Configuring OSPF

1
Configuring OSPF Part 1 of 2

• CIS 185 CCNP ROUTE
• Rick Graziani
• Cabrillo College
• graziani_at_cabrillo.edu
• Last Updated Fall 2010

2
Topics

• Review of OSPF
• Areas
• LSAs
• show ip ospf database (summary of link state
  database)
• show ip route
• Stub Areas
• Totally Stubby Areas
• E1 and E2 routes
• Default Routes
• Route Summarization
• NSSA (Not So Stubby Areas)
• Multiple ABR Scenario
• Multiple ASBR Scenario

3
Single Area OSPF - Review
4
Introduction to OSPF

• OSPF is
• Classless
• Link-state routing protocol
• Uses the concept of areas for scalability
• RFC 2328 defines the OSPF metric as an arbitrary
  value called cost.
• Cisco IOS software uses bandwidth to calculate
  the OSPF cost metric.

5
The network Command
Router(config-router) network network-address
wildcard-mask area area-id

• The area area-id refers to the OSPF area.
• A group of OSPF routers that share link-state
  information.
• All OSPF routers in the same area must have the
  same link-state information in their link-state
  databases.
• This is accomplished by routers flooding their
  individual link states to all other routers in
  the area.

6
Link State Concepts
1 Flooding of link-state information
5 Routing Table
3 SPF Algorithm
2 Building a Topological Database
4 SPF Tree
7
Neighbors and Adjacencies

• Before two routers can form an OSPF neighbor
  adjacency, they must agree on three values
• Hello interval
• Dead interval
• Both the interfaces must be part of the same
  network, including having the same subnet mask.
• IP MTU must match

8
Hello Intervals

• By default, OSPF Hello packets are sent
• 10 seconds on multiaccess and point-to-point
  segments
• 30 seconds on nonbroadcast multiaccess (NBMA)
  segments (Frame Relay, X.25, ATM).
• In most cases, use multicast address
  ALLSPFRouters at 224.0.0.5.

9
Dead Intervals

• Cisco uses a default of four times the Hello
  interval.
• 40 seconds - Multiaccess and point-to-point
  segments.
• 120 seconds - NBMA networks.
• Dead interval expires
• OSPF removes that neighbor from its link-state
  database.
• Floods the link-state information about the
  down neighbor out all OSPF-enabled interfaces.

10
Modifying OSPF Intervals
R1 show ip ospf neighbor Neighbor ID Pri
State Dead Time Address
Interface 10.3.3.3 0 FULL/ - 000035
192.168.10.6 Serial0/0/1 10.2.2.2 0
FULL/ - 000036 192.168.10.2
Serial0/0/0

• Dead time is counting down from 40 seconds.
• Refreshed every 10 seconds when R1 receives a
  Hello from the neighbor.

11
Modifying OSPF Intervals
Router(config-if) ip ospf hello-interval
seconds Router(config-if) ip ospf dead-interval
seconds
12
Basic OSPF Configuration

• Lab Topology
• The router ospf command
• The network command
• OSPF Router ID
• Verifying OSPF
• Examining the Routing Table

13
OSPF Router ID
Router ID?
Router ID?
Router ID?

• OSPF Router ID is an IP address used to uniquely
  identify an OSPF router.
• Also used in the DR and BDR process.
• 1. Use the IP address configured with the OSPF
  router-id command.
• 2. Highest IP address of any of its loopback
  interfaces.
• 3. Highest active IP address of any of its
  physical interfaces.

14
Verifying New Router IDs (Loopbacks)
R1 show ip protocols Routing Protocol is ospf
1 Outgoing update filter list for all
interfaces is not set Incoming update filter
list for all interfaces is not set Router ID
10.1.1.1 ltoutput omittedgt R2 show ip
protocols Routing Protocol is ospf 1 Outgoing
update filter list for all interfaces is not set
Incoming update filter list for all interfaces
is not set Router ID 10.2.2.2 ltoutput
omittedgt R3 show ip protocols Routing Protocol
is ospf 1 Outgoing update filter list for all
interfaces is not set Incoming update filter
list for all interfaces is not set Router ID
10.3.3.3 ltoutput omittedgt
15
Verifying OSPF
R1 show ip ospf neighbor Neighbor ID Pri
State Dead Time Address
Interface 10.3.3.3 1 FULL/ - 000030
192.168.10.6 Serial0/0/1 10.2.2.2 1
FULL/ - 000033 192.168.10.2
Serial0/0/0

• Neighbor ID The router ID of the neighboring
  router.
• Pri The OSPF priority of the interface.
• State The OSPF state of the interface.
• Dead Time
• Address The IP address of the neighbors
  interface
• Interface Local interface

16
Verifying OSPF
R1 show ip ospf interface serial
0/0/0 Serial0/0/0 is up, line protocol is up
Internet Address 192.168.10.1/30, Area 0
Process ID 1, Router ID 10.1.1.1, Network Type
POINT_TO_POINT, Cost 64 Transmit Delay is 1
sec, State POINT_TO_POINT, Timer intervals
configured, Hello 10, Dead 40, Wait 40,
Retransmit 5 ltoutput omittedgt
17
Verifying OSPF
R1 show ip protocols Routing Protocol is ospf
1 Outgoing update filter list for all
interfaces is not set Incoming update filter
list for all interfaces is not set Router ID
10.1.1.1 Number of areas in this router is 1. 1
normal 0 stub 0 nssa Maximum path 4 Routing
for Networks 172.16.1.16 0.0.0.15 area 0
192.168.10.0 0.0.0.3 area 0 192.168.10.4
0.0.0.3 area 0 Reference bandwidth unit is 100
mbps Routing Information Sources Gateway
Distance Last Update 10.2.2.2
110 112929 10.3.3.3 110
112929 Distance (default is 110)
OSPF Process ID
OSPF Router ID
Networks OSPF is advertising that are originating
from this router
OSPF Neighbors
Administrative Distance
18
Verifying OSPF
R1 show ip ospf ltsome output omittedgt Routing
Process ospf 1 with ID 10.1.1.1 Start time
000019.540, Time elapsed 113115.776
Supports only single TOS(TOS0) routes Supports
opaque LSA Supports Link-local Signaling (LLS)
Supports area transit capability Router is not
originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs Minimum
hold time between two consecutive SPFs 10000
msecs Maximum wait time between two consecutive
SPFs 10000 msecs Incremental-SPF disabled
Minimum LSA interval 5 secs Minimum LSA arrival
1000 msecs Area BACKBONE(0) Number of
interfaces in this area is 3 Area has no
authentication SPF algorithm last
executed 113031.628 ago SPF algorithm
executed 5 times
19
Verifying OSPF
R1 show ip ospf ltsome output omittedgt Initial
SPF schedule delay 5000 msecs Minimum hold time
between two consecutive SPFs 10000 msecs Maximum
wait time between two consecutive SPFs 10000 msecs

• Any time a router receives new information about
  the topology (addition, deletion, or modification
  of a link), the router must
• Rerun the SPF algorithm
• Create a new SPF tree
• Update the routing table
• The SPF algorithm is CPU intensive, and the time
  it takes for calculation depends on the size of
  the area.

