The following commands can
be used to modify OSPF behavior.
- Modifying the OSPF router ID to a loopback address
:
router(config-if)#interface loopback number
The highest IP address used as the router ID can be overridden by
configuring an IP address on a loopback interface. A loopback interface
is a virtual interface that is always active and does not go
"down" like a physical interface. As such, OSPF is more
reliable if a loopback interface is configured because it will remain
active, even if one of the physical interfaces fail. Thus, it is
recommended that you use the loopback address on all key routers in your
OSPF based network. If you plan to publish your loopback address with
the
network area
command, make sure you use a private IP address.
Note that a loopback address requires a different subnet for each
router.
Pros and cons exist in using private addresses as opposed to using
real Internet subnet addresses. Private addressing saves on real IP
addresses, however, care must be taken to keep the private addresses
from being released to the actual Internet. Often, your choice of a
"private" address may well belong to a legitimate address
outside your network. This decision represents a trade-off between the
ease of designing the network and conservation of address space.
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Lab
Activity |
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In
this lab, you will learn how to stabilize OSPF router ID with a
loopback interface. |
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The Figure shows a router that could be configured using the private
loopback address of 1.1.1.1 or the real subnet address of 131.108.17.5.
To determine the Router ID of a router, type
show
ip ospf interface.
- Modifying router priority
--- Changing the OSPF priority on an
interface is accomplished using the following interface command:
router(config-if)#ip ospf priority number (from 0
to 255)
The default is 1. A priority value of 0 indicates an interface cannot
be elected as DR or BDR.
--- Override the default cost value
assigned to an OSPF interface, as follows:
router(config-if)#ip ospf cost cost
cost
--- A number from 1 to 65535 that indicates the metric
assigned to the interface. Path cost is the total of the costs
assigned to all interfaces that forward traffic along the path to the
destination.
Cisco's OSPF default cost assignment is based on the bandwidth of the
link. Other vendors might use a different mechanism to assign OSPF cost
to a link, so you may have to change the default cost. The key to
remember is that all interfaces connected to the same link must agree on
the link's cost.
In general, the path cost in Cisco routers is calculated using the
formula 108/Bandwidth.
Using this formula, the following are some example default costs:
- 56-kbps serial link. Default cost is 1785.
- T1 (1.544-Mbps serial link). Default cost is 64.
- 10BASE-T Ethernet. Default cost is 10.
- 16-Mbps Token Ring. Default cost is 6.
- 100-Mbps Fast Ethernet. Default cost is 1.
Note: On
serial lines, the default bandwidth is 1.544 Mbps. If the line is a
slower speed, use the bandwidth command to specify the real link
speed. The cost of the link will then change to correspond to the
bandwidth you configured.
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Lab
Activity |
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In this lab, you will learn how
to configure OSPF area 0 authentication. |
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