Abstract
- Router stores info about network in LSA, which are organised in a structure called the LSDB
- We will use LSA Flooding to ensure router and its routing neighbours have the same routing information. Then we can use Dijkstra’s Algorithm to calculate the best route to route packets
LSA
┌───┐ "I'm R1, connected to R2 via 10.1.1.0/30"
│R1 │ "I'm R2, connected to R1,R3 via networks..."
└───┘ 🌐 "Network 192.168.1.0/24 has R1,R2,R3 attached" 📡 "To reach Area 1 networks, go via ABR R5"🌍 "To reach Internet 0.0.0.0/0, go via ASBR"- Stands for Link State Advertisements
- Contains info like the router ID, the IP address of the new network interface and the cost
LSDB
- A collection of LSA
LSA Flooding
Step 1: Router R1 detects link change
┌───┐ ❗
│R1 │ "My link to 192.168.1.0/24 is down!"
└─┬─┘
│
Step 2: R1 generates new LSA
▼
┌─────────────┐
│ NEW LSA │
│ Type-1, R1 │
│ Seq: 0x8001 │
└─────────────┘
│
Step 3: LSA Flooding (multicast to neighbors)
▼
┌───┐ ──flooding──> ┌───┐ ──flooding──> ┌───┐
│R1 │ │R2 │ │R3 │
└───┘ └───┘ └───┘
│ │ │
▼ ▼ ▼
Updates Updates Updates
LSDB LSDB LSDB- When a link is added or down in the network. A LSA is created and is flooded throughout the network until all routers have received it, resulting in all routers in the same OSPF area sharing the same LSDB
Aging timer
The LSA will be flooded again after timer expires. The timer expires every 30 mins.
OSPF Area
- An area is a set of routers that share the same LSDB
- We usually use it to divide up the network to make it more efficient to manage
- Each area has its own LSDB, so router in an area needs less memory to store it. And since we have less nodes in the LSDB, the routing calculation is also more efficient
Important
OSPF interfaces in the same subnet must be in the same area.
If they aren’t in the same area, they can’t be OSPF neighbours and can’t exchange information about the network they know.
Backbone Area
- All other OSPF areas must have a direct connection with the backbone areas via the Area Border Router
- All routers inside this area are known as backbone routers
Internal Router
- Router whose interfaces are all in the same OSPF Area
- The router in the Backbone Area is also known as a Internal Router
Area Border Router
- Routers whose interfaces are in multiple areas
Important
Area Border Routers (ABRs) maintain a separate LSDB for each area they are connected to.
It is recommended that you connect an ABR to a maximum of 2 areas, or we can overburden the router.
ASBR
[OSPF Domain] ←→ [ASBR] ←→ [BGP/Internet]
(Areas 0,1,2,3) (Different AS)
- Acts as a translator, taking routes learned via Border Gateway Protocol (or other protocols) and advertising them into the OSPF domain as external routes (Type-5 LSA)
Intra-area Route
- A route to a destination inside the same OSPF Area
Inter-area Route
- A route to a destination in a different OSPF Area