Why is Crossover Slower

[dttusa]

We recently made a change to the network moving all servers to
a data closet. We have accessibility using a crossover cable to the main cascaded stack.

I have about 6 Nortel 450-24t switches, 3 in a stack, and several located around a building. I have connectivity using crossover cables as opposed to MDX or MDA connections.

Users were complaining about sluggishness on the network. I downloaded Netperf, and discovered that using crossover cable between switches reduces speeds from 96 MB down to 8 MB.

Inter-nortel, Nortel / 3com , swapping cables and switches, all produce the same results.

What is the difference?

Why is a crossover so much slower?

 

[jimbopalmer]

Ok, here are some possible ways to link 450s, I was unable to guess which ones you tried.

1) the Cascade Cables that loop down the back of the 450 run at 2.5 gig, bidirectionally in a ring

2) the MDA slots come in 1 gig fiber and 1 gig copper, as well as 100 meg copper and 10/100 copper. At best they will be 5 times slower than Cascade Cables. They are point to point with no redundancy.

3) the 24 ports of 10/100 copper in the main switch (or in a 100/100 MDA) will be 50 times slower than Cascade Cables and 10 times slower than gig ports.

If at all possible, use the cascade cables to make stacks of 450's.


I tried to remain child-like, all I acheived was childish.

 

[anthonyanderberg]

In case you're using gigabit over copper and the numbers above really are megabytes (instead of megabits) its worth remembering that gigabit crossover cables use all 4 pair - 10/100 crossover cables only need pins 1/2+5/6.

http://en.wikipedia.org/wiki/Ethernet_crossover_cable

 

[anthonyanderberg]

Opps - I meant to say that 10/100 cables use 1/2+3/6

Pins 4/5 are unused to avoid problems with our telephone friends.