20
Verifying OSPF
R1 show ip ospf ltsome output omittedgt Initial
SPF schedule delay 5000 msecs Minimum hold time
between two consecutive SPFs 10000 msecs

• A flapping link can cause OSPF routers in an area
  to constantly recalculate the SPF algorithm,
  preventing proper convergence.
• If there is a route in the routing table the
  router will continue to forward the packet.
• SPF schedule delay.
• To minimize this problem, the router waits 5
  seconds (5000 msec) after receiving an LSU before
  running the SPF algorithm.
• Minimum hold time
• To prevent a router from constantly running the
  SPF algorithm, there is an additional hold time
  of 10 seconds (10,000 ms).
• The router waits 10 seconds after running the SPF
  algorithm before rerunning the algorithm.

21
Verifying OSPF
R1 show ip ospf interface serial
0/0/0 Serial0/0/0 is up, line protocol is up
Internet Address 192.168.10.1/30, Area 0
Process ID 1, Router ID 10.1.1.1, Network Type
POINT_TO_POINT, Cost 64 Transmit Delay is 1
sec, State POINT_TO_POINT, Timer intervals
configured, Hello 10, Dead 40, Wait 40,
Retransmit 5 ltoutput omittedgt
22
Examining the Routing Table
R1 show ip route Codes ltsome code output
omittedgt D - EIGRP, EX - EIGRP external, O
- OSPF, IA - OSPF inter area 192.168.10.0/30
is subnetted, 3 subnets C 192.168.10.0 is
directly connected, Serial0/0/0 C
192.168.10.4 is directly connected, Serial0/0/1 O
192.168.10.8 110/128 via 192.168.10.2,
142757, Serial0/0/0 172.16.0.0/16 is
variably subnetted, 2 subnets, 2 masks O
172.16.1.32/29 110/65 via 192.168.10.6,
142757, Serial0/0/1 C 172.16.1.16/28 is
directly connected, FastEthernet0/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2
masks O 10.10.10.0/24 110/65 via
192.168.10.2, 142757, Serial0/0/0 C
10.1.1.1/32 is directly connected, Loopback0

• Unlike RIPv2 and EIGRP, OSPF does not
  automatically summarize at major network
  boundaries.

23
The OSPF Metric

• OSPF Metric
• Modifying the Cost of the Link

24
OSPF Metric

• The OSPF metric is called cost. The following
  passage is from RFC 2328
• A cost is associated with the output side of each
  router interface. This cost is configurable by
  the system administrator. The lower the cost, the
  more likely the interface is to be used to
  forward data traffic.
• RFC 2328 does not specify which values should be
  used to determine the cost.

25
OSPF Metric
Cisco IOS Cost for OSPF 108/bandwidth in bps

• Cisco IOS software uses the cumulative bandwidths
  of the outgoing interfaces from the router to the
  destination network as the cost value.
• 108 is known as the reference bandwidth

26
Reference Bandwidth
R1(config-router) auto-cost reference-bandwidth
? 1-4294967 The reference bandwidth in terms of
Mbits per second. R1(config-router) auto-cost
reference-bandwidth 10000
To increase it to 10GigE (10 Gbps Ethernet)
speeds, you need to change the reference
bandwidth to 10,000.

• When this command is necessary, it is recommended
  that it is used on all routers so the OSPF
  routing metric remains consistent.

27
OSPF Accumulates Cost
Serial interfaces bandwidth value defaults to T1
or 1544 Kbps.
R1 show ip route O 10.10.10.0/24 110/65 via
192.168.10.2, 142757, Serial0/0/0

• T1 cost 64 Fast Ethernet cost 1 65
• The Cost 64 refers to the default cost of the
  serial interface, 108/1,544,000 bps 64, and not
  to the actual 64-Kbps speed of the link.

28
Default Bandwidth on Serial Interfaces
R1 show interface serial 0/0/0 Serial0/0/0 is
up, line protocol is up Hardware is GT96K
Serial Description Link to R2 Internet
address is 192.168.10.1/30 MTU 1500 bytes, BW
1544 Kbit, DLY 20000 usec, reliability
255/255, txload 1/255, rxload 1/255

• On Cisco routers, the bandwidth value on many
  serial interfaces defaults to T1 (1.544 Mbps).

29
Modifying the Cost of the Link
Router(config-if) bandwidth bandwidth-kbps
R1(config) inter serial 0/0/0 R1(config-if)
bandwidth 64 R1(config-if) inter serial
0/0/1 R1(config-if) bandwidth 256 R1(config-if)
end R1 show ip ospf interface serial
0/0/0 Serial0/0 is up, line protocol is
up Internet Address 192.168.10.1/30, Area
0 Process ID 1, Router ID 10.1.1.1, Network Type
POINT_TO_POINT, Cost 1562 Transmit Delay is 1
sec, State POINT_TO_POINT, ltoutput omittedgt
100,000,000/64,000 1562

• The bandwidth command is used to modify the
  bandwidth value used by the Cisco IOS software in
  calculating the OSPF cost metric.
• Same as with EIGRP

30
The ip ospf cost Command
R1(config) inter serial 0/0/0 R1(config-if)
bandwidth 64 R1(config-if) end R1 show ip ospf
interface serial 0/0/0 Serial0/0 is up, line
protocol is up Internet Address 192.168.10.1/30,
Area 0 Process ID 1, Router ID 10.1.1.1, Network
Type POINT_TO_POINT, Cost 1562 ltoutput omittedgt
100,000,000/64,000 1562
R1(config) interface serial 0/0/0 R1(config-if)
ip ospf cost 1562

• An alternative method to using the bandwidth
  command is to use the ip ospf cost command, which
  allows you to directly specify the cost of an
  interface.
• This will not change the output of the show ip
  ospf interface command,

31
OSPF and Multiaccess Networks

• Challenges in Multiaccess Networks
• DR/BDR Election Process
• OSPF Interface Priority

32
Solution Designated Router

• OSPF elects a Designated Router (DR) to be the
  collection and distribution point for LSAs sent
  and received.
• A Backup Designated Router (BDR) is also elected
  in case the DR fails.
• All other routers become DROthers.

