Lessons Learned:
OSPF Summarization
All devices within the same area must have the same LSDB
Implies summarization can occur.
-Between Areas
---#area (Source Area) range (address) (mask)
During redistribution
---#summary-address (Address) (mask)
Automatically generates discard route
-disabled with # no discard route (internal | external )
Can be used for TE via longest match routing
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Since OSPF is a link state protocol – all routers within the
same are have to have the same link state DB.
For summarization there are two different types
Inter-area summary
– for summary of LSA type 3 which is the network summary LSA
This is implemented with the area-range command on the area
border router.
Summary routes as they go into Area 0 or into another
non-transit area.
External Summary
– redistribution at the ASBR with the summary-address command.
Note: in a Not so
stubby area – the ABR can summarize type 7 information as it is being generated
as type 5. This technically counts as a redistribution form the type 7 LSA into
the type 5 LSA.
Technical there’s only two places
you can do this - On the ASBR – with the “summary-address command” or ABR – with the “area range: command .
Regardless of what type of summary
we generate - Just like EIGRP or BGP.
The process will automatically
generate the discard route. the discard route is the match for the summary that
is pointing to Null 0.
Note: The idea behind the discard
route is that if we lose one of the subnets that makeup the summary and we
receive packets that are going to one of those destinations, we’re going to
drop them locally instead of forwarding them on to a shorter match, like a
default route.
This would then mean if we did
want to use default routing for subnets that are inside one of our summaries,
we would have to remove the Null route with the
# no discard route (internal |
external ) command - under the OSPF
process.
Summarization can also be used for
Traffic Engineering based on the longest match principal.
Regardless of what the distance is
or the metric is to a particular destination, the router will always choose the path that has the most BITs in common
with the destination. – the longest match.
This means if we we’re to
summarize the destination on multiple ABR’s or ASBR’s – whichever one is
advertising the longer match would be the one that is preferred for the
destinations.
Traffic engineering –
Topology:
============
Router 4 has a local route of 4.4.4.0 –
From the database on R3 you can see that I have two paths to
that route – over R2 and R1.
R3#sh ip ospf database summary 4.4.4.0
OSPF
Router with ID (3.3.3.3) (Process ID 10)
Summary Net Link States (Area 0)
Routing Bit Set on
this LSA
LS age: 312
Options: (No
TOS-capability, DC, Upward)
LS Type: Summary
Links(Network)
Link State ID:
4.4.4.0 (summary Network Number)
Advertising Router:
1.1.1.1
LS Seq Number: 80000002
Checksum: 0xEC3D
Length: 28
Network Mask: /24
TOS: 0 Metric: 2
Routing Bit Set on
this LSA
LS age: 283
Options: (No
TOS-capability, DC, Upward)
LS Type: Summary
Links(Network)
Link State ID:
4.4.4.0 (summary Network Number)
Advertising Router:
2.2.2.2
LS Seq Number:
80000002
Checksum: 0xCE57
Length: 28
Network Mask: /24
TOS: 0 Metric: 2
OSPF
Router with ID (30.30.30.3) (Process ID 50)
R3#
We can also see most of – if not all – the traffic is
currenlt ygoing over R2’s FastEthernet0/1 interface.
R3#sh ip route | i IA
D - EIGRP, EX -
EIGRP external, O - OSPF, IA - OSPF inter area
O IA 2.2.2.0
[110/2] via 192.168.23.2, 00:44:28, FastEthernet0/1
O IA 4.4.4.0
[110/3] via 192.168.23.2, 00:44:28, FastEthernet0/1
O IA 4.4.5.0
[110/3] via 192.168.23.2, 00:00:53, FastEthernet0/1
O IA 4.4.7.0
[110/3] via 192.168.23.2, 00:00:43, FastEthernet0/1
O IA 20.20.20.0
[110/2] via 192.168.23.2, 00:44:28, FastEthernet0/1
O IA 10.1.14.0 [110/2] via 172.16.13.1, 00:44:28,
FastEthernet0/0
O IA 10.1.24.0
[110/2] via 192.168.23.2, 00:44:28, FastEthernet0/1
O IA 11.11.11.0
[110/2] via 172.16.13.1, 00:44:28, FastEthernet0/0
R3#
I want to force all 4.4.5.4 traffic to go over R3. The rest
to still go over R2.
Note: The problem with
using an IGP, to implement traffic engineering is that we cannot make changes
on a per prefix basis. In BGP this not true. In OSPF there’s no way to say
match a certain prefix and change the cost. If we change th cost on the
interface it will inherently change the cost for any route that is using that interface.
Instead the next best path
is to use the longest match for TE.
So on R2 can use a summary
range for the summary address and then on R3 use a more specific longer match
for the / 24 routes.
