Subject: RE: [Isis-wg] IS-IS on UPSR ring

From: "Philip Christian"<[email-masked]>
Date: Sat, 20 Apr 2002 12:45:43 +0100

I cannot answer the question about unidirectional rings. UPSR is a North
American thing. My expertise is International market SDH.

However I can give you an OPINION on the second question,

I would advice anyone building Integrated IS-IS into a SONET/SDH ADM to
address the next hop using the System Identifier (NSAP) of the next hop.

Because the routing instance on SONET/SDH ADMs is often running on very
limited resourses, it can be important to keep the size of routing tables
down.

One solution that people are looking at is to use a single prefix per ADM,
as a circuitless interface, and to run the PPP or LAPD over DCC links
unnumbered.  There is even some discussion around connecting ADMs together
using Ethernet cables, and not assigning any IP address/subnet mask to the
LAN ports either.

If a link is unnumbered then I suppose that you should put your circuitless
interface IP address in as the "Interface IP address" TLV in Hellos.  I
haven't figured out yet if this will enable ICMP redirects to work on LAN
ports.

In order to interwork with such a solution, your cannot rely on ARP to find
a neighbouring router (you have to use IS-IS to resolve MAC address to
NSAP), and you can't refer to the next hop as an IP address (it might not
have one). Better to use NSAPs.

Note that Integrated IS-IS does allow IP subnetting rules to be broken for
connecting routers together, but of course the router needs a correct subnet
to be able to ARP for hosts on a LAN port.  However if there are no hosts,
just two routers then you can't rely on ARP working.

You could argue that ARP and correct subnetting isn't needed for connecting
two routers together.

That said, if you want to guarantee interop with stand alone Integrated
IS-IS routers, then you had better support ARPing on LAN interfaces too (if
your customer has configured in an IP address / subnet mask). Note that SOME
standalone routers will not bring up an adjacency on LAN ports unless they
consider their neighbour to be in the same subnet as themselves.

That's my opinion.

Philip

-----Original Message-----
From: Lawrence Mao [mailto:[email-masked]]
Sent: Saturday, April 20, 2002 12:33 AM
To: [email-masked]
Subject: [Isis-wg] IS-IS on UPSR ring

Hi all,

If the IS-IS needs to be supported on a SONET
UPSR ring, throught its DCC channel (note that
the ring is uni-directional), say, if I have 4
nodes on a ring:

    A --> B --> C --> D -
    ^                    |
    |--------------------

what kind of modifications need to be made in
order to let the protocol work properly (note
all interfaces are p2p type)?

2nd question is, if the interface is unnumbered,
how is its routing table generated? Use routerID?

Thanks,
--
Lawrence

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