Filter
Top Products
Nokia Network Voyager for IPSO 4.0 Reference Guide 355
Area Border Routers
Routers called Area Border Routers (ABR) have interfaces to multiple areas. ABRs compact the
topological information for an area and transmit it to the backbone area. Nokia supports the
implementation of ABR behavior as outlined in the Internet draft of the Internet Engineering
Task Force (IETF). The definition of an ABR in the OSPF specification as outlined in RFC 2026
does not require a router with multiple attached areas to have a backbone connection. However,
under this definition, any traffic destined for areas that are not connected to an ABR or that are
outside the OSPF domain is dropped. According to the Internet draft, a router is considered to be
an ABR if it has more than one area actively attached and one of them is the backbone area. An
area is considered actively attached if the router has at least one interface in that area that is not
down.
Rather than redefine an ABR, the Nokia implementation includes in its routing calculation
summary
LSAs from all actively attached areas if the ABR does not have an active backbone
connection, which means that the backbone is actively attached and includes at least one fully
adjacent neighbor. You do not need to configure this feature; it functions automatically under
certain topographies.
OSPF uses the following types of routes:
Intra-area—Have destinations within the same area.
Interarea—Have destinations in other OSPF areas.
Autonomous system external (ASE)—Have destinations external to the autonomous
system (AS). These are the routes calculated from Type 5 LSAs.
NSSA ASE Router—Have destinations external to AS. These are the routes calculated
from Type 7 LSAs.
All routers on a link must agree on the configuration parameters of the link. All routers in an
area must agree on the configuration parameters of the area. A separate copy of the SPF
algorithm is run for each area. Misconfigurations prevent adjacencies from forming between
neighbors, and routing black holes or loops can form.
High Availability Support for OSPF
VRRP
IPSO supports the advertising of the virtual IP address of the VRRP virtual router. You can
configure OSPF to advertise the virtual IP address rather than the actual IP address of the
interface.
You must use monitored-circuit VRRP, not VRRP v2, when configuring virtual IP support for
OSPF or any other dynamic routing protocol.
If you enable this option, OSPF runs only on the
master of the virtual router; on a failover, OSPF stops running on the old master and then starts
running on the new master. A traffic break might occur during the time it takes both the VRRP
and OSPF protocols to learn the routes again. The larger the network, the more time it takes
OSPF to synchronize its database and install routes again. For more information on enabling the
advertising of a virtual IP address when running OSPF, see “Configuring OSPF,” step 14f.