Jaycee's Networking

January 5, 2009

Hubs and Switches

Filed under: Information, IOS — Tags: , — Jaycee @ 3:09 am

1. Cables:

(1) 10Base-5 = thicknet
N connectors
(2) 10Base-2 = thin-net, for cable TV
BNC connectors
(3) UTP = unshielded twisted pair cables
RJ45 connectors
(4) 10Base-T — there is no specific distance limitation, usually keep within 100M
describes certain characteristics that a cable should meet

2. Hubs:

(1) A hub connects Ethernet cables together, their signals can be repeated to every other connected cable on the hub

(2) a hub is a repeater, a repeater is not necessarily a hub

(3) A repeater repeats a signal, usually used to extend a connection to a remote host, or to connect a group of users who exceed the distance limitation of 10Base-T.

(4) A repeater may have only 2 connectors, a hub can have many more.

(5) 5-4-3 rule of Ethernet design – b/w any 2 nodes on an Ethernet network

a. there can be be only 5 segments
b. connected via 4 repeaters
c. only 3 of the segments can be populated

3. Collision  domains

(1) Collisions are limited to network segments, where devices can communicate using layer-2 MAC addresses.
(2) Collisions are limited to collision domains, where collisions can occur.

4. Broadcast domain:

(1) where a broadcast will be propagated.

(2) Broadcasts stay within a layer-3 network, which usually bordered by a layer-3 device such as a router.

(3) Broadcasts are sent through switches (layer-2 devices), but stop at routers.

(4) Broadcasts and IP networks are not limited to VLANs.

(5) Broadcast Storms

a. Causes: endless loop
b. Symptoms: every device essentially being unable to send any frames on the network due to constant network traffic, all status lights on the hubs staying on instantly instead of blinking normally.
c. Resolves: the only way to resolve a broadcast storm is to break the loop.

5. Frames

(1) TCP packet is encapsulated with layer-2 information to form a frame
(2) always refer to frames when speaking of hubs and swithches

6. Switch terms:

(1) Switch — the general term used for anything that can switch
(2) Ethernet Switch — any device that forwards frames based on their layer-2 MAC addresses using Ethernet.

a. An Ethernet switch creates a collision domain on each port
b. A hub generally expands a collision domain through all ports

(3) Layer-3 switch — a switch with routing capabilities. VLANS can be configured as virtual interfaces on a layer-3 switch.

(4) Multilayer switch — Same as a layer-3 switch, but also allow for control based on higher layers in packets.

(5) Switching — is the act of forwarding frames based on their destination MAC addresses.

a. In telecom, switching is the act of making a connection b/w 2 parties.
b. In routing, switching is the process of forwarding packets from one interface to another within a router.

7. CAM table (content-addressable memory) in Cat OS and MAC address table in IOS contain a map of what MAC addresses have been discovered on what ports.

8. When a station using IP needs to send a packet to another IP address on the same network, it must 1st determine the MAC address for the destination IP address:

a. IP send out an ARP (Address Resolution Protocol) request packet. This packet is a broadcast, so it’s sent out all switch ports.

b. The ARP packet, when encapsulated into a frame, now contains the requesting station’s MAC address, so the switch knows what port to assign for the source.

c. When the destination station replies that it owns the requested IP address, the switch knows which port the destination MAC address is located on (the reply frame will contain the replying station’s MAC address).

9. To display information about the MAC address table:

show mac-address-table


*in Cat OS, use show cam dynamic

10. Cisco Switch Types:

(1) Fixed-configuration switches

a. are smaller, usually 1 rack unit (RU) in size
b. typically contain nothing but Ethernet ports
c. includes the Cisco 2950, 3550, 3750
d. 3750 is capable of being stackedthe limitation of stacking is that the backplane of the stack is limited to 32 Gbps (Gigabits per seconds)
Benefits: price, size, weight, power–capable of operating on normal household power,some support a power distribution unit which can provide some power redundancy at additional cost.

(2) Modular chassis-based switches

a. can support 720 Gbps on their backplanes
b. more expensive

11. Modular Chassis-based Switches

a. Advantages:

(1) Expandability — 7x 3750s for an equal ports of the 6500 chassis, but speed of a stack is limited to 32Gbps while 6500 provide 720Gbps

(2) Flexibility — 6500 chassis will accept modules that provide services outside the range of a normal switch:

a. Firewall Services Modules (FWSMs)
b. Intrusion Detection System Modules (IDSMs)
c. Content Switching Modules (CSMs)
d. Network Analysis Modules (NAMs)
e. WAN modules (FlexWAN)

(3) Redundancy

a. Support multiple power supplies
b. Support dual supervisors

(4) Speed

a. 6500 chassis employing Supervisor-720 (Sup-720) processors supoorts up to 720 Gbps of throughput on the backplane.
b. The fastest fixed-configuration switch — Cisco 4948 — supports only 48Gbps.

i. 4948 switch is designed to be placed at the top of a rack in order to support the devices in the rack.
ii. it cannot be stacked, therefore it’s limited to 48 ports.

b. Disadvantages: heavy, take a lot of room, require a lot of power.

*Cisco’s two primary chassis-based switches: 4500 series and 6500 series.

12. Planning a Chassis-Based Switch Installation

(1) Rack space

a. 6513 — 19 RU
b. NEBS version of 6509 — 21 RU
c. 4506 — 10 RU
d. 7-foot telecom rack is 40 RU

(2) Power

a. add up the power requirements for all the modules
b. To provide redundancy, each of the power supplies in the pair should be able to provide all the power necessary to run the entire switch, including all modules.
c. For DC power supplies, make sure you specify A and B feeds.
e.g. if you need 40 amps of DC power, you’d request 40 amps DC — A and B feeds. This means that you’ll get 2 40-amps power circuits for failover purposes.
d. Most collocation facilities supply positive gound DC power.
e. For AC power supplies, you’ll need to specify the voltage, amperage, and socket needed for each feed.
e.g. the power cord for a power supply may come with a NEMA L6-20P plug. It will require NEMA L6-20R receptacles(插座).
f. The P and R on the ends of the part numbers describe whether the part is a plug or a receptacle. NEMAL6-20 is a twist-lock 250-volt AC 16-amp connector.
g. Always tighten the clamp to avoid the cable popping out of the receptacle when stressed.

(3) Cooling

a. On many chassis switches, cooling is done from side to side.
b. NEBS-compliant 6509 switch moves air vertically, and the modules sit vertically in the chassis.
c. Always make sure you leave ample (充裕的) space b/w chassis switches when installing them.

(4) Installing and removing modules

Any time you’re working with a chassis or modules, you should use a static strap.

(5) Routing cables

When routing cables to modules, remember that you may need to remove the modules in the future.


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