Category: IS-IS

Many service providers choosing IS-IS as their IGP use it within a single area (or at least run all routers as L1L2 routers). Multi-level IS-IS design is a royal pain, more so in MPLS environments where every PE-router needs a distinct route for every BGP next hop (but of course there’s a nerd knob to disable L1 default route in IS-IS). Moreover, MPLS TE is reasonably simple only within a single level (L1 or L2).

I’m positive at least some service providers do something as stupid as I usually did – deploy IS-IS with default settings using a configuration similar to this one:

My Junos versus Cisco IOS: Explicit versus Implicit received a huge amount of helpful comments, some of them slightly philosophical, others highly practical – from using interfaces all combined with interface disable in routing protocol configuration, to using configuration groups (more about that fantastic concept in another post).

However, understanding what’s going on is not the same as being able to explain it in one sentence ... and Dan (@jonahsfo) Backman beautifully nailed that one.

My first Junos labbing project was an IPv6 backbone; I wanted to create a simple single-area IS-IS/BGP-free backbone running LDP and MPLS, and using 6PE for IPv6 connectivity. Needless to say, even though I read the excellent Day One books (highly recommended: Exploring IPv6, Advanced IPv6 configuration and Deploying MPLS), I stumbled on almost every step.

A reader of my blog planning to migrate his network from a traditional BGP-everywhere design to a BGP-over-MPLS one wondered about potential unexpected consequences. The MTU implications of introducing MPLS in a running network are usually well understood (even though you could get some very interesting behavior); if you can, increase the MTU size by at least 16 bytes (4 labels) and check whether MTU includes L2 header. Another somewhat more mysterious beast is the interaction between IGP and LDP that can cause traffic disruptions after the physical connectivity has been reestablished.

Yap Chin Hoong has been looking at the OSI protocol stack I’ve published and asked an interesting question: “where is CLNS in that protocol stack?”

The OSI protocol stack has a major advantage over the TCP/IP stack: it defines both the protocols and the APIs between the layers. CLNS (Connection-less network Service) is the API (the function calls that allow transport layers to exchange datagrams across the network) while CLNP (Connection-less network Protocol) is the layer-3 protocol that implements CLNS. In my diagram, CLNS would be a thin line above CLNP between L3 and L4 boxes.

Someone started an interesting discussion on the NANOG mailing list. He inherited a network that extended its internal OSPF to its multihomed customers and wondered whether he should leave the network, change OSPF to IS-IS, or deploy BGP. Here are a few thoughts from my reply.

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:

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 entire 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 a device that is not on the shortest path results in 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?

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.