Distance vector routing protocols work by having routers periodically share their routing tables with neighboring routers. This allows each router to build up its own table indicating the distance to various destinations via each neighbor. Issues can arise from slow convergence of routing information leading to inconsistent entries. Techniques like split horizon, route poisoning, and holddown timers aim to prevent routing loops from occurring during this convergence process.
17. • The router keeps an entry for the “possibly down state” in the
network, allowing time for other routers to recompute for this
topology change.
Holddown Timers
18. • The router sends updates when a change in its routing
table occurs.
Triggered Updates
25. Summary
• Distance vector routing protocols generate periodic routing
updates addressed to directly connected routing devices.
Routers running a distance vector routing protocol send
periodic updates even if there are no changes in the network.
• When a router receives an update from a neighbor’s router, the
router compares the update with its own routing table. The
router adds the cost of reaching the neighbor’s router to the
path cost reported by the neighbor to establish a new metric.
• Routing inconsistencies occur if slow internetwork
convergence or a new configuration causes incorrect routing
entries.
26. Summary (Cont.)
• Distance vector protocols define infinity as some maximum
number. The routing protocol then permits the routing table
update loop until the metric exceeds its maximum allowed
value.
• There are five techniques for eliminating routing loops on
distance vector routing networks: split horizon, route
poisoning, poison reverse, holddown timers, and triggered
updates.
• All five techniques can be used together to eliminate routing
loops in area networks.