OSPF - Notes
OSPF - Open Shortest Path First
Link State Routing Protocol
OSPFv2 is for IPv4, and OSPFv3 is for IPv6.
Single-area OR Multi-area.
OSPF Benefits / Features
- Works with VLSM/CIDR.
- Changes trigger updated.
- Changes propagate quickly.
- Large/small network to single/multi area.
- Uses MDS authentication.
AD - Administrative Distance
Static - 1
RIP - 120
ISIS - 115
OSPF - 110
OSPF has three databases.
- Adjacency Database
- Neighbor Table
- Unique identifier for each router.
- Link State Database (LSDB)
- Topology Table
- Same with Area
- Forwarding Database
- Routing Table
- Unique for each router.
Routing protocol, two kinds for IPv4 and IPv6, doesn’t send automatic updates, single or multi-area, and can use MD5.
Generic Process to Reach Convergence
- Establish Neighbor Adjacencies
- Exchange Link-State Advertisements
- Builds the Topology Table
- Execute the SPF Algorithm (Shortest Path First)
- Creates the SPF Tree
- Creates the Routing Table entries.
OSPF uses the Link State Packets, Types:
Type 1 - Hello Packet
- Establish Adjacencies
Type 2 - Database Description Packet
- Shortened version of the LSDB.
Type 3 - Link State Request Packet
- If a router decides it needs more information, it will request more information about one of the packets above (Type 2 or Type 1).
Type 4 - Link State Update Packet
- Known as “LSU”.
- Raph to LSR
Type 5 - Link State Acknowledgement
There are several OSPF states, described as follows:
- Down State
- No Hello Packets Received; Down
- Router sends Hello packets.
- Transition to Init state.
- Init State
- Hello packets are received from the neighbor.
- They contain the sending router’s Router ID.
- Transition to Two-Way state.
- Two-Way State
- On Ethernet links, elect a DR, and a BDR.
- Transition to ExStart state.
- Exstart State
- Negotiate master / slave relationship and DBD packet sequence number.
- The master initiates the DBD packet exchange.
- Exchange State
- Routers exchange DBD packets.
- If additional router information is required then transition to Loading; otherwise, transition to Full.
- Loading State
- LSRs and LSUs are used to gain additional route information.
- Routes are processed using the SPF algorithm.
- Transition to the Full state.
- Full State
- Routers have converged.
DR & BDR
DR stands for “Designated Routing”.
BDR stands for “Backup Designated Routing”.
If we have five routers, how many adjacencies would we end up with? 10
Formula: n(n-1)/2 \\ n = Number of Routers
Each Router needs a RouterID.
Enable OSPF Routing.
Identity/Add connected networks.
1 - Router OSPF <#>
2 - Router-ID 220.127.116.11
3 - network 18.104.22.168 0.0.0.255 area 0
4 - Passive Interface G0/0
OSPF and IPv6 Integration
V2 & V3:
- Link State
- SPF Algorithm
- 5 Packet Types
- Router ID
- Must have IPv6 Unicast-Routing.
- Uses Link-Local (IPv6-- fe80::1, Link Local), same link local used from all interfaces.
- Enable IPv6 Routing
- Assign Addresses (Including Link Local)
- Assign Router ID
- Options - Bandwidth, cost…
- Enable for an Interface (IPv6 ospf 10 area 0)
Additional OSPF Notes
OSPF has 3 databases for IPv4, and 3 for IPv6
1. Link State Database ("Topology Table")
2. Adjacency Database ("Neighbor Table")
3. Forwarding Database ("Routing Table")
-- If we are converged, the topology should be the same in each of those routers.
-- We are able to 'see' the topology table via show commands.
Question: What does OSPF use to discover neighbors?
-- OSPF uses "HELLO" Packets to establish (and confirm) network adjacency.
Question: What would happen if the dead interval threshold is reached (or expires) before the router receives the hello packet?
-- OSPF will drop that route/network.
Question: What is the purpose of OSPF “Router ID”?
-- Router-ID allows for unique identification, and “elections”. See DB and DBR earlier in this page for their purposes.
Question: What are three ways the router can get a Router ID?
- The “router-id” command from IOS.
- It can also obtain a router ID via a loopback address.
- As well as taking the address of an active interface.
Question: You have an IP; “192.168.7.111/29”, what should your wildcard mask be?
Question: Which command should be used to check the OSPF process ID, the router ID, networks the router is advertising, the neighbors the router is receiving updates from, and the default administrative distance?
- The command “show ip protocol” will do this.
Question: What does “show ip ospf neighbor” do?
- Shows Router IDs and IPs of Neighbors.
Question: What does “show ip ospf” do?
- This will yield the Process, Router ID, and Area Information.
Question: What does “show ip ospf interface” do?
- Detailed interface about each OSPF interface.
Question: What does “show ip protocols” do?
- Shows Process, Router ID, Neighbors, and advertised networks. Basically everything.
Question: What does “R1(config)# ipv6 router ospf 64” do?
- It gives that router (R1) IPv6 OSPF (OSPFv3) with a process ID of 64.