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Category: IS-IS

What is a CTunnel interface?

You might have noticed that your IOS release supports a ctunnel interface (hint: your image has to support CLNS) and wondered what it could do. Well, it’s a GRE tunnel between a pair of NSAPs, so you can transport IP traffic across your well-engineered CLNS network without ever exposing the core routers to the dangers of IP.

But wait, it gets better: starting with IOS releases 12.3(7)T and 12.2(33)SRA, you can transport IPv6 across the ctunnel interface. Unfortunately, they haven’t implemented MPLS over GRE over CLNS yet (the mpls ip command is present, but does not work).

It looks like there's at least one potentially very large-scale application that could use this feature.

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IS-IS Is Not Running over CLNP

Numerous sources on the Internet claim that IS-IS runs on top of OSI’s Connectionless Network Protocol (CLNP). This is not the case; although IS-IS and CLNP share the same layer-2 Service Access Point (SAP), OSI provides an additional field (Network Layer Protocol Identifier; NLPID) in the first byte of the layer-3 header.

Contrary to the IP world where the identification of layer-3 protocol is based on Ethertype or PPP protocol ID, the identification of a layer-3 OSI protocol is performed based on layer-2 Service Access Point (DSAP = 0xFE) and the first byte of the layer-3 header, which has the following values:

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Why is OSPF (or IS-IS) afraid of unequal-cost load balancing

You might have wondered why no link-state routing protocols support unequal-cost load balancing (UCLB). Petr Lapukhov provides part of the answer in his Understanding Unequal-Cost Load-Balancing article: EIGRP is one of those few protocols that can ensure a neighbor is not using the current router as its next-hop.

However, one has to wonder: with OSPF and IS-IS having the full network topology (or at least the intra-area part of it) in the SPF tree, how hard would it be to detect that sending a packet to someone that is not on the shortest path would not generate a forwarding loop? Is the lack of OSPF or IS-IS UCLB in Cisco IOS the result of lip service to the standards (at least the OSPF one is way too prescriptive) or a shoddy implementation? What are your thoughts?

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SPF Events in OSPF and IS-IS

Link-state algorithms select the best routes through a two-step process:

  1. The topology of the area is analyzed using SPF algorithm, resulting in a shortest-path tree. This tree contains the shortest paths from the current router to any other node (router or transit LAN) in the current area. This step performed with the Shortest Path First (SPF) algorithm.
  2. The best routes are selected based on the advertisements from all routers in the area (including inter-area and external routes in case of OSPF). The route selection is a simple distance-vector operation where the router selects the minimum-cost IP prefixes from the set of all advertised IP prefixes.
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