Subject: [Isis-wg] Help Reqd !

From: Swati Rastogi <[email-masked]>
Date: Sun, 5 Aug 2001 22:07:46 -0700 (PDT)

Hi,
I am new to this protocol and i am going thru the RFC
1142 and am facing the following problems :-

(1) What does rfc 1142 mean by "adjaceny" .. Is it an
interface on which the packets are sent ?? If yes,
then what is a link ?
(2) What is a circuit ?
(3) I was going through the presentation given by Dave
Katz where he compares OSPF and ISIS. What is
troubling me is the fact that in one of his slides he
says that OSPF runs over IP while ISIS runs besides
IP. Now in Dual ISIS we encapsulate our ISIS packet
over the IP. So we are still using IP.
(4) As far as i understand we send CSNPs to list the
sequence numbers of one or more LSPs operating ... and
as an ACK. Why do we then send Partial SNPs ??

Thanks and regards,
Swati Rastogi

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---

From: "Manav Bhatia" <[email-masked]>
Date: Mon, 6 Aug 2001 14:10:32 +0530

> interface on which the packets are sent ?? If yes,
AFAIK adjacencies tell me what all routers/systems are connected to my
router. Link refers to the actual hardware interface.
> (2) What is a circuit ?
Adjacencies can be of many types .. end system,L1 or L2 reachable address
prefixes. Within each type based we have circuit ID which differentiaties as
to whether it is a non broadcast circuit OR broadcast circuit when running
level 1 decision process . .etc (Refer to 7.2.7 rfc 1142)
> over the IP. So we are still using IP.
No Idea !
> as an ACK. Why do we then send Partial SNPs ??
CSNPs are routing packets sent out of the designated router (on a LAN) to
summarize all the LSPs in its database. These are recieved by other direct
neighbouring ISs on the LAN and are used to maintain synchronization of link
state databases amongst all ISs.The synchronization procedure is different
on LANs and point to point links.
On a point to point link, all the LSPs transmitted must be explicitly
acknowledged by a Partial SNP from the other router. Unacknowledged LSPs are
retransmitted.

Regards,
Manav Bhatia

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---

From: "Jeff Learman" <[email-masked]>
Date: Mon, 6 Aug 2001 09:05:31 -0400

----- Original Message ----- 
From: Swati Rastogi <[email-masked]>
To: <[email-masked]>
Sent: Monday, August 06, 2001 1:07 AM
Subject: [Isis-wg] Help Reqd !

> 1142 and am facing the following problems :-

These terms are defined in ISO 10589, which must be read for a thorough
understanding of IS-IS.

> interface on which the packets are sent ?? If yes,

An adjacency is a (circuit, l2 address) pair identifying a means
of reaching an immediate neighbor.  So, an adjacency is like a
neighbor, except that you can have multiple adjacencies to a
single neighbor, e.g., if it has to L2 addresses on one link
or two links between you and it.

> (2) What is a circuit ?

A link.

> over the IP. So we are still using IP.

Nope.  The encapsulation does not change; IS-IS runs
directly over the link layer and does not use IP or OSI
layer 3 addressing in communications between neighbors.

> as an ACK. Why do we then send Partial SNPs ??

As ACKs on point-to-point links.  The rules are different for
point-to-point links and broadcast links and I don't have the
details on the top of my head, but in general, PSNPs are used
as ACKs, and CSNPs are used periodically.

---

From: "Ken Larmer" <[email-masked]>
Date: Mon, 6 Aug 2001 09:52:48 -0400

> interface on which the packets are sent ?? If yes,

From an IS perspective, a directly attached IS neighbour becomes an
adjacency if the Broadcast or Non-Broadcast initialization process succeeds
between the 2 neighbours. This includes IIH PDU Acceptance Tests, Area
Address Tests, Adjacency Tests and Adjacency Communication Tests. Once a
FULL adjacency has been formed, the 2 IS neighbours will exchange routing
information.

> (2) What is a circuit ?

A circuit references a link level connection, which connects at least 2 IS
neighbours together so they may form an adjacency. Circuit-IDs identify
individual links for purposes of path pruning and detection of 2-way
communication.

> over the IP. So we are still using IP.

