Category Archives: Cisco
Flash to See Spanning Tree Protocol Algorithm
Rules:
1. one ROOT Bridge per broadcast domain;
2. one ROOT PORT per NRB (non-root bridge);
3. one DESIGNATED PORT per segment;
4. non-DESIGNATED PORTS are not used;
Below you can see the STP algorithm in action:
or expand the flash file to see it bigger.
Different Types of Memory That a Computer Uses
The different types of memory that a computer uses, in order of fastest to slowest, are as follows:
- memory inside CPU – L1 cache
- memory in the processor housing – L2 cache
- memory on the motherboard – RAM
- hard drive space that is used as memory – virtual memory
Cache
Cache is a much faster memory that stores copies of the data from the most frequently used main memory locations.
So when the CPU needs to access the data, first it checks the cache to see if it exists there. If so the processor will read the information from the cache, which is much faster than accessing it from the main memory.
There are three three independent caches:
– an instruction cache to speed up executable instruction fetch;
– a data cache to speed up data fetch and store;
– a translation lookaside buffer (TLB) used to speed up virtual-to-physical address translation for both executable instructions and data;
Data cache is usually organized as a hierarchy of more cache levels:
- L1;
- L2;
- L3;
- L4;
- …
Read more at: Wikipedia Multi Level Caches
When to use Straight-Through cable and when to use Crossover cable
Devices Connected with Crossover and Straight-Through cables
On the left hand side are devices that use straight-through cable and on the right hand side are devices that use crossover cable.
These are the devices that use a straight-through cable:
- Switch – Router
- Switch – PC
- Switch – Server
- Hub – PC
- Hub – Server
These are the devices that use a crossover cable:
- Switch – Switch
- Switch – Hub
- Hub – Hub
- Router – Router
- Router – PC
- Router – Server
- PC – PC
But nowadays this standard is becoming obsolete due to the fact that Auto-MDIX (automatic medium-dependent interface crossover) technology let devices to choose between port signals. This means that you don’t need to know anymore what type of cable you must use when connecting two devices.
OSPF Process in Packet Tracer
When clearing the OSPF process in Packet Tracer instead of using the command clear ip ospf process, because it is not supported by Packet Tracer, save your config using copy ru sta and then reload, wait for the router to reload and the use sh ip o n.
And be patient because the DR election will take more time in Packet Tracer than in real life.
OSPF Network Types
- Non-Broadcast (NBMA) – RFC Standard
- Point-to-Multipoint – RFC Standard
- Point-to-Point – CISCO Proprietary
- Broadcast – CISCO Proprietary (you don’t have to statically configure the neighbs)
- Point-to-Multipoint, Non-Broadcast – CISCO Proprietary (same as P-M, but you have to statically configure the neighbs)
OSPF in Single Area
A router is a DR until it is rebooted.
Default OSPF net type for
- ethernet interface – broadcast
- serial interface – non broadcast
OSPF in Multi Areas
Internal Router – all interfaces are confined in the same area
ABR – Area Border Router – one interface in Area 0 (backbone) and another one in nonbackbone area
ASBR – Autonomous System Boundary Router – A router that connects to outside world
LSA Types
- Type 1 – O – R link LSA – flooded into an area
- Type 2 – O – net link LSA – flooded to whole area
- Type 3 – O IA – net summary link LSA – generated by ABR – sent between areas
- Type 4 – O IA – AS external ASBR summary link LSA – sent from ABR to ASBR
- Type 5 – [O E1|O E2] – external link LSA – flooded throught the AS by ASBRs
- Type 7 – NSSA external LSA – by ASBR residing in NSSA
How to remember what type is used by ABR or ASBR ?
- ABR – 3 letters => LSA Type 3
- ASBR – 4 letters => LSA Type 4
and the other you know them.
Areas
- Ordinary or Standard Area – this area connects to backbone
- Stub Area
- NO EXT SUMMARY Routes = NO Type 5 LSAs
- use of default route
- used in Hub-and-Spoke design
- Totally Stubby Area (or Total Stub Area)
- NO SUMMARY LSAs and NO EXT SUMMARY Routes = NO 3,4,5
- use of default route (network is 0.0.0.0)
- totally Cisco shop
- NSSA
- used when conn to ISPs or when redi is reqed
- NO 4 or 5 LSA, YES for LSA 7
- @NSSA ABR LSA 7 is translated in LSA 5
- RFC 1587 “The OSPF NSSA Option”
- Backbone Area (Area 0)
- can propagate all LSAs except LSA 7
Limitations
- 50 Rs per area;
- 60 Neighbs per R;
- 3 areas per R;
- a R may NOT be a DR or BDR for more than 1 LAN.
STUB and Totally Stubby Areas limitations
- NO EXT routes are allowed;
- NO virtual links are allowed;
- NO redi is allowed;
- NO ASBR Rs are allowed;
- the area is NOT the backbone area;
- All Rs are configed to be STUB Rs.
Commands
default-information originate //+ use of a default route
area range //only on ABRs
summary-address //only on ASBRs
//ASBR performs route redi
I kinda understood what is all about with ABR or ASBR, LSA Types, NSSA, stub and stuff. But it will take me some OSPF configs to understand how it works.
Subnet Zero and the All-Ones Subnet
RFC 1878 states:
“This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks.”
Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.
Subnetting Formula
November 2011 Webinar Schedule CCNA DHCP Deepdive
November 2011 Technical Webinars for Instructors
Registration is still open for the following technical webinar session for instructors during November 2011:
November 2011 Webinar Schedule
Date: Wednesday, November 23rd, 2011 (12:00 GMT)
Topic: CCNA DHCP Deepdive
Description:
This webinar focuses on the DHCP instructor professional development self-paced learning offering available. The session is designed to be interactive where initially we will review CCNA DHCP level concepts and then work through various troubleshooting examples. Not only does this session teach you about DHCP, but it is a great way for you to interact with the panelists and take away some packet tracer troubleshooting examples for use with your students.
Presenter: Jaskaran Kalsi & Anton Merckushov, Networking Academy Technical Advocacy Team
Registration:https://cisco.webex.com/cisco/onstage/g.php?t=a&d=206192255
For further assistance please contact: Jaskaran Kalsi (jkalsi@cisco.com).
Best wishes,
Technical Advocacy, Cisco Networking Academy
Cisco Networking Academy
Cisco Aspire Game
“Aspire is a game created by Cisco that helps you to teach not only computer networking, but human networking as well. In game, you can fullfill contracts, make contacts, and even build your own house. In this simulation world, you have to make your living a network specialist. The game is integrated into Packet Tracer 5, so you can easily create and confiure the networks without leaving the game.”
Cisco Packet Tracer Skills Assessment Environment Check
