Enterasys Networks Security Router X-PeditionTM - Manual (Page 115)

IP Routing Protocols
XSR User’s Guide 5-13
• 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 in the following manner:
• An update request is defined as a request to a peer 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.
• Update acknowledgments answer 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.
• An update packet with the flush flag set is received; all routes learned from that next hop
router are marked unreachable.
• Too many 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
occurs 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).