434
area3 is defined as a STUB area). Layer3 SwitchA, SwitchB, SwitchE, SwitchF, SwitchH,
SwitchI, SwitchL are in-area layer3 switches, SwitchC, SwitchD, SwitchG, SwitchJ and
SwitchK are edge layer3 switches of the area, SwitchE and SwitchG are edge layer3
switches of the autonomous system.
To area1, layer3 switches SwitchA and SwitchB are both in-area switches, area edge
switches SwitchC and SwitchD are responsible for reporting distance cost to all
destination outside the area, while they are also responsible for reporting the position of
the AS edge layer3 switches SwitchE and SwitchG, AS exterior link-state advertisement
from SwitchE and SwitchG are flooded throughout the whole autonomous system. When
ASE LSA floods in area 1, those LSAs are included in the area 1 database to get the
routes to network N11 and N15.
In addition, layer3 SwitchC and SwitchD must summary the topology of area 1 to the
backbone area (area 0, all non-0 areas must be connected via area 0, direct connections
are not allowed), and advertise the networks in area 1 (N1-N4) and the costs from
SwitchC and SwitchD to those networks. As the backbone area is required to keep
connected, there must be a virtual link between backbone layer3 SwitchJ and SwitchK.
The area edge layer3 switches exchange summary information via the backbone layer3
switch, each area edge layer3 switch listens to the summary information from the other
edge layer3 switches.
Virtual link can not only maintain the connectivity of the backbone area, but also
strengthen the backbone area. For example, if the connection between backbone layer3
SwitchH and SwitchJ is cut down, the backbone area will become incontinuous. The
backbone area can become more robust by establishing a virtual link between backbone
layer3 switches SwitchG and SwitchJ. In addition, the virtual link between SwitchG and
SwitchJ provide a short path from area 3 to layer3 SwitchG.
Take area 1 as an example. Assume the IP address of layer3 SwitchA is 10.1.1.1, IP
address of layer3 SwitchB interface VLAN2 is 10.1.1.2, IP address of layer3 SwitchC
interface VLAN2 is 10.1.1.3, IP address of layer3 SwitchD interface VLAN2 is 10.1.1.4.
SwitchA is connecting to network N1 through Ethernet interface VLAN1 (IP address
20.1.1.1); SwitchB is connecting to network N2 through Ethernet interface VLAN1 (IP
address 20.1.2.1); SwitchC is connecting to network N4 through Ethernet interface
VLAN3 (IP address 20.1.3.1). All the three addresses belong to area 1. SwitchC is
connecting to layer3 SwitchF through Ethernet interface VLAN1 (IP address 10.1.5.1);
SwitchD is connecting to layer3 SwitchE through Ethernet interface VLAN1 (IP address
10.1.6.1); both two addresses belong to area 1. Simple authentication is implemented
among layer3 switches in area1, edge layer3 switches of area 1 authenticate with the
area 0 backbone layer3 switches by MD5 authentication..
The followings are just configurations for all layer3 switches in area 1, configurations
