Configuring Ospf; Ospf Dynamic Cost - Avaya G250 Administration

Configuring OSPF

The Open Shortest Path First (OSPF) protocol enables routers to compute the path that an IP
packet should follow. Routers exchange routing information with OSPF to determine where to
send each IP packet on its next hop. RIP is an older protocol that serves a similar purpose. For
more information about RIP, see
OSPF is based on the shortest-path-first or link-state algorithm. It was introduced to overcome
the limitations of RIP in increasingly complex network designs. OSPF uses the cost of a path as
the criterion for comparing paths. In contrast, RIP uses the number of hops as the criterion for
comparing paths. Also, updates are sent when there is a topological change in the network,
rather than every 30 seconds as with RIP.
The advantage of shortest-path-first algorithms is that under stable conditions, there are less
frequent updates (thereby saving bandwidth). They converge quickly, thus preventing such
problems as routing loops and Count-to-Infinity, when routers continuously increment the hop
count to a particular network. These algorithms make a stable network. The disadvantage of
shortest-path-first algorithms is that they require a lot of CPU power and memory.
In OSPF, routers use link-state updates to send routing information to all nodes in a network by
calculating the shortest path to each node. This calculation is based on a topography of the
network constructed by each node. Each router sends that portion of the routing table that
describes the state of its own links, and it also sends the complete routing structure
(topography).
You can configure route redistribution between OSPF, RIP, and static routes. With route
redistribution, you can configure the G250/G350 to redistribute routes learned from one protocol
into the domain of the other routing protocol. For more information, see
page 441.

OSPF dynamic cost

An OSPF interface on the G250/G350 can dynamically set a Cost. The Cost represents the
price assigned to each interface for purposes of determining the shortest path.
By default the OSPF interface Cost is calculated based on the interface bandwidth, according to
the following formula:
Cost = 100,000/bandwidth (in kbps)
The result is that the higher the bandwidth, the lower the Cost.
To manually configure the Cost of an OSPF interface, use the ip ospf cost command from
the interface context. By using this option, dynamic bandwidth updates do not change the Cost.
Use the no ip ospf cost command to return to dynamic cost calculation on an interface.
Configuring RIP on page 435.
Configuring OSPF
Route redistribution
Issue 3 February 2007
on
439