OSPF is, mostly, a link-state routing protocol. In contrast to distance-vector protocols, such as RIP or BGP, where routers describe available paths (i.e. routes) to each other, in link-state protocols routers instead describe the state of their links to their immediate neighbouring routers.
Each router describes their link-state information in a message known as an LSA (Link State Advertisement), which is then propogated through to all other routers in a link-state routing domain, by a process called flooding. Each router thus builds up an LSDB (Link State Database) of all the link-state messages. From this collection of LSAs in the LSDB, each router can then calculate the shortest path to any other router, based on some common metric, by using an algorithm such as Edgser Dijkstra’s SPF (Shortest Path First).
By describing connectivity of a network in this way, in terms of routers and links rather than in terms of the paths through a network, a link-state protocol can use less bandwidth and converge more quickly than other protocols. A link-state protocol need distribute only one link-state message throughout the link-state domain when a link on any single given router changes state, in order for all routers to reconverge on the best paths through the network. In contrast, distance vector protocols can require a progression of different path update messages from a series of different routers in order to converge.
The disadvantage to a link-state protocol is that the process of computing the best paths can be relatively intensive when compared to distance-vector protocols, in which near to no computation need be done other than (potentially) select between multiple routes. This overhead is mostly negligible for modern embedded CPUs, even for networks with thousands of nodes. The primary scaling overhead lies more in coping with the ever greater frequency of LSA updates as the size of a link-state area increases, in managing the LSDB and required flooding.
This section aims to give a distilled, but accurate, description of the more important workings of OSPF which an administrator may need to know to be able best configure and trouble-shoot OSPF.
OSPF defines a range of mechanisms, concerned with detecting, describing and propogating state through a network. These mechanisms will nearly all be covered in greater detail further on. They may be broadly classed as:
The OSPF Hello protocol allows OSPF to quickly detect changes in two-way reachability between routers on a link. OSPF can additionally avail of other sources of reachability information, such as link-state information provided by hardware, or through dedicated reachability protocols such as BFD (Bi-directional Forwarding Detection).
OSPF also uses the Hello protocol to propagate certain state between routers sharing a link, for example:
The Hello protocol is comparatively trivial and will not be explored in greater detail than here.
At the heart of OSPF are LSA (Link State Advertisement) messages. Despite the name, some LSAs do not, strictly speaking, describe link-state information. Common LSAs describe information such as:
Routes entirely external to OSPF. Routers originating such routes are known as ASBR (Autonomous-System Border Router) routers.
Routes which summarise routing information relating to OSPF areas external to the OSPF link-state area at hand, originated by ABR (Area Boundary Router) routers.
OSPF defines several related mechanisms, used to manage synchronisation of LSDBs between neighbours as neighbours form adjacencies and the propogation, or flooding of new or updated LSAs.
See OSPF Flooding.
OSPF provides for the protocol to be broken up into multiple smaller and independent link-state areas. Each area must be connected to a common backbone area by an ABR (Area Boundary Router). These ABR routers are responsible for summarising the link-state routing information of an area into Summary LSAs, possibly in a condensed (i.e. aggregated) form, and then originating these summaries into all other areas the ABR is connected to.
Note that only summaries and external routes are passed between areas. As these describe paths, rather than any router link-states, routing between areas hence is by distance-vector, not link-state.
See OSPF Areas.
LSAs are the core object in OSPF. Everything else in OSPF revolves around detecting what to describe in LSAs, when to update them, how to flood them throughout a network and how to calculate routes from them.
There are a variety of different LSAs, for purposes such as describing actual link-state information, describing paths (i.e. routes), describing bandwidth usage of links for TE (Traffic Engineering) purposes, and even arbitrary data by way of Opaque LSAs.
All LSAs share a common header with the following information:
Different types of LSAs describe different things in OSPF. Types include:
The specifics of the different types of LSA are examined below.
The Router ID of the router originating the LSA, see ospf router-id.
The ID of the LSA, which is typically derived in some way from the information the LSA describes, e.g. a Router LSA uses the Router ID as the LSA ID, a Network LSA will have the IP address of the DR as its LSA ID.
The combination of the Type, ID and Advertising Router ID must uniquely identify the LSA. There can however be multiple instances of an LSA with the same Type, LSA ID and Advertising Router ID, see LSA Sequence Number.
A number to allow stale LSAs to, eventually, be purged by routers from their LSDBs.
The value nominally is one of seconds. An age of 3600, i.e. 1 hour, is called the MaxAge. MaxAge LSAs are ignored in routing calculations. LSAs must be periodically refreshed by their Advertising Router before reaching MaxAge if they are to remain valid.
Routers may deliberately flood LSAs with the age artificially set to 3600 to indicate an LSA is no longer valid. This is called flushing of an LSA.
