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Introduction to Routing Protocols

A routing protocol dynamically computes reachability between various end points. This enables communication to work around link and node failures, and additions and withdrawals of various addresses.

IP routing protocols are typically distributed; an instance of the routing protocol runs on each of the routers in a network.

Cumulus Linux does not support running multiple instances of the same protocol on a router.

Distributed routing protocols compute reachability between end points by disseminating relevant information and running a routing algorithm to determine the routes to each end station. To scale the amount of information that needs to be exchanged, routes are computed on address prefixes rather than on every end point address.

Configure Routing Protocols

A routing protocol needs to know three pieces of information, at a minimum:

  • Who am I (my identity)
  • Where to disseminate information
  • What to disseminate

Most routing protocols use the concept of a router ID to identify a node. Different routing protocols answer the last two questions differently.

The way they answer these questions affects the network design and thereby configuration. For example, in a link-state protocol such as OSPF (see Open Shortest Path First - OSPF) or IS-IS, complete local information (links and attached address prefixes) about a node is disseminated to every other node in the network. Since the state that a node has to keep grows rapidly in such a case, link-state protocols typically limit the number of nodes that communicate this way. They allow for bigger networks to be built by breaking up a network into a set of smaller subnetworks (which are called areas or levels) and by advertising summarized information about an area to other areas.

Protocol Tuning

Most protocols provide certain tunable parameters that are specific to convergence during changes.

Wikipedia defines convergence as the “state of a set of routers that have the same topological information about the network in which they operate. " It is imperative that the routers in a network have the same topological state for the proper functioning of a network. Without this, traffic can be blackholed and unable to reach its destination. It is normal for different routers to have differing topological states during changes, but this difference should vanish as the routers exchange information about the change and recompute the forwarding paths. Different protocols converge at different speeds in the presence of changes.

A key factor that governs how quickly a routing protocol converges is the time it takes to detect the change. For example, how quickly can a routing protocol be expected to act when there is a link failure. Routing protocols classify changes into two kinds: hard changes such as link failures and soft changes such as a peer dying silently. They are classified differently because protocols provide different mechanisms for dealing with these failures.

It is important to configure the protocols to be notified immediately on link changes. This is also true when a node goes down, causing all of its links to go down.

Even if a link does not fail, a routing peer can crash. This causes that router to delete the routes it has computed or worse, it makes that router impervious to changes in the network, causing it to go out of sync with the other routers in the network because it no longer shares the same topological information as its peers.

The most common way to detect a protocol peer dying is to detect the absence of a heartbeat. All routing protocols send a heartbeat (or hello) packet periodically. When a node does not see a consecutive set of these hello packets from a peer, it declares its peer dead and informs other routers in the network about this. The period of each heartbeat and the number of heartbeats that need to be missed before a peer is declared dead are two popular configurable parameters.

If you configure these timers very low, the network can quickly descend into instability under stressful conditions when a router is not able to keep sending the heartbeats quickly as it is busy computing routing state; or the traffic is so much that the hellos get lost. Alternately, configuring the timers to very high values also causes blackholing of communication because it takes much longer to detect peer failures. Usually, the default values initialized within each protocol are good enough for most networks. Do not adjust these settings.