33
224.0.0.5
224.0.0.6
DROther
DROther
DROther
DROther
DROther
DROther

• DROthers only form full adjacencies with the DR
  and BDR in the network.
• send their LSAs to the DR and BDR
• using the multicast address 224.0.0.6
  (ALLDRouters, all DR routers).
• R1 sends LSAs to the DR.
• The BDR listens, too.
• The DR is responsible for forwarding the LSAs
  from R1 to all other routers.
• DR uses the multicast address 224.0.0.5
  (AllSPFRouters, all OSPF routers).
• Only one router doing all the flooding.

34
DR/BDR Election
BDR
DR
DROther

• The following criteria are applied
• 1. DR Router with the highest OSPF interface
  priority.
• 2. BDR Router with the second highest OSPF
  interface priority.
• 3. If OSPF interface priorities are equal, the
  highest router ID is used to break the tie.
• Default OSPF interface priority is 1.
• Current configuration, the OSPF router ID is used
  to elect the DR and BDR.

35
Verifying Router States
RouterA show ip ospf interface fastethernet
0/0 FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.1/24, Area 0 Process
ID 1, Router ID 192.168.31.11, Network Type
BROADCAST, Cost 1 Transmit Delay is 1 sec,
State DROTHER, Priority 1 Designated Router
(ID) 192.168.31.33, Interface address
192.168.1.3 Backup Designated router (ID)
192.168.31.22, Interface address 192.168.1.2
Timer intervals configured, Hello 10, Dead 40,
Wait 40, Retransmit 5 ltoutput omittedgt
36
Timing of DR/BDR Election
If I booted first and started the election before
the others were ready, I would be the DR!
37
Timing of DR/BDR Election
DR failed! I am now the DR! Elections will now
happened for BDR
DR
I am now the BDR!
BDR

• When the DR is elected, it remains the DR until
  one of the following conditions occurs
• The DR fails.
• The OSPF process on the DR fails.
• The multiaccess interface on the DR fails.
• If the DR fails, the BDR assumes the role of DR,
  and an election is held to choose a new BDR.

38
Timing of DR/BDR Election
DR
BDR
I am a new router with the highest Router ID. I
cannot force a new DR or BDR election, so I am a
DROther.
DROther

• If a new router enters the network after the DR
  and BDR have been elected, it will not become the
  DR or the BDR even if it has a higher OSPF
  interface priority or router ID than the current
  DR or BDR.

39
Timing of DR/BDR Election
Im back but I dont get to become DR again. I am
now just a DROther.
DR
BDR
DROther
DROther

• A previous DR does not regain DR status if it
  returns to the network.

40
Timing of DR/BDR Election
DR
BDR
DROther
Amongst the DROthers I have the highest Router
ID, so I am the new BDR!
BDR

• If the BDR fails, an election is held among the
  DROthers to see which router will be the new BDR.

41
Timing of DR/BDR Election
DR
I am now the new BDR!
BDR
DROther
I am now the new DR!
BDR

• RouterB fails.
• Because RouterD is the current BDR, it is
  promoted to DR.
• RouterC becomes the BDR.

42
Timing of DR/BDR Election
To simplify our discussion, we removed RouterD
from the topology.
How can we make sure RouterB is the DR and
RouterA is the BDR, regarless of RouterID values?
Want to be DR
Highest Router ID
Want to be BDR

• We can change the OSPF interface priority to
  better control our DR/BDR elections.

43
OSPF Interface Priority
Router(config-if) ip ospf priority 0 - 255

• Control the election of these routers with the ip
  ospf priority interface command.
• Priority (Highest priority wins)
• 0 Cannot become DR or BDR
• 1 Default
• Therefore, the router ID determines the DR and
  BDR.
• Priorities are an interface-specific value, they
  provide better control of the OSPF multiaccess
  networks.
• They also allow a router to be the DR in one
  network and a DROther in another.

44
OSPF Interface Priority
RouterA show ip ospf interface fastethernet
0/0 FastEthernet0/0 is up, line protocol is up
Internet Address 192.168.1.1/24, Area 0 Process
ID 1, Router ID 192.168.31.11, Network Type
BROADCAST, Cost 1 Transmit Delay is 1 sec,
State DROTHER, Priority 1 Designated Router
(ID) 192.168.31.33, Interface address
192.168.1.3 Backup Designated router (ID)
192.168.31.22, Interface address 192.168.1.2
Timer intervals configured, Hello 10, Dead 40,
Wait 40, Retransmit 5 ltoutput omittedgt

• The OSPF interface priority can be viewed using
  the show ip ospf interface command.

45
Highest priority wins
Pri 100
Pri 200
RouterA(config) interface fastethernet
0/0 RouterA(config-if) ip ospf priority
200 RouterB(config) interface fastethernet
0/0 RouterB(config-if) ip ospf priority 100

• After doing a shutdown and a no shutdown on the
  Fast Ethernet 0/0 interfaces of all three
  routers, we see the result of the change of OSPF
  interface priorities.

46
Clarifications regarding DR/BDR

• Hello packets are still exchanged between all
  routers on a multi-access segment (DR, BDR,
  DROthers,.) to maintain neighbor adjacencies.
• OSPF LSA packets (coming) are packets which are
  sent from the BDR/DROthers to the DR, and then
  from the DR to the BDR/DROthers. (The reason for
  a DR/BDR.)
• Normal routing of IP packets still takes the
  lowest cost route, which might be between two
  DROthers.