It is not abnormal to see stale LSAs in the LSDB, this can occur where a router has shutdown without flushing its LSA(s), e.g. where it has become disconnected from the network. Such LSAs do little harm.
A number used to distinguish newer instances of an LSA from older instances.
Of all the various kinds of LSAs, just two types comprise the actual link-state part of OSPF, Router LSAs and Network LSAs. These LSA types are absolutely core to the protocol.
Instances of these LSAs are specific to the link-state area in which they are originated. Routes calculated from these two LSA types are called intra-area routes.
Each OSPF Router must originate a router LSA to describe itself. In it, the router lists each of its OSPF enabled interfaces, for the given link-state area, in terms of:
The output cost of that interface, scaled inversely to some commonly known reference value, See auto-cost reference-bandwidth.
A link to a multi-access network, on which the router has at least one Full adjacency with another router.
A link to a single remote router, with a Full adjacency. No DR (Designated Router) is elected on such links; no network LSA is originated for such a link.
A link with no adjacent neighbours, or a host route.
These values depend on the Link Type:
|Link Type||Link ID||Link Data|
|Transit||Link IP address of the DR||Interface IP address|
|Point-to-Point||Router ID of the remote router||Local interface IP address, or the ifindex (MIB-II interface index) for unnumbered links|
|Stub||IP address||Subnet Mask|
Links on a router may be listed multiple times in the Router LSA, e.g. a PtP interface on which OSPF is enabled must always be described by a Stub link in the Router LSA, in addition to being listed as PtP link in the Router LSA if the adjacency with the remote router is Full.
Stub links may also be used as a way to describe links on which OSPF is not spoken, known as passive interfaces, see passive-interface.
On multi-access links (e.g. ethernets, certain kinds of ATM and X.25 configurations), routers elect a DR. The DR is responsible for originating a Network LSA, which helps reduce the information needed to describe multi-access networks with multiple routers attached. The DR also acts as a hub for the flooding of LSAs on that link, thus reducing flooding overheads.
The contents of the Network LSA describes the:
As the LSA ID of a Network LSA must be the IP address of the DR, the Subnet Mask together with the LSA ID gives you the network address.
Each router fully-adjacent with the DR is listed in the LSA, by their Router-ID. This allows the corresponding Router LSAs to be easily retrieved from the LSDB.
Summary of Link State LSAs:
|LSA Type||LSA ID Describes||LSA Data Describes|
|Router LSA||The Router ID||The OSPF enabled links of the router, within a specific link-state area.|
|Network LSA||The IP address of the DR for the network||The Subnet Mask of the network, and the Router IDs of all routers on the network.|
With an LSDB composed of just these two types of LSA, it is possible to construct a directed graph of the connectivity between all routers and networks in a given OSPF link-state area. So, not surprisingly, when OSPF routers build updated routing tables, the first stage of SPF calculation concerns itself only with these two LSA types.
The example below (see OSPF Link-State LSA Example) shows two LSAs, both originated by the same router (Router ID 192.168.0.49) and with the same LSA ID (192.168.0.49), but of different LSA types.
The first LSA being the router LSA describing 192.168.0.49’s links: 2 links to multi-access networks with fully-adjacent neighbours (i.e. Transit links) and 1 being a Stub link (no adjacent neighbours).
The second LSA being a Network LSA, for which 192.168.0.49 is the DR, listing the Router IDs of 4 routers on that network which are fully adjacent with 192.168.0.49.
# show ip ospf database router 192.168.0.49 OSPF Router with ID (192.168.0.53) Router Link States (Area 0.0.0.0) LS age: 38 Options: 0x2 : *|-|-|-|-|-|E|* LS Flags: 0x6 Flags: 0x2 : ASBR LS Type: router-LSA Link State ID: 192.168.0.49 Advertising Router: 192.168.0.49 LS Seq Number: 80000f90 Checksum: 0x518b Length: 60 Number of Links: 3 Link connected to: a Transit Network (Link ID) Designated Router address: 192.168.1.3 (Link Data) Router Interface address: 192.168.1.3 Number of TOS metrics: 0 TOS 0 Metric: 10 Link connected to: a Transit Network (Link ID) Designated Router address: 192.168.0.49 (Link Data) Router Interface address: 192.168.0.49 Number of TOS metrics: 0 TOS 0 Metric: 10 Link connected to: Stub Network (Link ID) Net: 192.168.3.190 (Link Data) Network Mask: 255.255.255.255 Number of TOS metrics: 0 TOS 0 Metric: 39063 # show ip ospf database network 192.168.0.49 OSPF Router with ID (192.168.0.53) Net Link States (Area 0.0.0.0) LS age: 285 Options: 0x2 : *|-|-|-|-|-|E|* LS Flags: 0x6 LS Type: network-LSA Link State ID: 192.168.0.49 (address of Designated Router) Advertising Router: 192.168.0.49 LS Seq Number: 80000074 Checksum: 0x0103 Length: 40 Network Mask: /29 Attached Router: 192.168.0.49 Attached Router: 192.168.0.52 Attached Router: 192.168.0.53 Attached Router: 192.168.0.54
Note that from one LSA, you can find the other. E.g. Given the Network-LSA you have a list of Router IDs on that network, from which you can then look up, in the local LSDB, the matching Router LSA. From that Router-LSA you may (potentially) find links to other Transit networks and Routers IDs which can be used to lookup the corresponding Router or Network LSA. And in that fashion, one can find all the Routers and Networks reachable from that starting LSA.
