Hold down is sort of a receiver-side version of triggered updates: the receiver does not use new alternative routes for a period of time (perhaps two router-update cycles) following discovery of unreachability. This gives time for bad news to arrive. In the first example, it would mean that when A received B’s report ⟨D,2⟩, it would set this aside. It would then report ⟨D,∞⟩ to B as usual, at which point B would now report ⟨D,∞⟩ back to A, at which point B’s earlier report ⟨D,2⟩ would be discarded. A significant drawback of hold down is that legitimate new routes are also delayed by the hold-down period.
These mechanisms for preventing slow convergence are, in the real world, quite effective. The Routing Information Protocol (RIP, RFC 2453) implements all but hold-down, and has been widely adopted at smaller installations.
However, the potential for routing loops and the limited value for infinity led to the development of alternatives. One alternative is the link-state strategy, 9.5 Link-State Routing-Update Algorithm. Another alternative is Cisco’s Enhanced Interior Gateway Routing Protocol, or EIGRP, 9.4.4 EIGRP. While part of the distance-vector family, EIGRP is provably loop-free, though to achieve this it must sometimes suspend forwarding to some destinations while tables are in flux.
There is a significant problem with distance-vector table updates in the presence of broken links. Not only can routing loops form, but the loops can persist indefinitely! As an example, suppose we have the following arrangement, with all links having cost 1:
Now suppose the D–A link breaks:
If A immediately reports to B that D is no longer reachable (cost = ∞), then all is well. However, it is possible that B reports to A first, telling A that it has a route to D, with cost 2, which B still believes it has.
This means A now installs the entry ⟨D,B,3⟩. At this point we have what we called in 1.6 Routing Loops a linear routing loop: if a packet is addressed to D, A will forward it to B and B will forward it back to A.
Worse, this loop will be with us a while. At some point A will report ⟨D,3⟩ to B, at which point B will update its entry to ⟨D,A,4⟩. Then B will report ⟨D,4⟩ to A, and A’s entry will be ⟨D,B,5⟩, etc. This process is known as slow convergence to infinity. If A and B each report to the other once a minute, it will take 2,000 years for the costs to overflow an ordinary 32-bit integer.
9.2.1 Slow-Convergence Fixes
The simplest fix to this problem is to use a small value for infinity. Most flavors of the RIP protocol use infinity=16, with updates every 30 seconds. The drawback to so small an infinity is that no path through the network can be longer than this; this makes paths with weighted link costs difficult. Cisco IGRP uses a variable value for infinity up to a maximum of 256; the default infinity is 100.
There are several well-known other fixes:
126.96.36.199 Split Horizon
Under split horizon, if A uses N as its next_hop for destination D, then A simply does not report to N that it can reach D; that is, in preparing its report to N it first deletes all entries that have N as next_hop. In the example above, split horizon would mean B would never report to A about the reachability of D because A is B’s next_hop to D.
Split horizon prevents all linear routing loops. However, there are other topologies where it cannot prevent loops. One is the following:
Suppose the A-D link breaks, and A updates to ⟨D,-,∞⟩. A then reports ⟨D,∞⟩ to B, which updates its table to ⟨D,-,∞⟩. But then, before A can also report ⟨D,∞⟩ to C, C reports ⟨D,2⟩ to B. B then updates to ⟨D,C,3⟩, and reports ⟨D,3⟩ back to A; neither this nor the previous report violates split-horizon. Now A’s entry is ⟨D,B,4⟩. Eventually A will report to C, at which point C’s entry becomes ⟨D,A,5⟩, and the numbers keep increasing as the reports circulate counterclockwise. The actual routing proceeds in the other direction, clockwise.
Split horizon often also includes poison reverse: if A uses N as its next_hop to D, then A in fact reports ⟨D,∞⟩ to N, which is a more definitive statement that A cannot reach D by itself. However, coming up with a scenario where poison reverse actually affects the outcome is not trivial.
188.8.131.52 Triggered Updates
In the original example, if A was first to report to B then the loop resolved immediately; the loop occurred if B was first to report to A. Nominally each outcome has probability 50%. Triggered updates means that any router should report immediately to its neighbors whenever it detects any change for the worse. If A reports first to B in the first example, the problem goes away. Similarly, in the second example, if A reports to both B and C before B or C report to one another, the problem goes away. There remains, however, a small window where B could send its report to A just as A has discovered the problem, before A can report to B.