47
More OSPF Configuration

• Redistributing an OSPF Default Route
• Fine-tuning OSPF

48
Redistributing an OSPF Default Route
The static default route is using the loopback as
an exit interface because the ISP router in this
topology does not physically exist.
R1(config) interface loopback 1 R1(config-if)
ip add 172.30.1.1 255.255.255.252 R1(config-if)
exit R1(config) ip route 0.0.0.0 0.0.0.0
loopback 1 R1(config) router ospf
1 R1(config-router) default-information originate

• If the default-information originate command is
  not used, the default quad zero route will not
  be propagated to other routers in the OSPF area.

49
R3s Routing Table
R3 show ip route Gateway of last resort is
192.168.10.5 to network 0.0.0.0
192.168.10.0/30 is subnetted, 3 subnets O
192.168.10.0 110/1952 via 192.168.10.5,
000038, S0/0/0 C 192.168.10.4 is directly
connected, Serial0/0/0 C 192.168.10.8 is
directly connected, Serial0/0/1
172.16.0.0/16 is variably subnetted, 2 subnets, 2
masks C 172.16.1.32/29 is directly
connected, FastEthernet0/0 O 172.16.1.16/28
110/391 via 192.168.10.5, 000038, S0/0/0
10.0.0.0/8 is variably subnetted, 2 subnets, 2
masks C 10.3.3.3/32 is directly connected,
Loopback0 O 10.10.10.0/24 110/782 via
192.168.10.9, 000038, S0/0/1 OE2 0.0.0.0/0
110/1 via 192.168.10.5, 000027, Serial0/0/0
50
External Type 2 Route
R3 show ip route OE2 0.0.0.0/0 110/1 via
192.168.10.5, 000027, Serial0/0/0

• E2 denotes that this route is an OSPF External
  Type 2 route.
• OSPF external routes fall in one of two
  categories
• External Type 1 (E1)
• External Type 2 (E2)
• OSPF accumulates cost for an E1 route as the
  route is being propagated throughout the OSPF
  area.
• This process is identical to cost calculations
  for normal OSPF internal routes.
• E2 route is always the external cost,
  irrespective of the interior cost to reach that
  route.
• In this topology, because the default route has
  an external cost of 1 on the R1 router, R2 and R3
  also show a cost of 1 for the default E2 route.
• E2 routes at a cost of 1 are the default OSPF
  configuration.
• More later

51
Steps to OSPF Operation with States

• 1. Establishing router adjacencies (Routers are
  adjacent)
• Down State No Hello received
• Init State Hello received, but not with this
  routers Router ID
• Hi, my name is Carlos. Hi, my
  name is Maria.
• Two-way State Hello received, and with this
  routers Router ID
• Hi, Maria, my name is Carlos. Hi, Carlos, my
  name is Maria.
• 2. Electing DR and BDR Multi-access
  (broadcast) segments only
• ExStart State with DR and BDR
• Two-way State with all other routers
• 3. Discovering Routes
• ExStart State
• Exchange State
• Loading State
• Full State (Routers are fully adjacent)

4. Calculating the Routing Table 5.
Maintaining the LSDB and Routing Table
52
1. Establishing Adjacencies
Hello 10.6.0.1 10.5.0.1
Hello 10.6.0.1
Down
Init
Down
Init
2-way
2-way
Hello 10.5.0.1
Hello 10.5.0.1 10.6.0.1

• Down State - Init State Two Way State
• Down State - OSPF routers send Hello packets at
  regular intervals (10 sec.) to establish
  neighbors.
• When a router (sends or) receives its first Hello
  packet, it enters the init state.
• Hello packet contains a list of known neighbors.
• When the router sends a Hello packet (unicast
  reply) to the neighbor with its RouterID and the
  neighbor sends a Hello packet packet back with
  that Router ID, the routers interface will
  transition to the two-way state.
• Now, the router is ready to take the relationship
  to the next level.

53
Steps to OSPF Operation with States (cont)
Explanations in Notes Section
54
Couple of notes on link state flooding

• OSPF is a link state routing protocol and does
  not send periodic updates like RIP.
• OSPF only floods link state state advertisements
  when there is a change in topology (this includes
  when a routers are first booted).
• OSPF uses hop-by-hop flooding of LSAs an LSA
  received on one interface are flooded out other
  OSPF enabled interfaces.
• If a link state entry in the LSDB (Link State
  DataBase) reaches an age of 60 minutes (MaxAge)
  without being updated, it is removed and SPF is
  recalculated.
• Every 30 minutes (LSRefreshTime), OSPF routers
  flood only their link states to all other routers
  (in the area).
• This is known as a paranoid update
• These do not trigger SPF recalculations.
• Special note When a link goes down and a router
  wants to send a LSA to tell other routers to
  remove this link state, it sends this link state
  with a value of 60 minutes (MAXAGE).

55
Single Area OSPF End of Review

• CIS 185 Advanced Routing
• Rick Graziani
• Cabrillo College
• graziani_at_cabrillo.edu

56
Issues with large OSPF nets

• Large link-state table
• Each router maintains a LSDB for all links in the
  area
• The LSDB requires the use of memory
• Frequent SPF calculations
• A topology change in an area causes each router
  to re-run SPF to rebuild the SPF tree and the
  routing table.
• A flapping link will affect an entire area.
• SPF re-calculations are done only for changes
  within that area.
• Large routing table
• Typically, the larger the area the larger the
  routing table.
• A larger routing table requires more memory and
  takes more time to perform the route look-ups.
• Solution Divide the network into multiple areas

57
OSPF uses Areas

• Hierarchical routing enables you to separate
  large internetworks (autonomous systems) into
  smaller internetworks that are called areas.
• With this technique, routing still occurs between
  the areas (called inter-area routing).
• Some operations are restricted within an area
• Flooding of LSAs
• Recalculating the database
• Re-running the SPF algorithm

58
OSPF Router Types
59
OSPF Router Types

• Internal Routers with all their interfaces
  within the same area
• Backbone Routers with at least one interface
  connected to area 0
• ASBR (Autonomous System Boundary Router)
  Routers that have at least one interface
  connected to an external internetwork (another
  autonomous system)
• ABR (Area Border Router) Routers with
  interfaces attached to multiple areas.