Given the Router LSA instead, you have the IP address of the DR of any attached transit links. Network LSAs will have that IP as their LSA ID, so you can then look up that Network LSA and from that find all the attached routers on that link, leading potentially to more links and Network and Router LSAs, etc. etc.
From just the above two LSAs, one can already see the following partial topology:
--------------------- Network: ...... | Designated Router IP: 192.168.1.3 | IP: 192.168.1.3 (transit link) (cost: 10) Router ID: 192.168.0.49(stub)---------- IP: 192.168.3.190/32 (cost: 10) (cost: 39063) (transit link) IP: 192.168.0.49 | | ------------------------------ Network: 192.168.0.48/29 | | | Designated Router IP: 192.168.0.49 | | | | | Router ID: 192.168.0.54 | | | Router ID: 192.168.0.53 | Router ID: 192.168.0.52
Note the Router IDs, though they look like IP addresses and often are IP addresses, are not strictly speaking IP addresses, nor need they be reachable addresses (though, OSPF will calculate routes to Router IDs).
External, or "Type 5", LSAs describe routing information which is entirely external to OSPF, and is "injected" into OSPF. Such routing information may have come from another routing protocol, such as RIP or BGP, they may represent static routes or they may represent a default route.
An OSPF router which originates External LSAs is known as an ASBR (AS Boundary Router). Unlike the link-state LSAs, and most other LSAs, which are flooded only within the area in which they originate, External LSAs are flooded through-out the OSPF network to all areas capable of carrying External LSAs (see OSPF Areas).
Routes internal to OSPF (intra-area or inter-area) are always preferred over external routes.
The External LSA describes the following:
The IP Network number of the route is described by the LSA ID field.
The body of the External LSA describes the IP Network Mask of the route. This, together with the LSA ID, describes the prefix of the IP route concerned.
The cost of the External Route. This cost may be an OSPF cost (also known as a "Type 1" metric), i.e. equivalent to the normal OSPF costs, or an externally derived cost ("Type 2" metric) which is not comparable to OSPF costs and always considered larger than any OSPF cost. Where there are both Type 1 and 2 External routes for a route, the Type 1 is always preferred.
The address of the router to forward packets to for the route. This may be, and usually is, left as 0 to specify that the ASBR originating the External LSA should be used. There must be an internal OSPF route to the forwarding address, for the forwarding address to be useable.
An arbitrary 4-bytes of data, not interpreted by OSPF, which may carry whatever information about the route which OSPF speakers desire.
To illustrate, below is an example of an External LSA in the LSDB of an OSPF router. It describes a route to the IP prefix of 192.168.165.0/24, originated by the ASBR with Router-ID 192.168.0.49. The metric of 20 is external to OSPF. The forwarding address is 0, so the route should forward to the originating ASBR if selected.
# show ip ospf database external 192.168.165.0 LS age: 995 Options: 0x2 : *|-|-|-|-|-|E|* LS Flags: 0x9 LS Type: AS-external-LSA Link State ID: 192.168.165.0 (External Network Number) Advertising Router: 192.168.0.49 LS Seq Number: 800001d8 Checksum: 0xea27 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0
We can add this to our partial topology from above, which now looks like:
--------------------- Network: ...... | Designated Router IP: 192.168.1.3 | IP: 192.168.1.3 /---- External route: 192.168.165.0/24 (transit link) / Cost: 20 (External metric) (cost: 10) / Router ID: 192.168.0.49(stub)---------- IP: 192.168.3.190/32 (cost: 10) (cost: 39063) (transit link) IP: 192.168.0.49 | | ------------------------------ Network: 192.168.0.48/29 | | | Designated Router IP: 192.168.0.49 | | | | | Router ID: 192.168.0.54 | | | Router ID: 192.168.0.53 | Router ID: 192.168.0.52
Summary LSAs are created by ABRs to summarise the destinations available within one area to other areas. These LSAs may describe IP networks, potentially in aggregated form, or ASBR routers.