Link State Algorithm Alternative to distance-vector Distributed computation Broadcast link information to all routers Each router computes shortest paths to all destinations based on knowledge of full topology Uses Dijkstra’s algorithm to compute the paths Avoids problem where one router can damage the entire internet by passing incorrect information as all routers have knowledge of full topology Most common implementation: Open Shortest Path First (OSPF)

Link State Update All routers know the network topology by sharing their local link information with all other routers. Think of routers as nodes in a graph, and the networks connecting them as edges or links Pairs of directly-connected routers periodically Test link between them Broadcast status of link to all other routers All routers Receive link status messages from all routers Compute routes based on their local database of link information

Open Shortest Path First (OSPF) Uses Link State routing Each router acquires complete topology information using link state updates Link-state - what it means: Link: That’s the interface of a router. State: Description of that interface and how it’s connected to neighboring routers. Link state information must be flooded to all routers (uses multicasting) Cost metric used to calculate shortest paths. Metric can be any link or network parameter (time, congestion, bandwidth, $$, distance) or a function that combines several weighted parameters Guaranteed to converge

Link State Routing: Basic principles Routers establish a relationship (“adjacency”) with neighbors. Two types: full neighbors: allows exchange of routing information 2way neighbor: no routing information exchange 2. Each router generates link state advertisements (LSAs) which are distributed to all “adjacent” routers (after all routers have established adjacencies). LSA = (link id, state of the link, cost, neighbors of the link) 3. Each router maintains a database (LSDB) of all received LSAs (topological database or link state database), which describes the network as a graph with weighted edges 4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

Operation of a Link State Routing protocol Received LSAs Dijkstra’s Algorithm IP Routing Table Link State Database LSAs are flooded to other interfaces

Features of OSPF Provides authentication of routing messages Enables load balancing by allowing traffic to be split evenly across routes with equal cost Type-of-Service routing allows setup of different routes dependent on the TOS (DS) field in IP header Uses AREAs to subdivide large networks, providing a hierarchical structure and limits the multicast LSAs within routers of the same area. Area 0 is called the backbone area and all other areas connect directly to it. All OSPF networks must have a backbone area

OSPF Areas Area Border Routers (ABR) are any routers that have one interface in one area and another interface in another area

Link State Advertisements (LSA) LSAs are at the heart of OSPF operation OSPF routers use LSAs to describe the link state of all its interfaces. A Link State DataBase (LSDB) stores all received LSAs on a router. A router uses a Router LSA to describe its interface IP addresses. After OSPF is started on a router, it creates an LSDB that contains entries of this router’s Router LSAs

OSPF Operation contd. Link-state routing protocols generate routing updates only when a change occurs in the network topology. When a link changes state, the device that detected the change creates a link-state advertisement (LSA) concerning that link and sends it to all neighboring devices using a special multicast address. Each routing device reads the LSA. The LSA has a sequence number that allows the router to check to see if it has already seen that update. If old, it is discarded, if new, link-state database (LSDB) info updated and LSA passed along to next neighbors. The entire routing table (LSDB) is transmitted once every 30 minutes

Types of OSPF Messages There are five types of OSPF Link-State Packets (LSPs). Hello: are used to establish and maintain adjacency with other OSPF routers. They are also used to elect the Designated Router (DR) and BackupDesignated Router (BDR) on multi-access networks. Database Description (DBD or DD): contains an abbreviated list of the sending router’s link-state database and is used by receiving routers to check against the local link-state database to make sure it has the latest information

LSPs contd. Link-State Request (LSR): used by routers to request more information about any entry in the DBD Link-State Update (LSU): used to reply to LSRs as well as to announce new information. LSUs can contain 7 different types of Link-State Advertisements (LSAs) Link-State Acknowledgement (LSAck): sent to confirm receipt of an LSU message

OSPF Operation HELLO messages are used to maintain adjacency with neighbors. By default, OSPF routers send Hello packets every 10 seconds on broadcast networks and every 30 seconds on non-broadcast segments

OSPF Packet Format OSPF packets are not carried as UDP or TCP payload! OSPF has its own IP protocol number: 89 TTL: set to 1 (in most cases) Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters: (designated and backup designated only)

OSPF Packet Format 2: current version is OSPF V2 ID of the Area from which the packet originated Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID Address 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Standard IP checksum taken over entire packet

OSPF Operation via an Example Suppose OSPF has just been enabled on R1 & R2. Both R1 and R2 are very eager to discover if they have any neighbors nearby but before sending Hello messages they must first choose an OSPF router identifier (router-id) to tell their neighbors who they are. The Router ID (RID) is an IP address used to identify the router and is chosen using the following sequence: The highest IP address assigned to a loopback (logical) interface. If a loopback interface is not defined, the highest IP address of all the active router’s physical interfaces will be chosen. The router ID can be manually assigned if necessary

Example contd. In this example, suppose R1 has 2 loopback interfaces & 2 physical interfaces: Loopback 0: 10.0.0.1 Loopback 1: 12.0.0.1 eth0/0: 192.168.1.1 eth0/1: 200.200.200.1 The loopback interfaces are preferred to physical interfaces (because they are never down) so the highest IP address of the loopback interfaces is chosen as the router-id -> Loopback 1 IP address is chosen as the router-id.

