applies when the port is in a spanning-tree state allowing such an action. (Refer to STP section below).
- CAM (Content Addressable Memory) Table
> The CAM table is used on all Cisco Catalyst/Nexus switches today.
> A CAM table entry, as discussed above, list the MAC addresses to port mappings.
> Additionally the associated VLAN and a timestamp of the packet arrival are also stored in the CAM for each entry.
> If an already learned MAC arrives on a different port to what is stored in the CAM, a new entry with an updated timestamp is recorded and the old entry is removed.
> Every entry in the CAM table is deleted after a lifetime (default 300 sec) if not refreshed. These are called stale entries.
> MAC flapping or relearning is when one/multiple MAC addresses are continually learned on different ports causing the CAM table to be continuously updated.
> MAC flapping could have serious impacts on production switching environments when the CPUs can't keep up.
> MAC flapping is caused by asymmetric switching or persistent TCs (Topology Changes), malfunctioning switches, broken port-channels or faulty NIC teaming setups.
> TCs generally result in the invalidation of CAM table entries, followed by unknown unicast traffic flooding to relearn the MAC addresses.
- Full-Duplex
> Bridges and Switches localizes the collision domain to the port connecting the switch to the host.
> Full-duplex took it further by allowing both devices on a port to transmit and receive at the same time, effectively doubling the throughput.
> Full-duplex supersedes the need for collision detection and as a result is disabled on full-duplex links.
> For full-duplex connectivity both sides of the same wire must be configured/negotiated to use full-duplex.
- Unicast, Broadcast and Multicast Frames
> Unicast
>> Is the sending of data to a single host.
>> An unicast frame with a known destination MAC address will only be forwarded out the switch port the MAC address was previously learned on.
>> An unicast frame with an unknown destination MAC address will be flooded out every other port except the port which received on.
> Broadcast
>> Is the sending of data to all hosts simultaneously in a specific network.
>> Broadcasts frames will be flooded out every other port except the port which received on within the broadcast domain.
>> Broadcasts use a MAC address of FFFF.FFFF.FFFF.
>> Extreme amounts of broadcast traffic constitute a broadcast storm, which could potentially bring down a network.
>> Refer to the Advanced Switching Section to read more about Storm-Control on how to mitigate broadcast storms.
1-340-998-2840
4412 Indian Wells Dr, Greensboro, NC, USA
Ronald J Pitts
ronjazz @ gmail.com
Ronald J Pitts
ronjazz @ gmail.com
<routing-bits.com>
我的内容管理
展开