60
An advantage of Multiple Areas

• Question I understand the routing table is
  recalculated every time the router receives an
  new version of an LSA. Does OSPF recalculate its
  routing table when their is a topology change in
  another area? show ip ospf displays no change in
  SPF execution, but show ip ospf database shows a
  change in the topology?
• Answer Good question! OSPF areas are designed to
  keep issues like flapping links within an area.
• SPF is not recalculated if the topology change is
  in another area.
• The interesting thing is that OSPF distributes
  inter-area (between areas) topology information
  using a distance-vector method.
• OSPF uses link-state principles only within an
  area.
• ABRs do not announce topological information
  between areas, instead, only routing information
  is injected into other areas.
• ABRs relay routing information between areas via
  distance vector technique similar to RIP or
  EIGRP.
• This is why show ip ospf does not show a change
  in the number of times SPF has been executed when
  the topology change is in another area.
• Note It is still a good idea to perform route
  summarization between areas, announcing multiple
  routes as a single inter-area route. This will
  hide any changes in one area from affecting
  routing tables in other areas.

61
OSPF Packet Types

• In CCNA we discussed various OSPF packets

OSPF packet types
62
OSPF Type 4 - Link State Advertisements

• In CCNP we will look at OSPF Type 4 packets more
  closely

OSPF packet types
63
OSPF packet types
OSPF Type-4 packets have 7 LSA packets (later)
64
LSAs used for discovering routes and reaching
Full State, along with Maintain Routes
LSA Types
65
LSA Types

• LSA Types 1 through 5
• We will look at these in detail as we discuss
  areas in this chapter.
• LSA Type 6 MOSPF (Multicast OSPF)
• Not supported by Cisco.
• MOSPF enhances OSPF by letting routers use their
  link-state databases to build multicast
  distribution trees for the forwarding of
  multicast traffic.
• LSA Type 7 NSSA External Link Entry
• Next presentation!
• LSA Type 8 External attributes LSA for BGP
• Not supported by Cisco
• N/A
• LSA Type 9, 10, or 11 Opaque LSAs
• Future upgrades

66
Area Types

• Standard or Normal Areas
• Backbone
• Non-Backbone
• Stub Areas
• Stub Area
• Totally Stubby Area
• Not-so-stubby-area (NSSA)

67
Area Types
68
Part I - LSAs using all normal areas
Multi Area OSPF Normal Areas
What are the router types?
Backbone Area
ASBR
ABR
ABR
Internal
Internal
Internal
Internal
69
Part I - LSAs using all normal areas

• Routes Received on all OSPF Routers
• Overview of Normal Areas This will all be
  explained!
• Receives all routes from within A.S.
• Within the local area LSA 1 and LSA 2
• From other areas (Inter-Area) LSA 3, LSA 4, LSA
  5
• Receives all routes from External A.S.s
  (External AS means routes not from this OSPF
  routing domain)
• From external ASs LSA 5
• As long as routes are being redistributed by the
  ASBR (more later)
• Default Route
• Received only if default-information-originate
  command was used (later)
• If default-information-originate command is not
  used, then the default route is not received

70
1. OSPF Multi-Areas - All Normal Areas

• R33
• router ospf 1
• network 172.16.1.0 0.0.0.255 area 1
• network 172.30.1.0 0.0.0.255 area 1
• R22
• router ospf 1
• network 172.16.1.0 0.0.0.255 area 1
• network 172.30.2.0 0.0.0.255 area 1
• R1
• router ospf 1
• network 10.0.0.0 0.0.0.3 area 0
• network 9.0.0.0 0.0.0.3 area 0
• network 172.16.1.0 0.0.0.255 area 1
• network 172.16.2.0 0.0.0.255 area 1
• R2
• router ospf 1

• R3
• router ospf 1
• network 11.0.0.0 0.0.0.3 area 0
• network 9.0.0.0 0.0.0.3 area 0
• network 172.16.10.0 0.0.0.255 area 51
• network 172.16.11.0 0.0.0.255 area 51
• network 99.0.0.0 0.0.0.3 area 51
• R100
• router ospf 1
• network 99.0.0.0 0.0.0.3 area 51
• network 99.1.0.0 0.0.255.255 area 51
• network 99.0.0.4 0.0.0.3 area 51
• R200
• router ospf 1
• network 99.0.0.4 0.0.0.3 area 51
• network 99.0.0.0 0.0.255.255 area 51

ABR contains network statements for each area it
belongs to, using the proper area value.
71
Part I - LSAs using all normal areas
Multi Area OSPF Normal Areas
What are the router types?
Backbone Area
ASBR
ABR
ABR
Internal
Internal
Internal
Internal
72
Part I - LSAs using all normal areas
Multi Area OSPF Normal Areas
What are the router types?
Backbone Area
ASBR
ABR
ABR
Internal
Internal
Internal
Internal
73
Part I - LSAs using all normal areas
Multi Area OSPF Normal Areas
What are the router types?
Backbone Area
ASBR
ABR
ABR
Internal
Internal
Internal
Internal
74
Understanding LSAs (FYI ONLY)

• show ip ospf database
• This is not the link state database, only a
  summary.
• It is a tool to help determine what routes are
  included in the routing table.
• We will look at this output to learn the tool as
  well as become familiar with the different types
  of LSAs.
• To view the link state database use show ip ospf
  database routernetwork

LSA Header
0 1 2
3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
9 0 1 2 3 4 5 6 7 8 9 0 1 ------------
--------------------
LS age Options LS
type --------------------
------------
Link State ID
------------------------
--------
Advertising Router
------------------------
-------- LS
sequence number
------------------------
-------- LS checksum
length
------------------------
--------
75
LSA 1 - Router Link States

• LSA 1 Router LSA
• Generated by each router for each area it belongs
  to.
• Describes the states of the links in the area to
  which this router belongs.
• Flooded only within the area. On multi-access
  networks, sent to the DR.
• Denoted by just an O in the routing table or
  C if the network is directly connected.
• ABR will include a set of LSA 1s for each area
  it belongs to.
• When a new LSA 1 is received and installed in the
  LSDB, the router forwards that LSA, using
  hop-by-hop or asynchronous flooding.

Router As LSA 1s which are flooded to all other
routers in this area.
Leaf network
76
LSA 1 - Router Link States

• 0 1 2
  3
• 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
  1 2 3 4 5 6 7 8 9 0 1
• ---------------------
  -----------
• LS age
  Options 1
• ---------------------
  -----------
• Link State ID
  
• ---------------------
  -----------
• Advertising Router
  
• ---------------------
  -----------
• LS sequence number
  
• ---------------------
  -----------
• LS checksum
  length
• ---------------------
  -----------
• 0 VEB 0
  links
• ---------------------
  -----------
• Link ID
  
• ---------------------
  -----------
• Link Data
  
• ---------------------
  -----------

77
LSA 1 Router Link States
LSA 1s
LSA 1s
LSA 1s

• Each router floods their LSA 1s ONLY within their
  own area.
• LSA 1s only announce the links (networks) within
  the area.
• Router receives LSA 1s from neighbor, floods
  those LSA 1s to other neighbors within the same
  area.

