OSPF

General Notes

AD 110 for internal & external
Multicast 224.0.0.5 for all OSPF routers, 224.0.0.6 for all DR/BDRs, port number 89
Advertises all routes to all nodes in area to build a topology before deciding best path
Hello and hold timers must match for adjacency (unlike EIGRP)
Adjacency formed, DBD (database descriptor) packet sent describing LSDB (link-state database)
- Once other neighbor determines which LSAs they’re missing, LSR (link-state request) packet sent to ask for missing LSAs and sync database
- All OSPF routers in an area must share a common view of the database for OSPF to function
DR/BDR Election
- No real prepending - the only way to ensure a node does not become DR is by removing it from the election
- This is done with ip ospf priority 0 under the interface you don’t want to become DR/BDR
OSPF LSA Types
- Type 1 - Router LSA
  - Generated by all OSPF routers, advertised intra-area, describes routers’ own interfaces, neighbors
- Type 2 - Network LSA
  - Generated by DRs, advertised intra-area, describes routers on its segment
- Type 3 - Summary LSA
  - Generated by ABRs, advertised inter-area to describe reachable networks to another area
- Type 4 - ASBR Summary LSA
  - Generated by ABRs, adverised inter-area to describe how to reach an ASBR advertising external networks
- Type 5 - External Summary LSA
  - Generated by ASBRs, advertised inter-area to describe external routes
- Type 7 - NSSA External Summary LSA
  - Same as above but generated in a NSSA where Type 5 LSAs are not allowed, reflooded as a Type 5 LSA if recieved by an ABR
Use show ip ospf border-routers to see both ABRs, ASBRs in the OSPF process
For point-to-point links, add ip ospf network point-to-point under interface to save overhead on DR/BDR election
- Null output after the slash in State column of show ip ospf neighbor indicates point-to-point link
Loopbacks automatically advertised as /32s regardless of subnet mask
- Can make the loopback point-to-point to advertise the full subnet
By default all links greater than 100Mbps will have the same reference cost
- To change this, change reference bandwidth under router ospf config with auto-cost reference bandwidth <value>
- This should match on all devices in topology to prevent loops, but is not required for adjacency
Metric can be modified directly with cost on interface (ip ospf cost <cost>), or indirectly by changing interface bandwidth
Cost can also be configured per-neighbor under router ospf with neighbor <ip> cost <cost>
To form a neighborship, area, SN mask, stub flag, authentication and hello/dead timers must match
Neighborships in a good state in 2-way for drothers on broadcast segments or Full for all other links
- All other states indicate an issue if they persist
- Stuck in ExStart, Exchange states usually indicates an MTU issue

Features

Authentication

Supports plaintext (pointless), MD5 & HMAC-SHA auth
HMAC-SHA configuration done with key chain in global config
MD5 configuration done under interface or for an entire area under router ospf
Key-id numbers must match for successful auth

key chain OSPF_KEYS
 key 1
  key-string <psk>
  cryptographic-algorithm hmac-sha-256
!
int gi0/0
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 <psk>
!
int gi0/1
ip ospf authentication key-chain OSPF_KEYS

Summarization

Possible only at ABRs, ASBRs
To summarize LSA 3 as originated by ABR, use area <area#> range <ip & mask>
- The area used in the above command is the area the prefix is originated in - the summary will apply to all areas aside from this
- Can add not-advertise prefix to prevent advertising prefixes in range, form of route filtering ^ffa496
To summarize LSA 5 at ASBR, use summary-address <ip & mask>
Leak map can be used to leak longer mask prefixes included in summary
- summary-address 0.0.0.0 0.0.0.0 leak-map <route-map>
Null0 installed on router to match summary address for both LSA 3 and LSA 5 summaries

Route filtering

Inter-area filtering possible only at ABRs, filtering LSA3s
Performed with area <area#> filter-list prefix <prefix-list> [in|out]
Inter-area filtering also possible with area range and not-advertise flag as described above
Can also use distribute lists to filter inbound into the RIB, not the OSPF database
- Can match on many values with route-maps, such as interface, ip address, ip next-hop, ip route-source, metric, route-type and tag
- For example:
```
access-list 3 permit 155.1.146.0
access-list 4 permit 155.1.0.4
!
route-map DENY_VLAN146_FROM_R4 deny 10
 match ip address 3
 match ip next-hop 4
!         
route-map DENY_VLAN146_FROM_R4 permit 20
!
router ospf 1
 distribute-list route-map DENY_VLAN146_FROM_R4 in
```
- Note again distribute-lists ONLY AFFECTS LOCAL RIB, not OSPF database
- Easy to create traffic black holes if not careful with this feature

Default routing

Can be advertised with default-information originate [always]
always flag allows advertisement even if default not in local route table

Stub areas

Two types, stubby and totally stubby
- Stubby prevents advertisement of LSA 5 (Externals) into the area
  - Configured with area <area#> stub under router ospf
- Totally stubby prevents advertisements of LSA 3 (summary) & LSA 5 into the area
  - Configured with area <area#> stub no-summary under router ospf
    - Only ABR needs no-summary, rest of area just stub
ABR advertises default route into stub area for reachability
Must be configured on all routers in stub area

NSSA

Two types, NSSA and totally NSSA
- NSSA prevents LSA5 advertisements but allows redistribution of externals
  - Configured with area <area#> nssa under router ospf
  - Does not advertise default route from ABR by default
  - Can use area <area#> nssa default-information-originate to advertise default
- Totally NSSA prevents LSA3, 5 but allows redistribution of externals
  - Configured with area <area#> nssa no-summary under router ospf
  - Advertises default route from ABR
Externals originated in these areas will be LSA7s instead of LSA5s, translated to LSA5 at ABR

Virtual links

Can be used to repair discontiguous networks or for traffic engineering
Under OSPF, configure area <area#> virtual-link <neighbor RID> on both nodes
The IP used is the router ID, not the address used for OSPF peering
To keep virtual-links stable, it’s a good idea to manually define the router ID (as is best practice anyways)
See Redistribution page for details on that

Useful debugs/show commands

show ip ospf interface brief - Displays interfaces participating in OSPF, areas, cost, etc
show ip ospf neighbors - Displays neighbors and neighbor states
show ip ospf database - Displays OSPF database information
show ip ospf database router self-originate - Useful to see routes advertised by local device
show ip ospf - Displays detailed information about OSPF protocols, timers, metric reference values, etc
show ip protocols - Shows various information about active routing protocols
debug ip ospf adj - OSPF adjacency formation
debug ip ospf packet - All OSPF packets

Standard OSPF Config

int gi0
ip ospf network point-to-point
ip ospf 1 area 0

router ospf 1
 router-id 1.1.1.1
 passive-interface default
 no passive-interface gi0
 network 150.1.5.0 0.0.0.255 area 0