TCPmag.com
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July 24, 2007
Editor: Gladys Rama (grama 1105media.com)
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THIS ISSUE SPONSORED BY:
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IN THIS ISSUE OF TCPmag.com:
1. Q&A: Using 'Broadcast' in Layer 3 to Layer 2 Mapping
2. What's New on TCPmag.com
3. Interesting Employment in California, Virginia
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1. Q&A: Using 'Broadcast' in Layer 3 to Layer 2 Mapping
Send your toughest Cisco technical questions to editortcpmag.com
with the subject line "Attn: Scott."
Scott,
I am confused about few things. Could you please clarify
these points?
1. Should we use the "broadcast" keyword when
mapping a Layer 3 to
Layer 2 address in frame relay for both point-to-point
and
point-to-multipoint networks?
2. In IPv6 implementation over frame relay, why are we
mapping remote
link-local addresses to DLCI as link-local addresses are
local to
the segment, and why should we map these link-local
addresses to
DLCI in frame relay?
-- Zubedi
------------------------------
Zubedi,
Excellent questions. And by the diverse nature of them, I'm
going to
assume you're working on studying for the CCIE Routing &
Switching
lab exam (call it a hunch!).
So, let's see what we have, starting with question No. 1.
When you
talk about mapping Layer 3 to a Layer 2 address, let's think
about
why/when this needs to take place. Since you mention both a
point-to-point and a point-to-multipoint interface, we'll
think
about both.
In a point-to-multipoint world, there are many potential
Layer 2
destinations for any Layer 3 address. If our multipoint
interface
were an Ethernet segment, we would use ARP to say "Who
has x.x.x.x IP
address?" But in the non-broadcast, multiple-access
(NBMA) world of
frame relay, we can't do that.
Outside of the evil, twisted world that CCIE candidates live
in, we
would likely use something sane like inverse ARP to assist
us with
the mapping. But since we aren't allowed to do simple things
like
that, we'll have to manually map the address. Now, we are
manually
mapping what something like ARP would have done
automatically.
We're saying, "If I have a packet to this IP address,
it will go to
this Layer 2 (DLCI) address." So that's the first
piece. But you're
asking specifically about the "broadcast" keyword
on that frame
relay map statement. The short answer is: "It
depends."
Once we have our map committed (without
"broadcast"), we should be
able to ping the other side, because our router can now
resolve
from the Layer 3 address it knows is on a directly connected
interface down to a Layer 2 address which that interface
will use
to reach the destination. That will handle ANY unicast
operation.
The trick, of course, is that by default most of our routing
protocols use multicast or broadcast destination addresses.
And
without the use of the "broadcast" parameter, no
group addressing
is allowed.
This is true for IP multicast streams, as well, but the
routing
protocols are generally the more obvious impact of not
putting
the "broadcast" parameter on the map statement.
Using "debug IP
packet," you may see many "encapsulation
failed" messages on any
broadcast or multicast destination going out.
The next extension to this same question is: "How many
times do
I need that 'broadcast' parameter?" The answer is once
per DLCI.
Even if you have multiple IPs mapping to the same DLCI, only
put
the "broadcast" parameter on one of them. In a lab
environment,
we likely wouldn't care. But in real life (and doing labs is
always good practice for real life), overuse of
"broadcast" can
have a huge impact on a router. If we had more than one map
to
the same DLCI with "broadcast," that DLCI would
receive multiple
copies of every single multicast and take up processing and
bandwidth to do so.
For the point-to-point interfaces, though, there's no
ability to
use a "frame relay map" command or anything with
the "broadcast"
parameter. It's a different thought process. In
point-to-point
interfaces, broadcasts will inherently work. In thinking
through
things we'll find that the logic is: "If it's not my
address, it
must be yours." And that applies for unicasts,
broadcasts and
multicast packets (frame relay voodoo magic).
For question No. 2, IPv6 is a different protocol, but you
can follow
the same basic rules that we went over above. If there isn't
any
magic mapping for us, you'll need to manually map any
reachable
address. And while you note that the link-local (FE80::/64)
addresses are only good on a particular link, in the frame
relay
world the DLCI IS that link. So it's the same rules applied
to a
different protocol.
Everything that a router originates itself in IPv6 will be
generated
from the link-local address by default, so multiple mappings
may
be needed.
Hope that helps,
-- Scott
Scott Morris, quadruple CCIE, JNCIE and all-around
Uber-Geek, can often
be seen traveling around the world consulting and delivering
CCIE
training. He has recently stepped up as VP of Curriculum
Development
for IPexpert and will oversee a new consulting practice. For
more
information on him check out http://www.ipexpert.com.
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2. What's New on TCPmag.com
NEWS: "Behind Cisco's Push Into Broadband Mesh"
You might have missed it, but Cisco Systems Inc. last month
launched
its Cisco Cable ServiceMesh offering, a new line of Wi-Fi
mesh
offerings designed with cable operators in mind.
http://tcpmag.com/news/article.asp?editorialsid=1244
NEWS: "Juniper Takes Fight to Cisco With Branch Office
Push"
Cisco Systems Inc. archrival Juniper Networks this month
expanded its
line of branch office solutions -- and ratcheted up the
pressure on
Cisco -- by introducing its J2320 and J2350 J-series
services routers,
its Secure Services Gateway 320M (SSG 320M) and SSG 350M
security
platforms, two new network management appliances, and
Juniper NSMXpress.
http://tcpmag.com/news/article.asp?editorialsid=1245
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3. Interesting Employment in California, Virginia
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