78
R100 show ip ospf database OSPF
Router with ID (100.100.100.100) (Process ID 1)
Router Link States (Area 51) lt- Note
the Area (LSA 1 - Links in this
area.) Link ID ADV Router Age
Seq Checksum LinkCnt 3.3.3.3
3.3.3.3 42 0x80000004 0x00168d
4 100.100.100.100 100.100.100.100 10
0x80000005 0x00472f 4 200.200.200.200
200.200.200.200 10 0x80000002 0x00db5f 1

• LSA 1 - Router Link States
• For Router Links
• Link State ID Advertising Router ID
• Advertising Router Router ID of the router that
  created this LSA 1
• Bottom line Router Link States (LSA1s) should
  display all the RouterIDs of routers in that
  area, including its own.
• Ricks reminder LSA 1 -gt my one area

79
LSA 1 - Router Link States

• R100 show ip route
• 172.16.0.0/24 is subnetted, 4 subnets
• O 172.16.10.0 110/65 via 99.0.0.1,
  000830, Serial0/0
• O 172.16.11.0 110/65 via 99.0.0.1,
  000830, Serial0/0

• Denoted by just an O in the routing table, or a
  C
• Note Only partial routing tables will be shown

80
LSA 1 - Router Link States
LSA 1s
LSA 1s
LSA 1s
81
LSA 2 - Network Link States

• LSA 2 Network LSA
• Generated by the DR on every multi-access network
• Denoted by just an O in the routing table or
  C if the network is directly connected.
• Flooded only within the originating area.
• LSA 2s are in link state database for all
  routers within area, even those routers on not on
  multi-access networks or DRs on other
  multi-access networks in the same area.
• ABR may include a set of LSA 2s for each area it
  belongs to.

82
LSA 2 - Network Link States

• 0 1
  2 3
• 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
  1 2 3 4 5 6 7 8 9 0 1
• ---------------------
  -----------
• LS age
  Options 2
• ---------------------
  -----------
• Link State ID
  
• ---------------------
  -----------
• Advertising Router
  
• ---------------------
  -----------
• LS sequence number
  
• ---------------------
  -----------
• LS checksum
  length
• ---------------------
  -----------
• Network Mask
  
• ---------------------
  -----------
• Attached Router
  
• ---------------------
  -----------
• ...
  

83
LSA 2s
LSA 2s
LSA 2s

• LSA 2s flooded within area by DR.

84
LSA 2 - Network Link States

• R3 show ip ospf database
•  
• Net Link States (Area 51)
• Link ID ADV Router Age Seq
  Checksum
• 99.0.0.6 200.200.200.200 241
  0x80000002 0x006159

• Link ID IP address of DR on MultiAccess Network
• ADV Router Router ID of DR
• Bottom line Net Link States (LSA2s) should
  display the RouterIDs of the DRs on all
  multi-access networks in the area and their IP
  addresses.
• Ricks reminder LSA 2 -gt Ethernet Layer 2 or
  D R
• 
  1 2

85
LSA 2 - Network Link States
LSA 2s
LSA 2s
86
LSA 3 Summary Net Link States

• LSA 3 Summary LSA
• Originated by the ABR.
• Describes links between ABR and Internal Routers
  of the Local Area
• ABR will include a set of LSA 3s for each area
  it belongs to.
• LSA 3s are flooded throughout the backbone (Area
  0) and to other ABRs.
• Routes learned via LSA type 3s are denoted by an
  IA (Inter-area) in the routing table.

87
LSA 3 Summary LSAs
ABR
ABR
LSA 1s
LSA 3s
LSA 3s

• LSA 3 Summary LSA
• Originated by the ABR.
• Describes links between ABR and Internal Routers
  of the Local Area
• ABR will include a set of LSA 3s for each area
  it belongs to.
• LSA 3s are flooded throughout the backbone (Area
  0) and to other ABRs.
• Routes learned via LSA type 3s are denoted by an
  IA (Inter-area) in the routing table.

88
LSA 3 Summary LSAs
ABR
ABR
LSA 3s
LSA 1s
LSA 3s
89
LSA 3 Summary LSAs
LSA 3s
LSA 3s
LSA 1s
90
LSA 3 Summary Net Link States

• 0 1 2
  3
• 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
  1 2 3 4 5 6 7 8 9 0 1
• ---------------------
  -----------
• LS age
  Options 3 or 4
• ---------------------
  -----------
• Link State ID
  
• ---------------------
  -----------
• Advertising Router
  
• ---------------------
  -----------
• LS sequence number
  
• ---------------------
  -----------
• LS checksum
  length
• ---------------------
  -----------
• Network Mask
  
• ---------------------
  -----------
• 0 metric
  
• ---------------------
  -----------
• TOS TOS
  metric

91
LSA 3 Summary Net Link States
New or change, do not run SPF algorithm.
LSA 1s
LSA 3s
X
LSA 3s
Process using DV technique not LSA 1 Link States.

• Routers only see the topology of the area they
  belong to.
• When a link in one area changes, the adjacent
  routers originate in LSA 1s and flood them
  within the area, causing intra-area (internal)
  routers to re-run the SPF and recalculating the
  routing table.
• ABRs do not announce topological information
  between areas.
• ABRs only inject routing information into other
  areas, which is basically a distance-vector
  technique.

92
LSA 3 Summary Net Link States
LSA 1s
LSA 3s
LSA 3s

• ABRs calculate intra-area routes and announce
  them to all other areas as inter-area routes,
  using LSA 3s.
• OSPF ABRs will only announce inter-area routes
  that were learned from the backbone area, area 0.
• The backbone area serves as a repository for
  inter-area routes.
• This keeps OSPF safe from routing loops.

93
LSA 1s
LSA 3
LSA 3
Not ABR

• In normal operation, OSPF ABRs will only announce
  inter-area routes that were learned from the
  backbone area, area 0.
• RTC does not forward LSA 3s from Area 1 to Area
  51, and does not forward LSA 3s from Area 51 to
  Area 1.
• The backbone area serves as a repository for
  inter-area routes.
• This keeps OSPF safe from routing loops.