External Summarization:
This is going to be on a ASBR that is doing redistribution, when
we’re generating either an External 1 or 2, or and N1 or N2 route. The N routes
– are if we’re inside a not so stubby area.
Example:
From the topology – I have
a redist router that is redist EIGRP into OSPF.
router ospf 10
router-id 192.168.33.10
log-adjacency-changes
redistribute eigrp 10 subnets
network 192.168.33.0 0.0.0.255 area 0
This now means that R3 will be generating a type 5 LSA or
the External LSA describing the EIGRP subnets.
R3#sh ip route ospf
O E2 150.5.5.0
[110/20] via 192.168.33.10, 00:02:53, FastEthernet1/0
O E2 150.5.50.0
[110/20] via 192.168.33.10, 00:02:53, FastEthernet1/0
R3#
To verify the redistribution is actually working we can look
at the # Sh ip ospf database, and
look for Type 5 External routes. And we are.
This is the external LSA for the EIGRP destinations. These
are either the E1 or E2 routers.
Type-5
AS External Link States
Link ID ADV
Router Age Seq# Checksum Tag
150.5.5.0
192.168.33.10 371 0x80000001 0x009ECC 0
150.5.50.0
192.168.33.10 371 0x80000001 0x00AD90 0
OSPF Router with ID
(30.30.30.3) (Process ID 50)
You can read this like this:
192.168.33.10 is the origination router of the 150.5.x.x
(eigrp routes) 150.5 .x.x are the xatual prefixes.
R3# sh ip ospf database external 150.5.5.0
OSPF
Router with ID (3.3.3.3) (Process ID 10)
Type-5
AS External Link States
Routing Bit Set on
this LSA
LS age: 604
Options: (No
TOS-capability, DC)
LS Type: AS External Link
Link State ID: 150.5.5.0 (External Network Number )
Advertising Router:
192.168.33.10 –Advertising router
LS Seq Number:
80000001
Checksum: 0x9ECC
Length: 36
Network Mask: /24
Metric Type: 2
(Larger than any link state path) – Metric type is 2 – means an E2 route (bt default)
TOS: 0
Metric: 20 - Default metric for redistribution
Forward
Address: 0.0.0.0 – This mean that for anyone in my area that wants to route to this
destination – thy should route the same path that they use to reach my
router-ID (192.168.33.10)
External Route
Tag: 0
OSPF
Router with ID (30.30.30.3) (Process ID 50)
R3#
From Router 4 ‘s perspective
R4#sh ip route 150.5.5.0
Routing entry for 150.5.5.0/24
Known via "ospf
10", distance 110, metric 20,
type extern 2, forward metric 3 –Metric
of 20, external route - the forward metric
is the Intra-area SPF cost that the local router is using the reach the ASBR
Last update from
10.1.24.2 on FastEthernet0/0, 00:14:16 ago
Routing Descriptor
Blocks:
10.1.24.2, from
192.168.33.10, 00:14:16 ago, via FastEthernet0/0
Route metric is
20, traffic share count is 1
* 10.1.14.1, from 192.168.33.10, 00:14:16 ago, via
FastEthernet0/1 – learned from
Route metric is
20, traffic share count is 1
Note: if I have two paths to the destination we would use
the forward metric to look at what
was the best path to the ASBR. The forward metric is the cost inside the local
area.
The LSA 5 will basically go everywhere in the OSPF area.
Note: now that we have the Type 5 LSA- is we don’t have an
entry in the local DB about the ASB for that prefix – we will use the LSA type
4 – This assumes that the SPT has reachability to the ABR for the external
route and it will forward them to ABR (1.1.1.1 or 2.2.2.2) and assume they can
reach the destination.
Type 4 LSA’s
Summary
ASB Link States (Area 10)
Link ID ADV
Router Age
Seq# Checksum
192.168.33.10
1.1.1.1 1400 0x80000001 0x00EEB2
192.168.33.10
2.2.2.2 1400 0x80000001 0x00D0CC
Key – Is you want to apply Traffic Engineering the External
destinations you do need to take into account the transit path along the
way - not just the redistribution
metric. The only case that the redistribution metric will not matter, is if
there are multiple ASBR’s originating the same route.
So – if I wanted to summarize the 150.5.x.x router to a /16
on the ABR is could simply add the sum address to the process on the ABR.
R3(config)#router ospf 10
R3(config-router)#summary-address 150.5.0.0 255.255.0.0
Then Sh ip route for Null – I should see the summary address
is routed to Null0 and the summary command created the Null route.
REDIST-R1#sh ip route | i Null
O 150.5.0.0/16
is a summary, 00:00:09, Null0
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