User`s guide

XSR User’s Guide 79
Chapter 5 IP Routing Protocols
Configuring IP
 The latest changes are sent when:
– The routing database is modified by new data. The latest changes
are sent through all interfaces running triggered-on-demand RIP.
RFC-2091 also specifies how packet types are handled:
 An update request is defined as a request to a peer system to send its
entire routing database. It is sent:
– When the XSR is powered up;
– When an interface is brought up.
 An update response is defined as a message containing zero or more
routes; it is retransmitted at periodic intervals until an update
acknowledge is received. It is sent:
– In response to an update request. The first response contains no
routes. Other update responses will not be sent until an update
acknowledge is received. Then the routing database is sent.
– At power up. The first update response will contain no routes.
– When a port comes up. The first response contains no routes.
– When a port is brought down.
– When there is fresh routing information to be propagated.
 Each update response packet sent to a peer is given a sequence number,
a 16-bit unsigned integer.
 Responses must be received in order. Updates received with a
sequence number out of order is dropped. Packets are accepted if:
– A sequence number is one more than the previous;
– A sequence number is the same as the previous (occurs when the
ack for the previous was sent, but not received on the other side);
– The sequence number is 0 (could occur at startup or when it
wraps around).
– The response sequence number received will be saved and used
as a starting point.
 Resynchronization occurs with every update response.
 An update acknowledgment is sent in answer to every update response.
The RFC delineates route persistency in the routing database as follows. Entries
learned from a triggered response on participating WAN interfaces are
permanent, unless certain events occur, in which case entries are marked as
unreachable and the hold-down timer started. These events are:
 A circuit-down event has been received; all routes learned from that
next hop router are marked unreachable.