Archived from groups: comp.dcom.lans.ethernet,comp.dcom.sys.cisco (More info?)
I have been faithfully evolving our LAN within the hop-count limits
that I have read about in newsgroups and online -- namely that
one is permitted 4 intermediate hops between end-stations for 10 Mbps,
and 3 intermediate hops between end-stations for 100 Mbps, with
each router "resetting" the count.
Several months ago, I went to check the exact name of the "rule"
(which I knew as the "3-4-5 rule"), and discovered that the "5"
referred to permitted fan-outs in a star topology; and I knew from
my readings and from practical experience that fan outs beyond 5
are extremely common (e.g., 24 or 48 port switches, potentially
stacked into the hundreds of ports range.)
At that point I started to doubt that the rule was for the same -kind-
of LAN that is commonly deployed these days, and the more I thought about
those "rules", the more I have come to doubt that what I had always
"known" to be a hard "Thall Shalt Not" rule had any meaning for our
network.
I did some digging in the 802.3 specs last night, and I don't find
that "rule" anywhere, and what I did find seems to lend credence
to my thought processes. I would appreciate some confirmation or
refutation from those who know ethernet better than I.
The limits I found in 802.3 were on Class I and Class I repeaters
(which I tend to think of as 'hubs'). I found a sentance in the
100BaseTX section that indicated that when you are using the
classic star topology with duplex links with bridges between the
segments, then each segment is treated as an independant collision
domain, and the segment size limit is then bound just by the 100 metres
limit rather than by round-trip propogation limits. But then a later
sentance talks about limits on the "network" without any proximate
wording to clarify if "network" is referring to a single
collision domain.
Am, then, I now properly interpreting that in the case where every device
is connected to a switch port through a proper Cat5 cable of < 100 metres
each, and the switches are themselves connected together through
proper Cat5 < 100 metres (or through 100BaseFX or 1000BaseFX if over 100m),
then it is permitted to have an arbitrary number of switches in the
chain [with the higher level protocol timers then becoming the
operative limits]?
If that -is- true, that there -is- no 3 hop rule at 100BaseTX /
1000BaseTX that is meaningful for fully switched network, then
certain topological changes I have simmering would be much simplified.
I'm planning to migrate some equipment to a new gigabit switch that
doesn't stack with our existing multi-vlan'd 100 Mb swithces; if there
is no hop count limit, then the new device becomes simple to introduce,
and transition to; if there -is- a maximum of 3 switches before hitting
a router, then I'll have to do noticable internal renumbering to
ensure that no -actual- path exceeds 3 switches before routing.
--
The Knights Of The Lambda Calculus aren't dead --this is their normal form!
I have been faithfully evolving our LAN within the hop-count limits
that I have read about in newsgroups and online -- namely that
one is permitted 4 intermediate hops between end-stations for 10 Mbps,
and 3 intermediate hops between end-stations for 100 Mbps, with
each router "resetting" the count.
Several months ago, I went to check the exact name of the "rule"
(which I knew as the "3-4-5 rule"), and discovered that the "5"
referred to permitted fan-outs in a star topology; and I knew from
my readings and from practical experience that fan outs beyond 5
are extremely common (e.g., 24 or 48 port switches, potentially
stacked into the hundreds of ports range.)
At that point I started to doubt that the rule was for the same -kind-
of LAN that is commonly deployed these days, and the more I thought about
those "rules", the more I have come to doubt that what I had always
"known" to be a hard "Thall Shalt Not" rule had any meaning for our
network.
I did some digging in the 802.3 specs last night, and I don't find
that "rule" anywhere, and what I did find seems to lend credence
to my thought processes. I would appreciate some confirmation or
refutation from those who know ethernet better than I.
The limits I found in 802.3 were on Class I and Class I repeaters
(which I tend to think of as 'hubs'). I found a sentance in the
100BaseTX section that indicated that when you are using the
classic star topology with duplex links with bridges between the
segments, then each segment is treated as an independant collision
domain, and the segment size limit is then bound just by the 100 metres
limit rather than by round-trip propogation limits. But then a later
sentance talks about limits on the "network" without any proximate
wording to clarify if "network" is referring to a single
collision domain.
Am, then, I now properly interpreting that in the case where every device
is connected to a switch port through a proper Cat5 cable of < 100 metres
each, and the switches are themselves connected together through
proper Cat5 < 100 metres (or through 100BaseFX or 1000BaseFX if over 100m),
then it is permitted to have an arbitrary number of switches in the
chain [with the higher level protocol timers then becoming the
operative limits]?
If that -is- true, that there -is- no 3 hop rule at 100BaseTX /
1000BaseTX that is meaningful for fully switched network, then
certain topological changes I have simmering would be much simplified.
I'm planning to migrate some equipment to a new gigabit switch that
doesn't stack with our existing multi-vlan'd 100 Mb swithces; if there
is no hop count limit, then the new device becomes simple to introduce,
and transition to; if there -is- a maximum of 3 switches before hitting
a router, then I'll have to do noticable internal renumbering to
ensure that no -actual- path exceeds 3 switches before routing.
--
The Knights Of The Lambda Calculus aren't dead --this is their normal form!
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