User`s guide

80 XSR User’s Guide
IP Routing Protocols Chapter 5
Configuring IP
 An update packet with the flush flag set is received; all routes learned
from that next hop router are marked unreachable.
 An excessive number of retransmissions of an update go
unacknowledged. All routes learned from that next hop router are
marked unreachable.
 An update response for an expired route comes in. That route is
marked unreachable.
The XSR does not retain alternative routes as they are not needed for the
following scenarios:
 Dialer and dialer backup connections, which are not both up at the
same time. Dialer backup is implemented only when the dialer
interface goes down (the best route is lost; the back up interface is
brought up, then an update request and reply are issued and the new
route installed).
 Dial-on-demand connections.
Retransmissions are governed by the following conditions, among others:
 The retransmission timer is a periodic timer set to 5 seconds.
 A limit in the number of retransmissions will be set, after which the
routes learned through the specified circuit are marked as
unreachable. The maximum number of retransmissions is
configurable. The default value is 36.
 After the maximum number of retransmissions has been reached,
requests will continue to be sent out with a polling interval whose
default value is 30 seconds. This value is also configurable. Polling
will continue until a response is received.
OSPF
The Open Shortest Path First (OSPF) routing protocol is a link-state protocol
as defined by RFC-2328. It supports a replicated database approach to routing
where each router has a copy of the database and contributes information to
the database describing the local environment of linked routers.
All routers piece together the data to obtain a current map of the network.
The shortest path is calculated using an algorithm based on information in
the database.