RuggedCom RuggedRouter RX1100 - Manual (Page 161)

17. Configuring Dynamic Routing
Revision 1.14.3 161 RX1000/RX1100™
subnet or connected to that interface may exchange routes. A neighbor is a specific router to exchange
routes with specified by its IP address. For point to point links (T1/E1 links for example) one must
use neighbor entries to add other routers to exchange routes with. The maximum number of hops
between two points on a RIP network is 15, placing a limit on network size.
Link failures will eventually be noticed although it is not unusual for RIP to take many minutes for
a dead route to disappear from the whole network. Large RIP networks could take over an hour to
converge when link or route changes occur. For fast convergence and recovery, OSPF is a much
better choice. RIP is a fairly old routing protocol and has mostly been superseded by OSPF.
17.1.4. OSPF Fundamentals
The Open Path Shortest First (OSPF) protocol routing determines the best path for routing IP traffic
over a TCP/IP network based on link cost and quality. Unlike static routing, OSPF takes link failures
and other network topology changes into account. Unlike the RIP routing protocol, OSPF provides
less router to router update traffic.
RuggedRouter routing protocols are supplied by the Quagga routing package.
The RuggedRouter OSPF daemon (ospfd) is an RFC 2178 compliant implementation of OSPFv2.
The daemon also adheres to the RFC2370 (Opaque LSA) and RFC3509 (ABR-Types) extensions.
OSPF network design usually involves partitioning a network into a number of self contained areas.
The areas are chosen to minimize intra-area router traffic, making more manageable and reducing
the number of advertised routes. Area numbers are assigned to each area. All routers in the area
are known as Area routers. If traffic must flow between two areas a router with links in each area is
selected to be an Area Border router, and serves as a gateway.
17.1.4.1. Link State Advertisements
When an OSPF configured router starts operating it issues a hello packet. Routers having the same
OSPF Area, hello-interval and dead-interval timers will communicate with each others and are said
to be neighbors
After discovering its neighbors, a router will exchange Link State Advertisements in order to determine
the network topology.
Every 30 minutes (by default) the entire topology of the network must be sent to all routers in an area.
If the link speeds are too low, the links too busy or there are too many routes, then some routes may
fail to get re-announced and will be aged out.
Splitting the network into smaller areas to reduce the number of routes within an area or reducing the
number of routes to be advertised may help to avoid this problem.
In shared access networks (i.e. routers connected by switches or hubs) a designated router and a
backup designated are elected to receive route changes from subnets in the area. Once a designated
router is picked, all routing state changes are sent to the designated router, which then sends the
resulting changes to all the routers.
The election is decided based on the priority assigned to the interface of each router. The highest
priority wins. If the priority is tied, the highest router-id wins.