Router 1

Router 2

Next Step – Hello Msgs Now both the routers have the Router-ID so they will send Hello packets on all OSPF-enabled interfaces to determine if there are any neighbors on those links. The information in the OSPF Hello includes the OSPF Router ID of the router sending the Hello packet.

Hello Packet Exchange

Hello Packet Content * Indicates values that have to be the same for both routers if they are to establish an adjacency, i.e., become neighbors

Discovery of Neighbors Routers multicast OSPF Hello packets on all OSPF-enabled interfaces. If two routers share a link, they can become neighbors, and establish an adjacency. Certain conditions have to be met. In broadcast environments, adjacency is only established with Designated and BackupDesignated Routers. After becoming a neighbor, routers exchange their link state databases

States of Establishing Adjacency Init state – router has received Hello message from other OSFP router 2-way state – neighbor has received Hello message and replied with a Hello message of his own Exstart state – beginning of the LSDB exchange between both routers. Exchange state – DBD (Database Descriptor) packets are exchanged. DBDs contain LSAs headers. Routers see what LSAs they need. Loading state – one neighbor starts by sending LSRs (Link State Requests) for every network it doesn't know about. The other neighbor replies with the LSUs (Link State Updates) which contain information about requested links. After all the requested information has been received, the other neighbor goes through the same process Full state (adjacency) - both routers have the synchronized database and are fully adjacent with each other.

Hello Msg R1 to R2 R1 just comes up and R2 is already up and running. R1 wants to find out if it has any neighbor running OSPF it sends a Hello message to the multicast address 224.0.0.5. This is the multicast address for all OSPF routers and all routers running OSPF will process this message.

Establishing adjacency If an OSPF router receives an OSPF Hello packet it will check some required parameters to determine if adjacency can be established. If all is in order:  R2 will add R1 to its neighbor table and send a Hello packet to R1

Hello Msg Adjacency Parameters

Exchange DD or DBD packets R1 and R2 are neighbors now The neighbors must first determine who will be the master and who will be the slave. The router with higher Router-ID becomes master and initiates the link exchange. They start by sending Database Description (DD or DBD) packets which contain an abbreviated list of the sending router’s link-state database The receiver acknowledges a received DD packet by sending an identical DD packet back to the sender. Each DD packet has a sequence number and only the master can increment sequence numbers.

DD Msg Exchange

LSA Request R1 or R2 can send Request to get missing LSA from its neighbors

LSA Exchange R2 sends back an LSAck packet to acknowledge the packet

Creating LSDBs Note that routers first exchange DD msgs that only list the content of the LSDB but no details. Once a router gets that info, it can then check to see if it has that information in its LSDB. If it doesn’t it requests an LSA to fill in the details. Reliable transmission: when a router receives an Update, it sends an Ack to the Update sender. If the sender does not receive Ack within a specific period, it times out and retransmits Update. OSPF uses Update-Ack to implement reliable transmission. It does not use TCP!

Routing Data Distribution ACK Routing Data Distribution LSA-Updates are distributed to all other routers via Reliable Flooding using multicast addresses. Example: Flooding of LSA from 10.10.10.1 LSA LSA ACK ACK LSA Update database Update database Update database ACK

Dissemination of LSA-Update A router sends and re-floods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet) Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are no new updates. Acknowledgements of LSA-updates: explicit ACK, or implicit via reception of an LSA-Update from neighbor.

Filling the LSDB

LSA Sequence Numbers What do the sequence numbers look like for OSPF LSAs? There are 4 bytes or 32-bits. Begins with 0x80000001 and ends at 0x7FFFFFFF and wraps around Every 30 minutes each LSA will age out and will be flooded with a sequence number that is incremented by one.

Broadcast Environments - Designated & Backup Designated Routers

Broadcast Environments - Designated and Backup Designated Router To minimize OSPF traffic (LSAs) on broadcast networks, OSPF elects a Designated Router (DR) and a Backup DR (BDR) How do we select a DR/BDR? During the process of becoming OSPF neighbors: The router with the highest priority will become DR. The router with the second highest priority will become BDR. If the priority is the same the OSPF router ID is the tiebreaker. Higher wins. DR/BDR election is non-preemptive. This means if you change the priority or router ID you have to reset OSPF in order to select a new DR/BDR. Routers that are not DR or BDR show up as DROTHER. Only DR and BDR have adjacencies (full neighbor) with all routers on the broadcast segment. Other routers are two-way neighbors. If a non designated router has an update, the LSA is sent to the DR and BDR using the 224.0.0.6 address. The LSA is then sent by the DR to all the routers on the broadcast segment using multicast address 224.0.0.5.