94
Normal Areas
LSA 3
LSA 3
LSA 1s
Not ABR

• RTC does not forward the LSA 3s back into Area
  1, or routing loops may develop.
• Note RTC will create LSA 1s and flood them
  within the appropriate area.
• OSPF specification states that ABRs are
  restricted to considering LSA 3s only from the
  backbone area to avoid routing information loops.

95
Normal Areas
Update is sent to Area 0 and Area 51 routers
using a distance vector update technique. SPF
not re-run, but routers update routing table.
LSA 3
LSA 3
Area 1 routers re-run SPF, creates new SPF tree
and updates routing table.
LSA 1s
X

• Topology Change Down Link
• When a router detects a topology change it
  immediately sends out LSA 1s (Router LSAs) with
  the change.
• Age of the LSA is set to MaxAge (3,600 seconds)
  Routers remove this entry from their LSDB (Link
  State Data Base).
• Routers that receive the LSA 1s, within the area
  of the change
• Re-run their SPF algorithm
• Build a new SPF tree
• Update IP routing tables. (Continued next slide)

96
Normal Areas
Update is sent to Area 0 and Area 51 routers
using a distance vector update technique. SPF
not re-run, but routers update routing table.
LSA 3
Area 1 routers re-run SPF, creates new SPF tree
and updates routing table.
LSA 3
LSA 1s
X

• Topology Change Down Link
• ABR RTA receives the LSA 1 and recalculate their
  SPF for that area, Area 1.
• RTA floods the change as a LSA 3 within its other
  area, Area 0.
• RTB receives the LSA 3 and floods it within Area
  51.
• Area 0 and Area 51 routers do not recalculate
  their SPFs, but inject the change into their
  routing tables.

97
LSA 3 Summary Net Link States (INTERNAL)
ABR

• R33 show ip ospf database
• Summary Net Link States (Area 1)
• Link ID ADV Router Age Seq
  Checksum
• 10.0.0.0 1.1.1.1 130
  0x8000000c 0x00ec09
• 9.0.0.0 1.1.1.1 130
  0x8000000d 0x00ec09
• 192.168.2.0 1.1.1.1 130
  0x8000000e 0x00ec09
• 11.0.0.0 1.1.1.1 130
  0x8000000f 0x00ec09
• 172.16.10.0 1.1.1.1 130
  0x80000010 0x00ec09
• 172.16.11.0 1.1.1.1 130
  0x80000011 0x00ec09
• 99.0.0.0 1.1.1.1 130
  0x80000012 0x00ec09
• 99.0.0.4 1.1.1.1 130
  0x80000013 0x00ec09
• 99.1.0.0 1.1.1.1 130
  0x80000014 0x00ec09

• Link ID IP network addresses of networks in
  other areas
• ADV Router ABR Router ID sending the LSA-3
• Bottom line Should see networks in other areas
  and the ABR advertising that route.
• Ricks reminder LSA 3 -gt networks sent by the A
  B R
• 
  1 2 3

98
LSA 3 Summary Net Link States (ABR)

• R1 show ip ospf database
• Summary Net Link States (Area 1)
  lt- Per Area
• Link ID ADV Router Age Seq
  Checksum
• 10.0.0.0 1.1.1.1 255
  0x8000000c 0x00ec09
• 9.0.0.0 1.1.1.1 255
  0x8000000d 0x00ec09
• 192.168.2.0 1.1.1.1 255
  0x8000000e 0x00ec09
• 11.0.0.0 1.1.1.1 255
  0x8000000f 0x00ec09
• 172.16.10.0 1.1.1.1 255
  0x80000010 0x00ec09
• 172.16.11.0 1.1.1.1 255
  0x80000011 0x00ec09
• 99.0.0.0 1.1.1.1 255
  0x80000012 0x00ec09
• 99.0.0.4 1.1.1.1 255
  0x80000013 0x00ec09
• 99.1.0.0 1.1.1.1 255
  0x80000014 0x00ec09

• ABR will show all routes it is injecting into the
  other area including
• LSA 3s from other areas
• LSA 1s from its adjacent area it is injecting
  into this area
• Bottom line Should see networks in other areas
  and the ABR advertising that route.
• Ricks reminder LSA 3 -gt networks sent by the A
  B R
• 
  1 2 3

99
LSA 3 Summary Net Link States

• R2 show ip route
• 99.0.0.0/8 is variably subnetted, 3 subnets, 2
  masks
• O IA 99.0.0.0/30 110/1626 via 11.0.0.2,
  004301, Serial0/1
• O IA 99.0.0.4/30 110/1627 via 11.0.0.2,
  004301, Serial0/1
• O IA 99.1.0.0/16 110/1627 via 11.0.0.2,
  004301, Serial0/1
• 172.16.0.0/24 is subnetted, 4 subnets
• O IA 172.16.1.0 110/65 via 10.0.0.1,
  004221, Serial0/0
• O IA 172.16.2.0 110/65 via 10.0.0.1,
  004251, Serial0/0
• O IA 172.16.10.0 110/1563 via 11.0.0.2,
  004301, Serial0/1
• O IA 172.16.11.0 110/1563 via 11.0.0.2,
  004301, Serial0/1
• 172.30.0.0/24 is subnetted, 2 subnets
• O IA 172.30.1.0 110/66 via 10.0.0.1,
  004221, Serial0/0
• O IA 172.30.2.0 110/66 via 10.0.0.1,
  004221, Serial0/0

• Routes learned via LSA type 3s are denoted by an
  IA (Inter-Area Routes) in the routing table.

