PrepAway - Latest Free Exam Questions & Answers

Which two statements are true regarding the election process of the DIS in IS-IS? (Choose two.)

Which two statements are true regarding the election process of the DIS in IS-IS? (Choose two.)

PrepAway - Latest Free Exam Questions & Answers

A.
In the event of a priority tie, the DIS is the router with the lowest configured IP address.

B.
In the event of a priority tie, the DIS is the router with the highest MAC address.

C.
A user can configure a priority for DIS election from 0 to 255.

D.
The default DIS election priority is 64 for Level 1 and Level 2.

Explanation:
IS-IS
This protocol is limited by the maximum number of LSPs that each IS-IS router can issue. This is 256 as its LSP ID is 1 octet long. The total number of IP prefixes carried by IS-IS can be easily computed and it comes to O(31000). However, RFC 3786 describes mechanisms to relax the limit on the number of LSP fragments, thereby increasing the number of IP prefixes that can be carried within IS-IS.

I have however, never seen any network carrying more than O(30K) IP prefixes inside IS-IS. Do let me know if youre aware of one that does!

I say this because this is a reasonable number for any sane IS-IS deployment and it will not run out of space unless someone actually injects the entire BGP feed into the IGP. In that case we will run out of space at about 20% of the way into redistribution and not be able to advertise the rest. It is for this reason that this practice has now been deprecated and the RFC 1745 which lays down the rules for BGP- OSPF interaction, has been moved to the HISTORICAL status.

There are 8 bits to define a pseudonode number in the LSPID which means that a router can be a Designated Intermediate System (DIS) for only 256 LANs. Additionally there is also a limitation on the number of routers that can be advertised in pseudonode LSP of the DIS. Dont worry RFC 3786 fixes this!

RFC 3373 OTOH proposes a new TLV thats carried in the IIH PDUs that can increase the number of point-to-point adjacencies from 256 on a single router.

The Remaining lifetime field which gives the number of seconds before LSP is considered expired is 16 bits wide.
This gives the life time of the LSP as 2^16/60/60 Hrs = 18.7 Hrs
Thus the LSP issued by a router needs to be refreshed after every 18.7 Hrs.


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