Re: [rbridge] TRILL control frames communication

[Donald Eastlake <[e-mail]>]

Hi Muhammad,

On Thu, Apr 5, 2012 at 1:55 AM, Muhammad Mohsin Sardar
<[e-mail]> wrote:

> Hi!
>
> Given in RFC6325 section 4.2.3
> TRILL IS-IS frames are never forwarded by an RBridge but are locally
> processed on receipt.  (Such processing may cause the RBridge to send
> additional TRILL IS-IS frames.)
> Given in RFC6325 section 4.6.2
> 1. If the Outer.MacDA is All-IS-IS-RBridges and the Ethertype is L2-IS-IS,
> the frame is handled as described in Section 4.6.2.1.
> 4.6.2.1. TRILL Control Frames: The frame is processed by the TRILL IS-IS
> instance on RBn and is not forwarded.
>
> As IS-IS frames are never forwarded by an RBridge, why All-IS-IS-RBridges
> multicast address is used? Please clarify.

The All-IS-IS-RBridges multicast Ethernet address is used on Ethernet
links to be sure that the frame reaches all the interfaces on the
link. Keep in mind that in TRILL terms a "link" can be a bridged LAN.
TRILL has always supported multi-access links.

> As per my understanding TRILL IIH frames are sent to the neighbors,
> bounded by RBridges ( Broadcast domain is link) Am I right?

Bounded by RBridges, Layer 3 routers, and end stations.

Your figure below was garbled in transmission. Trying to send wide
ASCII art through email is not such a good idea... I've attempted to
reconstruct it narrower below and have also added RB5.

+-----+  +-------+  +-------+     +-------+  +------+  +-----+
| ESa +--+ RB1   +--+ RB3   +--+--+ RB4   +--+ RB2  +--+ ESb |
|     |  |ingress|  |transit|  |  |transit|  |egress|  |     |
+-----+  +-------+  +-------+  |  +-------+  +------+  +-----+
                               |
                             +-+-+
                             |RB5|
                             +---+

> If all RBridges are just up, following pattern will be followed:
> RB1 send TRILL IIH to RB3
> RB2 send TRILL IIH to RB4
> RB3 send TRILL IIH to RB1 and RB4
> RB4 send TRILL IIH to RB2 and RB3
> Please correct me, if I am misinterpreting it.

RB1 also sends Hellos to ESa and RB2 also sends Hellos to ESb. Beause
Ethernet supports multi-access links and the Hellos are sent to a
multicast address, in my expanded version of your diagram, Hellos sent
by RB3 on the link it shares with RB4 are also delivered to RB5,
Hellos sent by RB4 on that link are also delivered to RB5, and Hellos
send by RB5 are delivered to both RB3 and RB4. It does not matter
particulrly if the link connecting RB3, RB4, and RB5 is some
inherently multi-access technology or perhaps consists of three
point-to-point physical Ethernet links connected by a bridge or
whatever.

> TRILL IS-IS LSP's are used to communicate the link information to
> the campus. That's how, RBridges know each other.
> As RBridges do not forward TRILL IS-IS frames, how information
> (TRILL IS-IS LSP) of RB1 is communicated to RB2?

"not forward" just means that the frames are not automatically
forwarded at a low level. If every RBridge that received an LSP
immediately forwarded it out all of its ports, other than the one it
received it on, for any non-tree topology you would quickly find your
network saturated with loooping LSPs. The ISO standard update process
for the link state database is described in Section 7.3 of ISO/IEC
10589-2002. The techniques used in Layer 3 IS-IS are generally
applicable to TRILL. Commonly, if an LSP is received that is not a
duplicate of one alrady held by the RBridge it will be stored in the
link state database and flags are set so it will be promptly
forwarded.

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   [phone] (cell)
 155 Beaver Street, Milford, MA 01757 USA
 [e-mail]

> Thank you!
>
> --
> Best Regards,
> Muhammad Mohsin Sardar

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