100
LSA 1s
LSA 3s
LSA 3s
101
LSA 4 ASBR Summary Link States

• LSA 4 ASBR Summary LSA
• Originated by the ABR.
• Flooded throughout the area.
• Describes the reachability to the ASBRs
• Advertises an ASBR (Router ID) not a network
• Included in routing table as an IA route.
• Exceptions
• Not flooded to Stub and Totally Stubby networks.
• More on this later

102
LSA 4 ASBR Summary Link States

• 0 1 2
  3
• 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
  1 2 3 4 5 6 7 8 9 0 1
• ---------------------
  -----------
• LS age
  Options 3 or 4
• ---------------------
  -----------
• Link State ID
  
• ---------------------
  -----------
• Advertising Router
  
• ---------------------
  -----------
• LS sequence number
  
• ---------------------
  -----------
• LS checksum
  length
• ---------------------
  -----------
• Network Mask
  
• ---------------------
  -----------
• 0 metric
  
• ---------------------
  -----------
• TOS TOS
  metric

103
LSA 4 ASBR Summary Link States
LSA 1s (e bit)
LSA 4
LSA 4

• How does the ABRs know about the ASBR?
• ASBR sends a type 1 Router LSA with a bit
  (external bit e bit) that is set to identify
  itself as the ASBR.

104
LSA 4 ASBR Summary Link States (ABR)
ABR

• R1 show ip ospf database
• Summary ASB Link States (Area 1)
• Link ID ADV Router Age Seq
  Checksum
• 2.2.2.2 1.1.1.1 1482
  0x8000000b 0x00ec09

ASBR
(This) ABR

• Link ID - Router ID of ASBR
• ADV Router - Router ID ABR advertising route
• Bottom line Routers in non-area 0, should see
  Router ID of ASBR and its ABR to get there .
• Ricks reminder LSA 4 -gt Reachability to the A
  S B R
• 
  1 2 3 4

105
LSA 4 ASBR Summary Link States (INTERNAL)
ABR

• R33 show ip ospf database
• Summary ASB Link States (Area 1)
• Link ID ADV Router Age Seq
  Checksum
• 2.2.2.2 1.1.1.1 130
  0x8000000b 0x00ec09

ASBR
(Advertising) ABR

• Link ID - Router ID of ASBR
• ADV Router - Router ID ABR advertising route
• Bottom line Routers in non-area 0, should see
  Router ID of ASBR and its ABR to get there .
• Ricks reminder LSA 4 -gt Reachability to the A
  S B R
• 
  1 2 3 4

106
LSA 4 ASBR Summary Link States
LSA 1s e bit
LSA 4
LSA 4
107
LSA 5 - AS External Link States

• LSA 5 AS External LSA
• Originated by the ASBR.
• Describes destination networks external to the
  Autonomous System (This OSPF Routing Domain)
• Flooded throughout the OSPF AS except to stub and
  totally stubby areas
• Denoted in routing table as E1 or E2 (default)
  route (soon)
• ASBR Router which redistributes routes into
  the OSPF domain.
• Exceptions
• Not flooded to Stub and Totally Stubby networks.
• More on this later

108
LSA 5 - AS External Link States

• 0 1
  2 3
• 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
  1 2 3 4 5 6 7 8 9 0 1
• ---------------------
  -----------
• LS age
  Options 5
• ---------------------
  -----------
• Link State ID
  
• ---------------------
  -----------
• Advertising Router
  
• ---------------------
  -----------
• LS sequence number
  
• ---------------------
  -----------
• LS checksum
  length
• ---------------------
  -----------
• Network Mask
  
• ---------------------
  -----------
• E 0 metric
  
• ---------------------
  -----------
• Forwarding address
  

109
Added -gt
ASBR
R2 (ASBR) router ospf 1 redistribute static ip
route 57.0.0.0 255.0.0.0 ser 0/3
110
R2 (ASBR) router ospf 1 redistribute static ip
route 57.0.0.0 255.0.0.0 ser 0/3
LSA 5
LSA 5s
LSA 5

• Redistribute command creates an ASBR router.
• LSA 5s
• Originated by the ASBR.
• Describes destination networks external to the
  OSPF Routing Domain
• Flooded throughout the OSPF AS except to stub and
  totally stubby areas

111
ASBR

• R1 show ip ospf database
• Type-5 AS External Link States lt-
  Note, NO Area!
• Link ID ADV Router Age Seq
  Checksum Tag
• 0.0.0.0 2.2.2.2 2088
  0x80000003 0x00ddeb 1
• 57.0.0.0 2.2.2.2 2089
  0x80000003 0x00ddeb 0

R2 (ASBR) router ospf 1 redistribute static
default-information originate ip route 0.0.0.0
0.0.0.0 ser 0/2 ip route 57.0.0.0 255.0.0.0 ser
0/3
Note Packet Tracer does not support LSA 5s for
redistributed routes

• Link ID External Networks
• ADV Router Router ID of ASBR
• Note For ABRs There is only one set of AS
  External Link States in database summary. In
  other words, an ABR router will only show one set
  of AS External Link States, not one per area.
  
• Bottom line All Routers should see External
  networks and the Router ID of ASBR to get there .
  
• Ricks reminder LSA 5 -gt O T H E R networks
• 
  1 2 3 4 5

112
LSA 5 - AS External Link States

• R1 show ip route
• O E2 57.0.0.0/8 110/20 via 10.0.0.2, 001602,
  Serial0/0
• OE2 0.0.0.0/0 110/1 via 10.0.0.2, 001602,
  Serial0/0

• Designated by E2
• Notice that the cost is 20 for all redistributed
  routes, we will see why later.
• It has to do with E2 routes and where the default
  cost is 20.
• Redistribute command (Route Optimization
  chapter) If a value is not specified for the
  metric option, and no value is specified using
  the default-metric command, the default metric
  value is 0, except for OSPF where the default
  cost is 20.
• Cost of 1 for the redistributed route.

113
LSA 5 - AS External Link States

• R33 show ip ospf database
• Type-5 AS External Link States lt- Note, NO Area!
• Link ID ADV Router Age Seq
  Checksum Tag
• 0.0.0.0 2.2.2.2 278
  0x80000003 0x00ddeb 1
• 57.0.0.0 2.2.2.2 1187
  0x80000003 0x00ddeb 0

• R33 show ip route
• O E2 57.0.0.0/8 110/20 via 10.0.0.2, 001602,
  Serial0/0
• OE2 0.0.0.0/0 110/1 via 10.0.0.2, 001602,
  Serial0/0

114
LSA 5 - AS External Link States

• E1 vs. E2 External Routes
• External routes fall under two categories
• external type 1
• external type 2 (default)
• The difference between the two is in the way the
  cost (metric) of the route is being calculated.
• The cost of a type 2 route is always the external
  cost, irrespective of the interior cost to reach
  that route.
• A type 1 cost is the addition of the external
  cost and the internal cost used to reach that
  route.
• A type 1 route is always pre