31. Dynamic Routing
ROX™ v2.2 User Guide
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RuggedBackbone™ RX5000
The ROX™ 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.
31.1.3.1. 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 other 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.
31.1.4. Key OSPF And RIP Parameters
31.1.4.1. Network Areas
Network areas determine the regions within which routes are distributed to other routers. The subnets
at a particular router can be added to its OSPF Area. The router will advertise these subnets to all
routers in its area.
OSPF areas must be designed such that no single link failure will cause the network to be
split into two disjoint networks.
A router can be part of multiple areas and function as a gateway between areas. When multiple areas
are used on a network, area 0 is the backbone area. All areas must have a router connecting them
to area 0.
31.1.4.2. Router-ID
Defines the ID of the router. By default this is the highest IP assigned to the router. It is often a good
idea to configure this value manually to avoid the router-id changing if interfaces are added or deleted
from the router. During elections for designated router, the router-id is one of the values used to pick
the winner. Keeping the router-id fixed will avoid any unexpected changes in the election of the master
router.
