Information Technology Reference
In-Depth Information
Invalid timer
: If an update has not been received to refresh an existing route after 180 seconds (the
default), the route is marked as invalid by setting the metric to 16. The route is retained in the routing
table until the flush timer expires.
Flush timer
: By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the
invalid timer. When the flush timer expires, the route is removed from the routing table.
Hold-down timer
: This timer stabilizes routing information and helps prevent routing loops during
periods when the topology is converging on new information. When a route is marked as unreachable,
it must stay in hold-down mode long enough for all routers in the topology to learn about the unreach-
able network. By default, the hold-down timer is set for 180 seconds. The hold-down timer is dis-
cussed in more detail later in this chapter.
What two commands will show timer values?
show ip route
show ip protocols
Which command shows the most information about timer values?
show ip protocols
EIGRP Updates
Unlike other distance vector routing protocols, EIGRP does not send periodic updates. Describe the
type of updates EIGRP uses.
EIGRP uses the following types of updates:
Nonperiodic
: They are not sent out on a regular basis.
■
Partial
: These updates are sent only when there is a change in topology that influences routing
information.
■
Bounded
: The propagation of partial updates is automatically bounded so that only those
routers that need the information are updated.
■
Triggered Updates
To speed the convergence when there is a topology change, RIP uses triggered updates.
What is a triggered update?
A triggered update is a routing table update that is sent immediately in response to a routing change.
Triggered updates do not wait for update timers to expire.
When are triggered updates sent?
Triggered updates are sent when
An interface changes state (up or down)
■
A route has entered (or exited) the “unreachable” state
■
A route is installed in the routing table
■
Random Jitter
Describe random jitter and explain the problem it solves concerning routing updates.
Over time, the update interval across the routers in the topology can become synchronized, which can
cause problems. As more routers' timers become synchronized, more collisions of updates and more
delays occur in the network.
