Network Bulls
www.networkbulls.com
Best Institute for CCNA CCNP CCSP CCIP CCIE Training in India
M-44, Old Dlf, Sector-14 Gurgaon, Haryana, India
Call: +91-9654672192
OSPF allows summarization at both ABRs and ASBRs but not on other OSPF routers. The
main reason is again that the LSDB must be the same for all routers in a single area. So, if
summarization is needed, the summary prefixes should be created at the edge of an area
(ABR or ASBR) and flooded throughout that area. However, the idea of summarizing on a
www.CareerCert.info
232 CCNP ROUTE 642-902 Official Certification Guide
router internal to an area, hoping that some routers in the area use the summary route, and
others in the same area do not, cannot be done with OSPF.
Good planning of route summaries can overcome the restriction of performing the summarization
only on ABRs and ASBRs. A good OSPF area design includes consideration of
future address summaries, and a good OSPF route summarization design considers the
ABR locations. Although it is rare to design a large internetwork from scratch, an addressing
plan that assigns all or most subnets in an area from one large address block does make
address summarization easier.
OSPF summarization differs slightly on ABRs versus ASBRs. This section first examines
route summarizations on ABRs and then ASBRs.
Manual Summarization at ABRs
The more difficult task with OSPF route summarization occurs when planning the design
of IP address blocks and OSPF areas. When the IP addressing plan and OSPF design have
been completed, if the subnet numbers inside an area happen to be from the same general
range, and none of the subnet in that range exist in other OSPF areas, then a reasonable
summary route can be created at the ABRs connected to that area. Without first having
such a reasonable block of addresses, route summarization may not be a useful option.
After a range of subnets has been chosen for summarization, the parameters in the area
range command must be planned. This command defines the parameters for the summary
route, most notably the origin area from which the subnets exist, and the subnet number/
mask that defines the summary route that should be advertised. The generic version of
the command is listed next, followed by some notes about the various parameters:
area area-id range ip-address mask [cost cost]
■ The configured area number refers to the area where the subnets exist; the summary
will be advertised into all other areas connected to the ABR.
■ The ABR compares the summary route’s range of addresses with all intra-area OSPF
routes, in the origin area, for which the ABR is creating Type 3 LSAs. If at least one
subordinate subnet exists (subnets that sit inside the range), then the ABR advertises
the summary route as a Type 3 LSA.
■ The ABR does not advertise the subordinate subnet’s Type 3 LSAs.
■ The ABR assigns a metric for the summary route’s Type 3 LSA, by default, to match
the best metric among all subordinate subnets.
■ The area range command can also explicitly set the cost of the summary.
■ If no subordinate subnets exist, the ABR does not advertise the summary.
For example, Figure 7-3 (earlier in this chapter) lists three subnets on the right side of the
figure, noted as Data Center subnets: 10.16.1.0/24, 10.16.2.0/24, and 10.16.3.0/24. ABR R1
could be configured to summarize these routes as 10.16.0.0/22, which includes all three
subnets. (10.16.0.0/22 implies a range from 10.16.0.0–10.16.3.255.) The ABRs (R1 and R2)
Key
Topic
www.CareerCert.info
Chapter 7: OSPF Route Summarization, Filtering, and Default Routing 233
R1
Type 3
Metric 11
10.16.1.0/24
Type 3
Metric 13
10.16.3.0/24
Type 3
Metric 11
10.16.0.0/22
Type 3
Metric 12
10.16.2.0/24
ABR
or
area 0 range 10.16.0.0/22
Figure 7-5 OSPF Area Summarization–Consolidating Type 3 LSAs
could be configured to advertise a summary route using the area 0 range 10.16.0.0
255.255.252.0 router subcommand.
Behind the scenes, ABR route summarization causes the ABR to no longer advertise the
subordinate routes’ Type 3 LSAs, but to instead advertise one Type 3 LSA for the summary
prefix. Figure 7-5 shows this concept on ABR R1, assuming the area 0 range
10.16.0.0 255.255.252.0 router subcommand has been configured. The three Type 3
LSAs that would normally have been advertised are shown above the ABR, and the one
Type 3 LSA for the summary route, which replaces the upper LSAs, is shown under the
ABR.
Example 7-4 shows some show command output related to this example. All route filtering
in earlier examples has been removed, and both R1 and R2 have configured OSPF to
summarize 10.16.0.0/22 with the area 0 range 10.16.0.0 255.255.252.0 router OSPF subcommand.
However, in R2’s case, the metric 12 parameter was used.