What Dave means here is that OSPF control packets are encapsulated in IP
packets. Whereas IS-IS control packets are encapsulated in CLNS packets.
Both OSPF and ISIS forward Data packets in their native format, IP packets
for IP data and CLNS packets for OSI data. A Dual ISIS will forward both IP
and CLNS Data packets.

> as an ACK. Why do we then send Partial SNPs ??

This is dependant upon the link type, Broadcast (LAN) or Non-Broadcast
(Point-to-Point).

On a Broadcast  circuit we elect a Designated Router for purposes of
synchronizing the LSP Databases amongst a common set of LAN ISs. The DR
periodically advertises the state of its LSP Database to the rest of the LAN
via CSNPs. If a CSNP does not contain a particular LSP, which another IS on
the LAN currently possesses in its LSP Database, the detecting IS will flood
that LSP onto the LAN. However, if the CSNP contains a LSP reference to an
LSP, which is not present in an adjacent LAN IS LSP Database, that detecting
IS will send a PSNP requesting that particular LSP. The DR, upon receipt of
the PSNP, will then flood that LSP onto the given LAN.

On a Non-Broadcast circuit, CSNPs are sent at initialization and/or during
circuit restart. This lets the adjacent IS know what the remote IS possesses
in its LSP database(s). Both sides of the link set the SRMflag for all LSPs
contained within their respective LSP databases prior to sending the CSNPs
(there are some rules which apply). The CSNP LSP Entries are then processed
to determine if the LSP Databases are synchronized. CSNP referenced LSP
Entries which are non-referenced or newer in the remote LSP Database are
requested via a PSNP and/or transmitted as a result of the SRMflag being
set. CSNP referenced LSP Entries which are non-existent or older in the
remote LSP Database are updated by the local IS as a result of receiving a
PSNP request or as a result of the SRMflag being set (one in the same). No
action is taken for CSNP referenced LSP Entries which are the same (no PSNP
sent, just clear the SRMflag, receipt of the same LSP is equal to receipt of
a PSNP acknowledgement). Once an LSP has been received on a non-broadcast
link, a PSNP is sent as an acknowledgement to that LSP.

Hope this helps?

Cheers,
Ken Larmer

---

From: "Chris Flores" <[email-masked]>
Date: Sun, 5 Aug 2001 21:32:21 -0500

My response is in-line. I hope it helps.

Thanks.

Chris

-----Original Message-----
From: [email-masked]
[mailto:[email-masked]]On Behalf Of Swati Rastogi
Sent: Monday, August 06, 2001 12:08 AM
To: [email-masked]
Subject: [Isis-wg] Help Reqd !

Hi,

cf: The IS-IS protocol builds (and maintains) adjacencies between IS's or
routers. A pair of neighboring IS's are said to be adjacent after they
exchange CNSPs and synchronize their databases. A separate adjacency is
created for each neighbor (on a circuit) and for each level of routing (on
broadcast media).

A link is a communication path between two neighbors.

(2) What is a circuit ?

cf: See RFC 1195. The IS-IS PDUs are transmitted directly over the
underlying link layer services without the need for mutual encapsulation.

cf: A PSNP is used to request the latest LSP from another IS. In addition,
the PSNP is an ACK for the receipt of a new LSP.

---

From: Jeff Learman <[email-masked]>
Date: Mon, 06 Aug 2001 11:01:46 -0400

>FULL adjacency has been formed, the 2 IS neighbours will exchange routing

Well put.  Don't forget that end systems can also have adjacencies,
although this may be less important for IP than for OSI, where it is
critical.

>individual links for purposes of path pruning and detection of 2-way

Again, End System adjacencies can appear on a circuit, even if there
are no other ISs on the link.  This may be of no importance for
an IP-only router.

>for IP data and CLNS packets for OSI data. A Dual ISIS will forward both IP

Be careful when you read "encapsulated in CLNS packets".  It is
absolutely correct, but does NOT mean that they are encapsulated in
CLNP packets.  It does mean that they are OSI connectionless layer
three packets and therefore begin with a Network Layer Protocol ID
(variously called NPID, NLPI, or NLPID).  After that first octet,
IS-IS and CLNP headers have little in common.

Regards,
Jeff