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OSPF
89 (LSDB/LSAs
stores topoplogy data and formed from LSAs which sent…
OSPF
89
Properties
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Link-state / operates with SPF algorithm
only changes to Type 1 and 2 LSAs require an SPF calculations
COST(metric) = reference Bandwidth(100Mbit by default) / Interface Bandwidth :pencil2:
Equal Cost = Load Balancing
If two Route-costs are equal, the will be added at routing table.
the default setting of maximum-paths 4 and can be adjusted to 16 or 32
IntraArea Metric:
- Metric between routers inside an Area.
- calculated by total cost of outgoing interfaces to the destination
InterArea Metric:
- Metric from one router in an area to a router in another Area
- calculated by adding the intraArea metric to the next ABR + Metric calculated by ABR (type 3 LSA)to the destination from intraArea
Metric Tunning
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Changing the reference Bandwidth
auto-cost reference Bandwidth
- default reference BW = 100 Mbps(100 x 1000 == 100 000 kbps)
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Default Route
- two purposes:
- to drive destined-internet traffic to internet-connected Router in enterprise network
- to drive traffic of internal routers in area to ABR
- advertised through ABR and ASBR
default-information originate
- without paramters, defalut route must be configured on ABR
- always: default route will be advertised even there is no defaut route on ABR
- metric: define the metric of the default route (default metric 1)
- metric-type; external type 1 or External type 2(default)
Stub Area:
- ABR in stubby Area advertise a default Route to all routers
- fewer LSAa in LSDBs with less memory and processing
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Router types
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ABR(Area Border Router):
Router between Areas, keep a copy of LSDB of each attached area, but doesn't advertise them to other areas, just advertise subnets, calculate routes for each area
- ABRs generate Type 1, Type 3 and Type 4 LSAs
- use default-information originate command to advertise the default route into internal area
ASBR(Autonomous System Boundary Router):
Router between OSPF Areas and another Network running another protocol
DR(Designated Router): :star:
- Router elected on Multi-Access Network and acts as the focal point to all routers in the muti-access network
multiaccess network—a network
with more than two routers connected to the same subnet
- highest priority
- if priorities are equal, the ID will be Tiebreaker
- the default priority of 1 manipulated with command ip ospf priority command under the interface
- changes take effect with clear ip ospf process command
- the election is non-primitive, per Multi-access segment not the Area :!:
- send multicat to 224.0.0.6
- no election occured until the DR and BDR fail.
- it genertaes network link advertisements (NLA) or type 2 LSA
Backup Designated Router(BDR):
- acts as Backup for DR.
- second highest Priority
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Packets:
v:2== ospf v2
v:3 == ospf v3
T:1== packet Nr1
L:48== packet legnth
RID == Router ID
CHK== ckecksum
AUT:0== Authentication
(0=no auth, 1 = cleartext, 2 = md5)
AUK== infos about authentication
Hello
- for Neighbor discovery and Neighbor adjacency
- sent every 10 sec on Broadcast and P2P/ dead intervals 40 :red_cross:
- sent every 30 sec on NBMA/ dead intervals 120 :red_cross:
- sent with two requirements:
- ospf enabled on interfaces by network command or ip ospf area command under interface
- it isn't passive interface
*ROUTER ID:must be unique :red_flag:
highest Loopback Interface otherwise the highest IP Address on any active interface
1- advertised Loopback: used to connect to router
2- unadvertised Loopback:** not used to connect the router :
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IP MTU has to match, otherwise the neighbor will not appear in LSDB, but will in sh ip ospf neighbors
Passive-interface:
the router stops sending routing updates and it will still receive routing updates and send hellos even as it.
DBD(Database Description)
a brief description and summary of LSDB to neighbor to compare Infos/ LSA headers
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Area types
sh ip ospf
Backbone Area: area 0, all areas have to be connected to it.
- can't be configured as Stub area
Standard Area: normal Area next to area 0, connected to Area 0 via ABR.
Stub Area: prevent type 5/ external LSAs to be advertised, no ASBR used, default Route used, at least one ABR.
- all routers in the Area have to be configured as stub.
Totally stub Area: prevent type 5/ external LSAs and type 3/ summary LSAs to be advertised, no ASBR used, at least one ABR.
- ABR configured with area _ stub no-summary
- all routers in the Area have to be configured as stub.
Not-so-stubby Area(NSSA):prevent type 5/ external LSAs to be advertised, ASBR used, type 7 external LSA
- can redistribute external LSAs.
Totally Not-so-stubby-Area(Totally NSSA):prevent type 5/ external LSAs and type 3/ summary LSAs to be advertised, ASBR used
- can redistribute external LSAs.
