- All OSPF routers will be one of four router types:
1. Internal - Are routers whose interfaces all belong to the same area. These routers have a single link-state database. exchange LSAs only with routers in the same area.
2. Backbone - Are routers with all interfaces attached to the backbone.
3. ABR - Connect one or more areas to the backbone and act as a gateway for inter-area traffic. Has at least one interface, which belongs to the backbone, and must maintain a separate link-state database for each of its connected areas.
4. ASBR - Is a gateway to external traffic. It injects routes into the OSPF domain that was learned (redistributed) from another external protocol.
LSA (Link State Advertisements)
- LSA is the OSPF data structure used to describe topology information.
- LSAs are aged as they reside in the link-state database.
- MaxAge (1 hour) is the time if reached when LSAs are flushed from the OSPF domain.
- LSRefreshTime (every 30 min): The router that originated the LSA floods a new copy of the LSA with an incremented
sequence number and an age of zero.
LSA types:
1. Router LSAs (intra Area)
- Are produced by every router for all its own connected interfaces.
- Lists all of a router's links, or interfaces, the state and outgoing cost of each link, and any known OSPF neighbors on the link.
- Local area flooding scope.
- Describes the intra-area routes (Displayed as 'O' routes in the RIB)
- Can be seen with "show ip ospf database router".
2. Network LSAs (intra area)
- Are produced by the DR on every multi-access network.
- Lists all attached routers, including the DR itself.
- Local area flooding scope.
- Describes who is the designated routers on a segment.
- Can be seen with "show ip ospf database network".
3. Network Summary LSAs (inter area)
- Are originated by ABRs.
- Are sent into a single area to advertise destinations outside that area, but still internal to the OSPF
autonomous system.
- Default routes external to the area, but internal to the OSPF autonomous system, are also advertised
by LSA type 3.
- Inter-Area flooding scope.
- Describes the inter-area routes (Displayed as 'O*IA' routes in the RIB)
- Can be seen with "show ip ospf database summary".
- on different area, next-hop to reach the other network is via ABR
4. ASBR Summary LSAs
- Are originated by ABRs.
- Are identical to network summary LSAs, except that the destination they advertise is an ASBR, not a
network.
- Inter-Area flooding scope.
- Describes who is doing the redistribution.
- Can be seen with "show ip ospf database asbr-summary".
area0
-------------ABR-------------------ASBR
type4<------| type1<-------|
Note: ABR will take type 1 from ASBR & convert it to type 4 (domain wide) for next-hop reachability to ASBR.
5. AS External LSAs (domain wide)
- Are originated by ASBRs.
- They advertise either a destination external to the OSPF autonomous system, or a default route external to the OSPF autonomous system.
- AS External LSAs are the only LSA type that are not associated with a particular area.
- Autonomous system wide flooding scope.
- Describes what routes were redistributed (Displayed as 'O*E1' or 'O*E2' routes in the RIB)
- Can be seen with "show ip ospf database external".
6. MOSPF
- Cisco routers do not support LSA Type 6 (MOSPF), and generates syslog messages if such packets are
received.
- It might be necessary to configure a router to ignore these packets and to prevent a large number of syslog messages
- Configured with "ospf ignore lsa mospf"
7. NSSA External LSAs
- Are originated by ASBRs within not-so-stubby areas (NSSAs).
- Similar to an AS External LSA, except NSSA External LSAs are flooded only within the not-so-stubby
areas in which it was originated.
- Describe redistributed routes within a NSSA area (Displayed as 'O*N1' or 'O*N2' routes in the RIB).
- Can be seen with "show ip ospf database nssa-external".
area0 nssa
-------------ABR-------------------
type5<------| type7<----
note: type 7 only exists in nssa
10. Opaque LSAs
- Have been used to add various extensions to OSPF, such as traffic engineering parameters for MPLS networks.
OSPF Link-State Database Overload Protection with MAX-LSA
- Allows you to limit the number of nonself-generated LSAs for a given OSPF process.
- Used to prevent excessive LSA's generated by other routers in the OSPF domain from substantially draining the CPU and memory resources of the router.
- Configured with "max-lsa"
OSPF LSA Throttling
- Provides a dynamic mechanism to slow down link-state advertisement (LSA) updates in OSPF during times of network instability.
- Also allows faster OSPF convergence by providing LSA rate limiting in milliseconds.
- Configured with "timers throttle lsa all"
commands:
show commands:
- Displays the number of LSAs in a link-state database by area and by LSA type
show ip ospf database database-summary
- Shows a list of the different LSAs in a link-state database
show ip ospf database [router|netw|sum|asbr-sum|ext|nssa-ext]
ospf process commands:
router ospf {pid}
- Allows more LSA's to be grouped together before being flooded (default=4min)
timers pacing lsa-group
- Changes the group pacing interval of LSA's
timers lsa-group-pacing {seconds}
- Changes the delay time between receiving a topology change and SPF calculation
timers spf {spf-delay} {spf-holdtime}
- Ignore MOSPF LSA packets, stops generating syslog messages.
ospf ignore lsa mospf
- Block the flooding of OSPF LSA packets only to a specific neighor
neighbor {ip} database-filter all out
- limit LSAs recieved from neighbor
max-lsa {max-no} [threshold-%] [warning-only] [ignore-time] [ignore-count] [reset-time]
- {max number}: of non-self-generated LSA’s that can be kept in the OSPF LSDB
- [threshold]: Percentage at which a warning message is logged. The default is 75%
- [warning-only]: OSPF process never enters ignore state. (Def = Disabled)
- [ignore-time]: time to ignore neighbors after the limit's exceeded. (Def = 5 min)
- [ignore-count]: number of times consecutively to enter ignore state. (Def = 5)
- [reset-time]: time before ignore count gets reset (Def = 10 min)
- limit LSAs from redistribution
redistribute maximum-prefix (#prefix) [opt]
- withdraw - stops redistribution if max reached.OSPF throttling
timers throttle lsa all {start-interval} {hold-interval} {max-interval}
- Sets the rate-limiting values (in milliseconds) for LSA generation
- {start-interval}: (Def = 0 ms)
- {hold-interval}: (Def = 5000 ms)
- {max-interval}: (Def = 5000 ms)
interface command:
interface s0/0
ip ospf database-filter all out - Block the flooding of OSPF LSA packets out the interface
Flood reduction
we have LSA updates, every 30 minutes it will refresh entire database to ensure stability, in a large network it will cause a blip in the network.
note: will affect voice traffic which is critical in the network.
command:
int e0/0
ip ospf flood-reduction
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