Router Status Full neighbor state Two-way neighbor state And router Susan (the BDR) sees the DR and DROTHER.

Choosing DR and BDR We can change which router becomes the DR/BDR by playing with the priority. You change the priority if you like by using the ip ospf priority command: The default priority is 1. A priority of 0 means you will never be elected as DR or BDR. You need to use clear ip ospf process before this change takes effect. Let’s turn router Nancy into the DR:

Donna is still the DR, we need to reset the OSPF neighbor adjacencies so that we’ll elect the new DR and BDR. Susan is now Drother anc Donna is ghd BDR Nancy is now DR

By Multiple Access not By Area Something you need to be aware of is that the DR/BDR election is per multi-access segment…not per area! Here we have 2 multi-access (broadcast) segments. Between router Donna and Nancy, and between router Donna and Susan. For each segment there will be a DR/BDR election. You can see that router Nancy is the DR for the 192.168.12.0/24 segment and router Susan is the DR for the 192.168.23.0/24 segment.

Point to Point Links For a point-to-point link running say HDLC. You can see that we have a neighbor but we didn’t do an election for DR or BDR. Makes sense because there is always only one router on the other side. 192.168.12.0 .1 .2

Link Cost and Path Choice What about the link metric? OSPF uses a metric called cost which is based on the bandwidth of an interface, it works like this: Cost = Reference Bandwidth / Interface Bandwidth The reference bandwidth is a default value on Cisco routers which is a 100Mbit interface. You divide the reference bandwidth by the bandwidth of the interface and you’ll get the cost. Example: If you have a 100 Mbit interface what will the cost be? Cost = Reference bandwidth / Interface bandwidth 100 Mbit / 100 Mbit = COST 1 Example: If you have a 10 Mbit interface what will the cost be? 100 Mbit / 10 Mbit = COST 10 Example: If you have a 1 Mbit interface what will the cost be? 100 Mbit / 1 Mbit = COST 100 The lower the cost the better the path is -> minimize route cost If we have links that are > 100M (e.g. 1G) the reference bandwidth is changed to always have a link cost that is >1

OSPF LSA Types OSPF has many different types of LSAs: LSA Type 1: Router LSA LSA Type 2: Network LSA LSA Type 3: Summary LSA LSA Type 4: Summary ASBR LSA LSA Type 5: Autonomous system external LSA LSA Type 6: Multicast OSPF LSA (NOT USED) LSA Type 7: Not-so-stubby area LSA LSA Type 8: External attribute LSA for BGP

Router LSA Each router within the area will flood a type 1 router LSA within the area. In this LSA you will find a list with all the directly connected links of this router. The router LSA will always stay within the area.

Network LSA The network LSA or type 2 is created for multi-access network that have a DR/BDR. If this is the case you will see these network LSAs being generated by the DR. The other routers in the segment generate a type 1 LSA to inform the DR of an update. In the type 2 LSA we will find all the routers that are connected to the multi-access network, the DR, BDR, and the prefix and subnet mask. The network LSA always stays within the area.

Multi Area LSAs Type 1 router LSAs always stay within the area. OSPF however works with multiple areas and you probably want full connectivity within all of the areas. Router Nancy is flooding a router LSA within the area so Area Border Router (ABR) Donna will store this in her LSDB. Router Mary and Susan need to know about the topology in Area 2. Router Donna is going to create a Type 3 summary LSA and flood it into area 0. This LSA will flood into all the other areas of our OSPF network. This way all the routers in other areas will know about the prefixes from other areas.

An outside RIP Router In this example we have router Nancy who is redistributing information from the RIP router into OSPF. This makes router Nancy an ASBR (Autonomous System Border Router). Router Nancy will flip a bit in her router LSA to identify herself as an ASBR. When router Donna who is a ABR receives this router LSA she will create a type 4 summary ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so all OSPF routers know where to find the ASBR.

Outside Network Same topology but we’ve added a prefix (5.5.5.0 /24) at our RIP router. This prefix will be redistributed into OSPF. Router Nancy (our ASBR) will take care of this and create a type 5 external LSA for this that will contain the external network prefix.

OSPF Tables There are 3 type of tables stored at a Router: Neighbor Topology Routing

Neighbor Table Contain information about the neighbors Neighbor is a router which shares a link on same network Some neighbor relationships are “adjacencies” LSA updates are exchanged only when adjacency is established

OSPF Topology Table Contains information about all networks and paths to reach any network All LSA’s are entered into the topology table When topology changes, LSA’s are generated and router sends new LSA’s Using the topology table a shortest path connectivity graph is created (routing table), the algorithm is known as SPF or Dijkstra’s algorithm

Routing Table Also known as forwarding database Generated when an algorithm is run on the topology database Routing table for each router is unique