OSPF - jibingl/CCNA-CCNP GitHub Wiki
| Version | RFC |
|---|---|
| OSPFv2 | rfc2328 |
| OSPFv3 | rfc2740 (IPv6) |
Steps to form LSDBs (Link State Database):
- Become neighbors with other routers connected to the same segment;
- Exchange LSAs (Link State Advertisement) with neighbor routers;
- Calculate the best routes to each destination, and insert them into the routing table.
Topology example and Key-info:
WAN_144.14.14.1/30
.---area 0------------------------------|-----------.
| R2-id_2.2.2.2 R1|id_1.1.1.1 |
192.168.2.0/24 | DR 10.0.10.0/30 |g0/0 | 172.16.1.1 .---------------------area 1-----------------------------.
[=]---|-g1/1-(+)-g0/0-DR------------BDR-g1/1-(+)-f1/0-DR-.2----[=] | R8-id_8.8.8.8 R7-id_7.7.7.7 |
| BDR|f1/0 BDR|g2/2 | | (+)-g0/0--------\ /--------g0/0-(+)-g1/1---[=] |
| | | | | DR \ / BDR DR |
| | 10.0.20.0/30 | | | \ / |
| | 10.0.40.0/30 | | | [=] 172.16.0.0/24 |
| | | | | / \ |
| DR|f1/0 DR|g2/2 | 10.0.50.0/30 | DROther / \ DROther DR |
[=]---|-g1/1-(+)-g0/0-BDR------------DR-g0/0-(+)-s1/0-----|-ospf-ppp-net-|-s1/0-(+)-g0/0-------/ \--------g0/0-(+)-g1/1---[=] |
192.168.3.0/24 | DR 10.0.30.0/30 DR|g1/1 | | R5-id_5.5.5.5 R6-id_6.6.6.6 |
| R3-id_3.3.3.3 R4|id_4.4.4.4 | `-------------------ospf-broadcast-net-------------------`
`---------------ospf-broadcast-net------|-----------`
[=]192.168.4.0/24
R2#show ip ospf database *Routes cost calculation (reference-bw 100000):
.-------------------------------------. ..........................................................
| LSDB (area0) | .---routers generate-. | Route 1 = R1_g1/1 -> R2_g1/1_192.168.2.0 |
| Router LSAs | | RID:3.3.3.3 | | Cost 200 = 100 + 100 |
| .------. .------. .------. .------. | | Net:192.168.3.0/24 | | Route 2 = R1_g1/1 -> R2_lookback |
| | LSA1 | | LSA1 | | LSA1 | | LSA1 |-|------>| Cost:100 | | Cost 101 = 100 + 1 |
| `------` `------` `------` `------` | `--------------------` | Route 3 = R1_g1/1 -> R2_f1/0 -> R3_g1/1_192.168.3.0 |
| .------. .------. .------. .------. | .---routers generate-. | Cost 1200 = 100 + 1000 + 100 |
| | LSA1 | | LSA1 | | LSA1 | | LSA1 |-|------>| RID:3.3.3.3 | ``````````````````````````````````````````````````````````
| `------` `------` `------` `------` | | Net:10.0.20.0/30 | *Cost forluma = reference-bw / exit-interface-bw
| Network LSAs | | Cost:1000 |
| .------. .------. .------. | `--------------------`
| | LSA2 | | LSA2 | | LSA2 |----------|------>.---DR generates-----.
| `------` `------` `------` | | DR-RID:3.3.3.3 |
| AS-external LSA | `--------------------`
| .------. | .---ASBR generates---.
| | LSA5 |----------------------------|------>| ASBR RID:1.1.1.1 |
| `------` | `--------------------`
`-------------------------------------`
*ASBR (Autonomous system boundary router) is R1 that connects to external network, normally Internet.
*ABR (Area Border Router) is R5 or R4, depends on context, that connects two areas.
*In OSPF broastcast network, DR/BDR election rule (High-Low): interface-priority --> router-id --> interface-ip. "First placement" rule.
