OSPF Summarization and Scalability 223
The following examples list the OSPF cost:
·
Fast Ethernet--10
8
/ 10
8
= 1
·
Ethernet--10
8
/ 10
7
= 10
·
T1--10
8
/ 1,544,000 = 65
Using the formula, the following costs have been derived:
·
56 Kbps serial link--Default cost is 1785
·
64 Kbps serial link--Default cost is 1562
·
E1 (2.048 Mbps serial link)--Default cost is 48
·
4 Mbps Token Ring--Default cost is 25
·
16 Mbps Token Ring--Default cost is 6
·
FDDI--Default cost is 1
The cost can be set by the following commands:
·
ip ospf cost
·
bandwidth bandwidth
·
auto-cost reference bandwidth
The auto-cost reference bandwidth command is used to change the default of 10
8
. Changing
the default affects the cost of every OSPF interface on the router. The following command sets
the reference bandwidth on a Cisco router:
RTR (config-router) #a
a
a
au
u
u
ut
t
t
to
o
o
o-
-
-
-c
c
c
co
o
o
os
s
s
st
t
t
t
r
r
r
re
e
e
ef
f
f
fe
e
e
er
r
r
re
e
e
en
n
n
nc
c
c
ce
e
e
e
b
b
b
ba
a
a
an
n
n
nd
d
d
dw
w
w
wi
i
i
id
d
d
dt
t
t
th
h
h
h?
?
?
?
14294967 is the reference bandwidth in Mbps.
NOTE
To ensure stability, the backbone area should be as small as practicality will allow. All areas
depend on the backbone to supply LSAs.
OSPF Summarization and Scalability
Summarization increases the network's stability and scalability. Summarization allows several
subnetworks to be summarized at a bit boundary before being advertised to other areas. So, 16
subnet routes can be advertised with a single route. A benefit is that link-state changes on a
subnetwork do not generate LSAs in the other area.
87200333.book Page 223 Wednesday, August 22, 2001 2:37 PM