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.

• 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

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.

And be patient because the DR election will take more time in Packet Tracer than in real life.

ena
conf t
int l0
no ip add
int fa0/0
no ip add
int s2/0
no ip add
int s3/0
int s0/0/0
no ip add
int s0/0/1
no ip add
int s0/1/0
no ip add
int s0/1/1
no ip add
no router rip
no router ospf
end
sh ru
copy ru sta

ena
conf t
no ip domain-lookup
end
sh ru
copy ru sta

A router is a DR until it is rebooted.

Default OSPF net type for

• ethernet interface – broadcast
• serial interface – non broadcast

LSA Types

1. Type 1 – O – R link LSA – flooded into an area
2. Type 2 – O – net link LSA – flooded to whole area
3. Type 3 – O IA – net summary link LSA – generated by ABR – sent between areas
4. Type 4 – O IA – AS external ASBR summary link LSA – sent from ABR to ASBR
5. Type 5 – [O E1|O E2] – external link LSA – flooded throught the AS by ASBRs
6. 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.