*When DR is down, current BDR becomes new DR even though existing of another higher-priority interface that will become new BDR.
CLI configuration example:
R1(config)#router ospf 1 //Process ID is locally significant. Routers with different process IDs can become neighbors.
R1(config-router)#network 10.0.10.0 0.0.0.3 area 0
R1(config-router)#network 10.0.40.0 0.0.0.3 area 0
R1(config-router)#network 172.16.1.2 0.0.0.0 area 0
R1(config-router)#passive-interface f1/0
R1(config-router)#default-information originate
R1(config-router)#router-id 1.1.1.1
R1(config-router)#maximum-path 8 //Change load-balancing pathes from 4 to 8
R1(config-router)#distance 85 //Change administrative distance from 110 to 85
R1(config-router)#auto-cost reference-bandwidth 100000 //Default is 100Mbits/s
R1(config-router)#passive-interface default //Set all interfaces to passive, then
R1(config-router)#no passive-interface g1/1 //specify an interface as active
R1(config-router)#shutdown //Shut the OSPF process 1 down
R1(config)#interface g1/1
R1(config-if)#ip ospf 1 area 0 //Active OSPF directly on an interface
R1(config-if)#ip ospf cost <1-65535> //Manualy set the interface OSPF cost, or
R1(config-if)#bandwidth <1-10000000> //Change the interface bandwidth (It won't change speed of the interface)
R1(config-if)#ip ospf priority <0-255> //Change interface priority (defualt is 1)
R1(config-if)#ip ospf hello-interval <1-65535> //Change Hello message sending interval (default 10s)
R1(config-if)#ip ospf dead-interval <1-65535> //Change Dead timer-count (default 40s)
R1(config-if)#ip ospf network broadcast //Set as broadcast OSPF network. Ohter types are point-to-point, non-broadcast, and point-to-multipoint
R1(config-if)#ip ospf authentication-key <string> //Set password for OSPF neighbors communication
R1(config-if)#ip ospf authentication //Enable
- An OSPF area is a set of routers and links that share the same LSDB.
- The backbone area (area 0) is an area that all other areas must directly connect to.
- All areas must have at least one ABR connect to the backbone area. (Best-practice for design)
- An ABR connnect maximum of 2 areas. (Best-practice for design)
- OSPF interfaces in the same subnet must be in the same area. (Best-practice for design)
- OSPF area should be contigous.
- Cost to a destination is the total cost of the outgoing/exit interfaces' costs.
NEIGHBOR REQUIREMENTS:
- Area number must match
- Interfaces must be the same subnet
- No shutdown to OSPF process
- Hello/Dead intervals must match
- Router ID must unique
- Authentication settings must match
- OSPF Network type must match
- IP MTU settings must match
Under 7 and 8, neighbors can be formed but OSPF operate inproperly.
WRONG OSPF TOPOLOGY DESIGNS:
.--------------------------------------area0 (backbone)-.
|-----R0(+)-----------------[=]---------------[=] |
`----------------------------|-----------------|--------`
R1(+)---------------(+)R2 (+)R3
.------/|\-area1. .------/|\-area2. .------/|\-area3.
| / | \ | | / | \ | | / | \ |
| / | \ | | / | \ | | / | \ |
| (+) (+) (+) | | (+) (+) (+) | | (+) (+) (+) |
| | | | | | | | | | | | | | |
`---------------` `---------------` `---------------`
Issues: area 1 has no ABR connect to backbone area (area 0); R2 connects to too many areas (three areas).
.--------------------------------------area0 (backbone)-.
|-----R0(+)--------[=]------------------------[=] |
`----------------/-----\-----------------------|--------`
R1(+) (+)R2 (+)R3
.area1----/-----. .-----\----area2. .-------|--area1.
| / /\ | | /\ \ | | / | \ |
| / | \ | | / | \ | | / | \ |
| (+) (+) (+) | | (+) (+) (+) | | (+) (+) (+) |
| | | | | | | | | | | | | | |
`---------------` `---------------` `---------------`
Issues: area 1 is not contigous, instead it is divided into two parts.