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SwitchE, SwitchF, SwitchG, SwitchH, 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 external 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 switch 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 switch 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
switch SwitchG 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 switch SwitchG.
Take area 1 as an example. Assume the IP address of layer3 switch SwitchA is
10.1.1.1, IP address of layer3 switch SwitchB interface VLAN2 is 10.1.1.2, IP address of
layer3 switch SwitchC interface VLAN2 is 10.1.1.3, IP address of layer3 switch 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 switch SwitchF through Ethernet interface
VLAN1 (IP address 10.1.5.1); SwitchD is connecting to layer3 switch 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
