Virtual Links; Router Id - Lenovo ThinkSystem NE2552E Application Manual

Virtual Links

Router ID

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Usually, all areas in an OSPF AS are physically connected to the backbone. In some
cases where this is not possible, you can use a virtual link. Virtual links are created
to connect one area to the backbone through another non‐backbone area (see
Figure 41 on page 390).
The area which contains a virtual link must be a transit area and have full routing
information. Virtual links cannot be configured inside a stub area or NSSA. The
area type must be defined as transit using the following command:
NE2552E(config-router-ospf)# area <area index> type transit
The virtual link must be configured on the routing devices at each endpoint of the
virtual link, though they may traverse multiple routing devices. To configure a
NE2552E as one endpoint of a virtual link, use the following command:
NE2552E(config-router-ospf)# area-virtual-link <link number> neighbor-router
<router ID>
where <link number> is a value between 1 and 3, <area index> is the OSPF area index
of the transit area, and <router ID> is the IP address of the virtual neighbor (nbr),
the routing device at the target endpoint. Another router ID is needed when
configuring a virtual link in the other direction. To provide the NE2552E with a
router ID, see the following section, Router ID.
For a detailed configuration example on Virtual Links, see "Example 2: Virtual
Links" on page 407.
Routing devices in OSPF areas are identified by a router ID, expressed in IP
address format. The router ID is not required to be part of any IP interface range or
in any OSPF area, and may even use the NE2552E loopback interface (see
"Loopback Interfaces in OSPF" on page
The router ID can be configured in one of the following two ways:
 Dynamically (the default)—OSPF protocol configures the router ID as the lowest
IP loopback interface IP address, if available, or else the lowest IP interface IP
address, if available. Once dynamically configured, the router ID does not nor‐
mally undergo further updates.
 Statically—Use the following command to manually configure the router ID:
NE2552E(config-router-ospf)# ip router-id <IPv4 address>
To change the router ID from static to dynamic, set the router ID to 0.0.0.0, save the
configuration, and reboot the NE2552E. To view the router ID, enter:
NE2552E(config-router-ospf)# show ip ospf
403).
Chapter 28: OSPF
399