January 2002
Beginner
238 pages
6h 20m
English
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Stub, Totally Stubby, and Not So Stubby Areas
External LSAs are flooded through the OSPF backbone as well as
through all regular areas. Let’s test this using
TraderMary’s network of Figure 6-10. A static route for
192.168.3.0 is defined (pointing to
null0) on Chicago and
redistributed into OSPF. Router Chicago then
advertises an external LSA with a link state ID of
192.168.3.0:
hostname Chicago ! router ospf 10redistribute static metric 100 metric-type 1 subnetsnetwork 172.16.0.0 0.0.255.255 area 0 !ip route 192.168.3.0 255.255.255.0 Null0
The LSA is flooded to all routers in the network. Let’s check Paris as an instance:
Paris#sh ip ospf database external
OSPF Router with ID (192.168.1.5) (Process ID 10)
AS External Link States
Routing Bit Set on this LSA
LS age: 158
Options: (No TOS-capability)
LS Type: AS External Link
Link State ID: 192.168.3.0 (External Network Number )
Advertising Router: 192.168.1.3
LS Seq Number: 80000001
Checksum: 0x8F67
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 100
Forward Address: 0.0.0.0
External Route Tag: 0The route to 192.168.3.0 also appears in the
routing table:
Paris#sh ip route
...
Gateway of last resort is not set
...
O E1 192.168.3.0/24 [110/302] via 10.0.1.2, 00:02:08, Serial1
...Flooding external LSAs throughout an OSPF network may be a waste of resources. Stub areas block the flooding of external LSAs, as we will see in the next section.
Stub Areas
Referring to Figure 6-1, the router ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.