Subject: [Isis-wg] doubts on IS-IS topology and addressing

From: "Selvam Murugesan" <[email-masked]>
Date: 10 Aug 2001 10:23:04 -0000

Hi,
   I need a small clarification on the IS-IS topology and addressing formats. 

1. Is it must for Level-1 IS to have a adjacency maintained with a Level-2 IS or can the Level-1 IS communicate with a Level-2 IS via Level-1 IS?

2.Regarding the ISO DCC addressing format adopted for Is-IS addresses using semi-octet decimal digits representation, the DSP is allocated 0-35 digits. Assuming CFI =1 (single digit), then CDI = 3digits,ESI = 31 digits, which adds up the total digits in the DSP to 35 digits. How is SEL field of the DSP accomodated in this case?

Kindly help me on this reagrds

Thanks in advance
Regards
Manigandan

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From: "Manav Bhatia" <[email-masked]>
Date: Fri, 10 Aug 2001 16:33:56 +0530

>1. Is it must for Level-1 IS to have a adjacency maintained with a
>Level-2 IS or can the Level-1 IS communicate with a Level-2 IS via
>Level-1 IS?

When a L1 IS receives a packet, it examines the destination address in the
network layer header. If its within its area then routes based on the ID
portion of the address. Otherwise, the IS forwards the packet to the closest
level 2 IS and if an entry does not exist for it then the packet is either
dropped or forwarded to some default IS designated for such purposes.
Thus a level 1 IS can communicate with a level 2 IS via a level 1 IS acting
as a gateway to the "last resort" !

--
manav

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