Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
New Member

what is the order of a, b, c for an OSPF route to enter in the routing table? a) less cost, b) longest prefix, c) preffered path type ?

what is the order of a, b, c for an OSPF route to enter in the routing table? a) less cost, b) longest prefix, c) preffered path type ?

In the Doyle book and RFC 2328 states that b is preffered, then c. In what point is looked at the cost?

1 REPLY
Anonymous
N/A

Re: what is the order of a, b, c for an OSPF route to enter in t

Non-policy IPv4 routing always works by longest-match. If you have a route for 10.2.5.0/24 that goes over an OC12 and a route for 10.2.5.192/30 that goes over a 28.8K modem, both routes will be installed in the IP routing table - there is no conflict, it is perfectly legal to have a more specific route. However, IP traffic destined for 10.2.5.193 will always pass over the modem - the router will always pick the route that most exactly matches your destination, the "longest-match". Where OSPF costs come into play is as a tiebreaker when you have two routes for the same prefix. Imagine that both your OC12 and your modem have routes for 10.2.5.192/30. OSPF will consult its cost for that route (based on the bandwidth of each link used to get to that destination) to establish the winner and insert it in the routing table. If the cost happens to be equal (if you had two OC12s to that destination, for instance) then OSPF may choose to install both routes in the table and the router will load-balance between them. The answer to your question is then b) longest prefix, using a) less cost as a tiebreaker when two identical routes exist.

90
Views
0
Helpful
1
Replies
CreatePlease to create content