Convergence occurs when all the routers in the routing domain agree on the routes that are
available. Convergence time is the time that it takes for every router’s routing table to
synchronize after there has been a change in the network topology.
It is important to ensure that the time taken is as short as possible because while the routers
disagree on the available networks, they cannot route data correctly or efficiently.
Each routing protocol has chosen a different method of updating the routing table. This affects
convergence time. Some new concepts are introduced in the following comparison in protocol
convergence methods. This is simply to show the relative merits of each approach.
RIPv1 Convergence
- The steps for RIPv1 convergence are as follows:
- Step 1: - When the local router sees a connected route disappear, it sends a
flash update and removes the route entry from its table. This is
called a triggered update with poison reverse.
- Step 2: - The receiving routers send flash updates and put the affected route
in holddown.
- Step 3: - The originating router queries its neighbor for alternative routes.
If the neighbor has an alternative route, it is sent; otherwise, the
poisoned route is sent.
- Step 4: - The originating router installs the best alternative route that it
hears because it has purged the original routes.
- Step 5: - Routers that are in holddown ignore the alternative route.
- Step 6: - When the other routers emerge from holddown, they will accept
the alternative route.
- Step 7: - Convergence takes the time for detection, plus holddown, plus the
number of routing updates (equal to the hop-count diameter of the network). This could take a long time.
- Step 1: - When the local router sees a connected route disappear, it sends a
flash update and removes the route entry from its table. This is
called a triggered update with poison reverse.
- Step 2: - The receiving routers send flash updates and put the affected route
in holddown.
- Step 3: - The originating router queries its neighbor for alternative routes.
If the neighbor has an alternative route, it is sent; otherwise, the
poisoned route is sent.
- Step 4: - The originating router installs the best alternative route that it
hears because it has purged the original routes.
- Step 5: - Routers that are in holddown ignore the alternative route.
- Step 6: - When the other routers emerge from holddown, they will accept
the alternative route.
- Step 7: - Convergence takes the time for detection, plus holddown, plus the
number of routing updates (equal to the hop-count diameter of the network). This could take a long time.
IGRP Convergence
- The steps for IGRP convergence are as follows:
- Step 1: - When the local router sees a connected route disappear, it sends a
flash update and removes the route entry from its table. This is
called a triggered update with poison reverse.
- Step 2: - The receiving routers send flash updates and put the affected route in holddown.
- Step 3: - The originating router queries its neighbor for alternative routes.
If the neighbor has an alternative route, it is sent; otherwise, the
poisoned route is sent.
- Step 4: - The originating router installs the best alternative route that it
hears because it has purged the original routes. It sends a new flash
update. This is either the routing table, with or without the
network available, stating the higher metric.
- Step 5: - Routers that are in holddown ignore the alternative route.
- Step 6: - When the routers come out of holddown, they accept the alternative route.
When the other routers emerge from holddown, they will accept the alternative route.
- Step 7: - Convergence takes the time for detection, plus holddown, plus the
number of routing updates (equal to the hop-count diameter of the network). Because the update timer is 90 seconds, this could take a very long time.
- Step 1: - When the local router sees a connected route disappear, it sends a
flash update and removes the route entry from its table. This is
called a triggered update with poison reverse.
- Step 2: - The receiving routers send flash updates and put the affected route in holddown.
- Step 3: - The originating router queries its neighbor for alternative routes.
If the neighbor has an alternative route, it is sent; otherwise, the
poisoned route is sent.
- Step 4: - The originating router installs the best alternative route that it
hears because it has purged the original routes. It sends a new flash
update. This is either the routing table, with or without the
network available, stating the higher metric.
- Step 5: - Routers that are in holddown ignore the alternative route.
- Step 6: - When the routers come out of holddown, they accept the alternative route.
When the other routers emerge from holddown, they will accept the alternative route.
- Step 7: - Convergence takes the time for detection, plus holddown, plus the
number of routing updates (equal to the hop-count diameter of the network). Because the update timer is 90 seconds, this could take a very long time.
EIGRP Convergence
- The steps for EIGRP convergence are as follows:
- Step 1: - When the local router sees a connected route disappear, it checks
the topology table for a feasible successor.
- Step 2: - If no feasible successor exists, it moves into active state.
- Step 3: - The originating router queries its neighbor for alternative routes,
the receiving router acknowledges.
- Step 4: - If an alternative exists, it is sent.
- Step 5: - If the router receives an acceptable successor, it adds the route to the table.
- Step 6: - A flash update of the path with the higher metric is sent out.
- Step 7: - Updates are acknowledged.
- Step 1: - When the local router sees a connected route disappear, it checks
the topology table for a feasible successor.
- Step 2: - If no feasible successor exists, it moves into active state.
- Step 3: - The originating router queries its neighbor for alternative routes,
the receiving router acknowledges.
- Step 4: - If an alternative exists, it is sent.
- Step 5: - If the router receives an acceptable successor, it adds the route to the table.
- Step 6: - A flash update of the path with the higher metric is sent out.
- Step 7: - Updates are acknowledged.
Convergence is very quick because it is the detection time, plus query, reply, and update time.
If there is a feasible successor, then convergence is almost instantaneous.
OSPF Convergence
- The steps for OSPF convergence are as follows:
- Step 1: - When a router detects a link failure, an LSA is sent to its
neighbors. If the router is on a multiaccess link, then the update is
sent to the DR and BDR, not to all neighbors.
- Step 2: - The path is removed from the originating router’s tables.
- Step 3: - On receipt of the LSA, all routers update the topology table and
flood the LSA out its interfaces.
- Step 4: - The Dijkstra algorithm is run to rebuild the routing table.
- Step 1: - When a router detects a link failure, an LSA is sent to its
neighbors. If the router is on a multiaccess link, then the update is
sent to the DR and BDR, not to all neighbors.
- Step 2: - The path is removed from the originating router’s tables.
- Step 3: - On receipt of the LSA, all routers update the topology table and
flood the LSA out its interfaces.
- Step 4: - The Dijkstra algorithm is run to rebuild the routing table.
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