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CCIE Network Design FAQ: IS-IS Network Design

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CCIE Network Design FAQ: IS-IS Network Design

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1: What protocol was IS-IS originally designed to provide routing information for?

Answer: Connectionless Network Service (CLNS).

2: Where can summarization take place in IS-IS?

Answer: On any L2 router.

3: How many levels of routing are there in an IS-IS network?

Answer: Two. L1 and L2.

4: How many pseudonodes are allowed in an IS-IS area?

Answer: 255.

5: Is it possible to overflow the LSP database on a router? What are the indications this is occurring?

Answer: Yes. The overflow bit will set in LSAs advertised by the router with the database that has overflowed.

6: What is the range of internal metrics in IS-IS? Are they external? Why is this a problem in a large-scale network?

Answer: Internals 0–63, Externals 64–127. With this small of a range of metrics, you may not be able to configure the costs of each interface so that the most optimum route is always taken through the network.

7: Why isn’t it good to have a dial backup dial into a router behind a summarization point for the networks behind the dial backup router?

Answer: Because when the dial backup is connected due to a link failure, the routes through the dial backup link cannot be summarized. This produces a lot of possible confusion and effort in the core and distribution layers of the network.

8: Will routers in different areas form L1 neighbor adjacencies?

Answer: No, they will form L2 adjacencies only.

9: Should you just let all the routers in your network run both L1 and L2 routing?

Answer: No, this incurs unnecessary overhead.

10: Will IS-IS automatically repair a partitioned L2 routing domain?

Answer: Although the mechanisms have been defined for doing so, most implementations do not support this.

11: Will routers running integrated IS-IS, which are in the same area but different IP subnets, form an adjacency? What could you look at, and what would you see to determine this is happening?

Answer: No. When you look at a router’s CLNS neighbors, you would see the following:

A#show clns neighbor
System Id Interface SNPA State Holdtime
Type Protocol
00C0.1465.A460 Se0 *HDLC* Up 297 IS
ES-IS

Note that the protocol is ES-IS rather than IS-IS; you would expect an IS-IS adjacency between these two neighbors. Because they are ES-IS neighbors, they will not exchange routing tables.

12: Must all L2 routers form one contiguous group of routers?

Answer: Yes. By definition, all L2 routers must form a contiguous core. In other words, two L2 routers cannot be separated by an L1 router someplace in the middle. It is important to leave enough redundancy between the L2 routers so that a single link failure will not cause the core to be partitioned.

13: How often does IS-IS flood link-state packets? Is this adjustable?

Answer: 20 minutes. Yes, the rate at which LSPs are flooded can be set using the lsprefresh-interval command.

14: How do you advertise a default route in IS-IS?

Answer: Use the default-information originate command under router IS-IS.

15: How do you configure a router so that a default route is advertised only under some conditions?

Answer: You can attach a route map to the default information originate command to conditionally advertise a default route.

16: What is the effect of an LSP that is corrupted at the data link layer, but the error correction codes are correct?

Answer: A possible LSP update storm.

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