Example 7-4 R3’s distribute-list to Filter 10.16.1.0/24
! On Router R1, before the summarization:
R1#sh ip route ospf | incl 10.16
O 10.16.2.0/24 [110/12] via 10.10.17.7, 00:00:24, FastEthernet0/0
O 10.16.3.0/24 [110/13] via 10.10.17.7, 00:00:24, FastEthernet0/0
O 10.16.1.0/24 [110/11] via 10.10.17.7, 00:00:34, FastEthernet0/0
www.CareerCert.info
234 CCNP ROUTE 642-902 Official Certification Guide
! Next, configuring the summarization:
router ospf 1
area 0 range 10.16.0.0 255.255.252.0
! Next, on R2, configuring the same summary
router ospf 2
area 0 range 10.16.0.0 255.255.252.0 cost 12
! Next, from R3
R3#show ip ospf database summary 10.16.0.0
OSPF Router with ID (3.3.3.3) (Process ID 3)
Summary Net Link States (Area 34)
Routing Bit Set on this LSA
LS age: 124
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.16.0.0 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000001
Checksum: 0x878F
Length: 28
Network Mask: /22
TOS: 0 Metric: 11
LS age: 103
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.16.0.0 (summary Network Number)
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0x739E
Length: 28
Network Mask: /22
TOS: 0 Metric: 12
R3#show ip route 10.16.0.0 255.255.0.0 longer-prefixes
! legend omitted for brevity
10.0.0.0/8 is variably subnetted, 16 subnets, 4 masks
O IA 10.16.0.0/22 [110/658] via 10.10.13.1, 00:03:46, Serial0/0/0.1
The example demonstrates the theory of what happens behind the scenes. R3 lists only
two Type 3 LSAs related to the 10.16.1.0/24, 10.16.2.0/24, and 10.16.3.0/24 subnets: the
Type 3 LSAs created by R1 and R2 for 10.16.0.0/22. However, the output does not denote
that this LSA represents a summarized route–it simply looks like yet another Type 3 LSA.
www.CareerCert.info
Chapter 7: OSPF Route Summarization, Filtering, and Default Routing 235
(Any mention of the word “summary” in the output refers to the fact that Type 3 LSAs are
called summary LSAs.) In this case, R3’s path to reach both R1 and R2 ties, but the LSA
for R1’s 10.16.0.0/22 summary was injected with metric 11, based on the lowest metric subordinate
route on R1, whereas R2’s uses the explicitly configured metric 12. As a result,
R3’s best route for 10.16.0.0/22 uses R1, as shown in the route at the end of the example.
The first show command in the example shows R1’s metrics for the three subordinate subnets,
specifically metrics 11, 12, and 13. As such, R1’s summary for 10.16.0.0/22, as shown
in R3’s show ip ospf database summary 10.16.0.0 command, confirms that by default R1
gave the summary route’s Type 3 LSA the best metric among the component subnets.
Note: Although not discussed in depth here, the optional not-advertise option on the
area range command tells the ABR to not advertise the Type 3 LSA for the summary
route, making it possible to do the equivalent of Type 3 LSA filtering with the area range
command.
Manual Summarization at ASBRs
OSPF defines an ASBR as a router that redistributes routes into OSPF from some other
routing source. When redistributing the routes, the ASBR creates a Type 5 External LSA
for each redistributed subnet, listing the subnet number as the LSID and listing the mask
as one of the fields in the LSA. The LSA also lists the ASBR’s RID as the advertising router
and a cost metric for the route. For the purposes of route summarization, you can think of
a Type 5 LSA as working much like a Type 3 LSA, except for routes learned externally.
Chapter 9 discusses external OSPF routes in more depth, including some additional background
on Type 5 LSAs. However, to keep the discussion of OSPF route summarization
together, this section describes ASBR route summarization, which has many similarities to
summarization by an ABR.
If you add the summary-address prefix mask OSPF subcommand, OSPF will then attempt
to summarize the external routes by creating a Type 5 LSA for the summary route,
and by no longer advertising the Type 5 LSAs for the subordinate subnets. When looking
for potential subordinate subnets inside the summary, the ASBR looks at all routes being
redistributed into OSPF from all outside route sources, and if any subordinate subnets exist,
the ASBR performs the route summarization.
Notably, this command works very much like the area range command on ABRs, with the
main exception being that the summary-address command cannot explicitly set the metric
of the summary route. The list of features is as follows:
■ The ASBR compares the summary route’s range of addresses with all routes redistributed
into OSPF on that ASBR to find any subordinate subnets (subnets that sit inside
the summary route range). If at least one subordinate subnet exists, the ASBR advertises
the summary route.
■ The ASBR does not advertise the subordinate subnets.
■ To create the summary, the ASBR actually creates a Type 5 LSA for the summary route.
Key
Topic
www.CareerCert.info
236 CCNP ROUTE 642-902 Official Certification Guide
Table 7-2 OSPF Route Summarization Commands
Where Used Command
ASBR summary-address {{ip-address mask} | {prefix mask}} [not-advertise]
ABR area area-id range ip-address mask [advertise | not-advertise] [cost
cost]
Key
Topic
■ The ASBR assigns the summary route the same metric as the lowest metric route
amongst all subordinate subnets.
■ If no subordinate subnets exist, the ASBR does not advertise the summary.
■ Unlike the area range command, the summary-address command cannot be used to
directly set the metric of the summary route.
The summary-address OSPF subcommand defines the summary route on the ASBR, with
similar syntax and parameters as compared to the area range command seen on ABRs.
Table 7-2 lists the two commands for comparison and study.
No comments:
Post a Comment