- configured with area area-number nssa no-summary
OSPF Network Types
non-broadcast NBMA
- with multi-access we have to select DR and BDR manually, we select the hub to be DR and Spokes with prio 0.
- non-broadcast means we have to configure neighbors manually
Broadcast:
- with multi-access we have to select DR and BDR manually, we select the hub to be DR and Spokes with prio 0.
- we don't have to configure neighbors, Multicast will be used to discover neighbors automatically.
Point-to-Multipoint:
- automatic Neighbor discovery
- no DR/BDR election, as it is about p2p
- single ip subnet is used
Point-to-Multipoint non-broadcast:
- no DR/BDR election, as it is about p2p
- single ip subnet is used
- Neighbors configured manually
Point-to-Point:
- automatic neighbor discovery
- no DR/BDR election
- single ip subnet used
- subinterfaces used
Route Filtering
Filtering Type 3 LSA on ABR & Type 5 LSA on ASBR
to prevent some users to reach some subnets
filter type 3 LSA from going IN or OUT an Area
- prefix-list has to be created first
- then [ area _ filter-list prefix prefix-listname IN/OUT]
Filtering Routes that router itself add to its routing table
- Filtering with ditribute list, prevent routers to forward packet to that route
- with distribute-list in Router subcommand
- must refer to ACL or Route map
OSPF Summarization
- conducted only on ABR and ASBR
- only summarize for type 3 and 5 LSAs
Interarea Route summarization/type3 LSA
- created on ABR which creates NULL0 0 as loop
prevention mechanism
- summay will be advertised if i have at least one subnet
falls in summary range
- the least cost of any route will be the cost of the summary
- created wirh area_ range _ command
- can be used to set the metric of summary route
External summarization/type 5 LSA
- created on ABR or ASBR which creates null 0 as
loop prevension mechanism
- created with summary address command
- can't be used to set the metric of summary route
Virtual Link:
- Link created to connect a discontiguous Area to area 0
- discontiguous backbone areas because of Link failure
- to merge tow OSPF domains(tow companys)
- created between ABRs and LSAs sent as unicast between them.
- LSA marked with DNA(Don't Age) avoiding reflooding every 30 sec.
- with command [area virtual link another ABR ID]
- Transit area must not be stubby area
OSPF 3
LSAs
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Type 9 / Intra-Area Prefix LSA:
- send information about ipv6 networks attached to the router like(type 1 in ipv4)
- send information about the ipv6 networks attached to a segment inside an area like(type 2 in ipv4)
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OSPF Verifying
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sh ip ospf interface brief
- confirming on which interfaces ospf enabled / omitting passive interfaces
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LSDB/LSAs
stores topoplogy data and formed from LSAs which sent every 30 Min.
- if LSA with Seq Nr. isn't in LSDB, it added to LSDB and flooded to Neighbors and LSAack sent back.
- if LSA with same Seq Nr. in LSDB it will be discarded.
- if LSA coming with higher Seq Nr. it will be treated as an LSU.
- if LSA coming with lower Seq Nr. it will be treated as obsolete and the newer will be sent back as LSU.
LSA treated as recent if:
- seq nr. is higher
- checksum nr. is higher
- age is equal to maximum age
- link-state age is younger
TO LIMLIT THE LSAs:
max-lsa number subcommand
Type 1/ Router LSA: each Router create its own, sent between routers in same Area.
Lists all directly connected Routers in the Area.
- ABR create two LSAs, one for each Area.
Type 2/ Network LSA: sent between Routers in Multi-access Network in the same area, generated by DR.
lists all routers connected to DR in Routing table
Type 3/ Summary LSA:summary LSA is created by ABR and flooded into other Areas.defines the subnets not the topology, cost to each subnet, RID od ABR
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Type 5/ External LSA: created by ASBR and advertises about external redistributed Routes. E1 and E2 in routing table.
includes:
- LSID (Subnet Number)
- Metric
- RID(advertising Router)
- E1 or E2
E2:
when the design choose the best route based on the external metric(cost of ASBR to subnet), it ignores the internal metric, unless there are tow equal E2s, we can adjust the redistributed routes on ASBRs to make one cost lower than the other, to be finally preferred, as it doesn't depend on internal costs
E1:
by adding the internal metric to ASBR to the metric of ASBR to the destination. we can set it , if we want router to choose best route based on internal cost and as load balance also
- for a given prefix/length, OSPF always prefers an E1 route over an E2
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Type 8/ Link LSAs:
related to ipv6, advertises the local link addresses to the other routers and lists all ipv6 addresses associated with a Link.
Type 9/ Intra Area Prefix LSAs:
for ipv6, play the roles of type 1 and 2 in IPV4
Type 10,11/opaque:
for future uses