OSPFv2

OSPF version 2 is a routing protocol which is described in RFC 2328. OSPF is an IGP. Compared with RIP, OSPF can provide scalable network support and faster convergence times. OSPF is widely used in large networks such as ISP backbone and enterprise networks.

OSPF Fundamentals

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, which is then propagated through to all other routers in a link-state routing domain, by a process called flooding. Each router thus builds up an LSDB 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 Edsger Dijkstra’s SPF algorithm.

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 Mechanisms

OSPF defines a range of mechanisms, concerned with detecting, describing and propagating state through a network. These mechanisms will nearly all be covered in greater detail further on. They may be broadly classed as:

The Hello Protocol

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.

OSPF also uses the Hello protocol to propagate certain state between routers sharing a link, for example:

  • Hello protocol configured state, such as the dead-interval.

  • Router priority, for DR/BDR election.

  • DR/BDR election results.

  • Any optional capabilities supported by each router.

The Hello protocol is comparatively trivial and will not be explored in more detail.

LSAs

At the heart of OSPF are LSA messages. Despite the name, some LSA s do not, strictly speaking, describe link-state information. Common LSA s describe information such as:

  • Routers, in terms of their links.

  • Networks, in terms of attached routers.

  • Routes, external to a link-state domain:

    External Routes

    Routes entirely external to OSPF. Routers originating such routes are known as ASBR routers.

    Summary Routes

    Routes which summarise routing information relating to OSPF areas external to the OSPF link-state area at hand, originated by ABR routers.

LSA Flooding

OSPF defines several related mechanisms, used to manage synchronisation of LSDB s between neighbours as neighbours form adjacencies and the propagation, or flooding of new or updated LSA s.

Areas

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. 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.

OSPF LSAs

The core objects in OSPF are LSA s. 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 LSA s, for purposes such as describing actual link-state information, describing paths (i.e. routes), describing bandwidth usage of links for TE purposes, and even arbitrary data by way of Opaque LSA s.

LSA Header

All LSAs share a common header with the following information:

  • Type

    Different types of LSA s describe different things in OSPF. Types include:

    • Router LSA

    • Network LSA

    • Network Summary LSA

    • Router Summary LSA

    • AS-External LSA

    The specifics of the different types of LSA are examined below.

  • Advertising Router

    The Router ID of the router originating the LSA.

See also

ospf router-id A.B.C.D.

  • LSA 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 sequence number.

  • Age

    A number to allow stale LSA s to, eventually, be purged by routers from their LSDB s.

    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.

  • Sequence Number

    A number used to distinguish newer instances of an LSA from older instances.

External LSAs

External, or “Type 5”, LSA s 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 LSA s is known as an ASBR. Unlike the link-state LSA s, and most other LSA s, which are flooded only within the area in which they originate, External LSA s are flooded through-out the OSPF network to all areas capable of carrying External LSA s (Areas).

Routes internal to OSPF (intra-area or inter-area) are always preferred over external routes.

The External LSA describes the following:

IP Network number

The IP Network number of the route is described by the LSA ID field.

IP Network Mask

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.

Metric

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.

Forwarding Address

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 usable.

Tag

An arbitrary 4-bytes of data, not interpreted by OSPF, which may carry whatever information about the route which OSPF speakers desire.

AS External LSA Example

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

Summary LSAs are created by ABR s to summarise the destinations available within one area to other areas. These LSAs may describe IP networks, potentially in aggregated form, or ASBR routers.

Routers

To start OSPF process you have to specify the OSPF router.

router ospf [{(1-65535)|vrf NAME}]

Enable or disable the OSPF process.

Multiple instances don’t support vrf NAME.

ospf router-id A.B.C.D

This sets the router-ID of the OSPF process. The router-ID may be an IP address of the router, but need not be - it can be any arbitrary 32bit number. However it MUST be unique within the entire OSPF domain to the OSPF speaker - bad things will happen if multiple OSPF speakers are configured with the same router-ID! If one is not specified then ospfd will obtain a router-ID automatically from zebra.

ospf abr-type TYPE

type can be cisco|ibm|shortcut|standard. The “Cisco” and “IBM” types are equivalent.

The OSPF standard for ABR behaviour does not allow an ABR to consider routes through non-backbone areas when its links to the backbone are down, even when there are other ABRs in attached non-backbone areas which still can reach the backbone - this restriction exists primarily to ensure routing-loops are avoided.

With the “Cisco” or “IBM” ABR type, the default in this release of FRR, this restriction is lifted, allowing an ABR to consider summaries learned from other ABRs through non-backbone areas, and hence route via non-backbone areas as a last resort when, and only when, backbone links are down.

Note that areas with fully-adjacent virtual-links are considered to be “transit capable” and can always be used to route backbone traffic, and hence are unaffected by this setting (area A.B.C.D virtual-link A.B.C.D).

More information regarding the behaviour controlled by this command can be found in RFC 3509, and draft-ietf-ospf-shortcut-abr-02.txt.

Quote: “Though the definition of the ABR in the OSPF specification does not require a router with multiple attached areas to have a backbone connection, it is actually necessary to provide successful routing to the inter-area and external destinations. If this requirement is not met, all traffic destined for the areas not connected to such an ABR or out of the OSPF domain, is dropped. This document describes alternative ABR behaviors implemented in Cisco and IBM routers.”

ospf rfc1583compatibility

RFC 2328, the successor to RFC 1583, suggests according to section G.2 (changes) in section 16.4 a change to the path preference algorithm that prevents possible routing loops that were possible in the old version of OSPFv2. More specifically it demands that inter-area paths and intra-area backbone path are now of equal preference but still both preferred to external paths.

This command should NOT be set normally.

log-adjacency-changes [detail]

Configures ospfd to log changes in adjacency. With the optional detail argument, all changes in adjacency status are shown. Without detail, only changes to full or regressions are shown.

passive-interface default

Make all interfaces that belong to this router passive by default. For the description of passive interface look at ip ospf passive [A.B.C.D]. Per-interface configuration takes precedence over the default value.

timers throttle spf (0-600000) (0-600000) (0-600000)

This command sets the initial delay, the initial-holdtime and the maximum-holdtime between when SPF is calculated and the event which triggered the calculation. The times are specified in milliseconds and must be in the range of 0 to 600000 milliseconds.

The delay specifies the minimum amount of time to delay SPF calculation (hence it affects how long SPF calculation is delayed after an event which occurs outside of the holdtime of any previous SPF calculation, and also serves as a minimum holdtime).

Consecutive SPF calculations will always be separated by at least ‘hold-time’ milliseconds. The hold-time is adaptive and initially is set to the initial-holdtime configured with the above command. Events which occur within the holdtime of the previous SPF calculation will cause the holdtime to be increased by initial-holdtime, bounded by the maximum-holdtime configured with this command. If the adaptive hold-time elapses without any SPF-triggering event occurring then the current holdtime is reset to the initial-holdtime. The current holdtime can be viewed with show ip ospf, where it is expressed as a multiplier of the initial-holdtime.

router ospf
timers throttle spf 200 400 10000

In this example, the delay is set to 200ms, the initial holdtime is set to 400ms and the maximum holdtime to 10s. Hence there will always be at least 200ms between an event which requires SPF calculation and the actual SPF calculation. Further consecutive SPF calculations will always be separated by between 400ms to 10s, the hold-time increasing by 400ms each time an SPF-triggering event occurs within the hold-time of the previous SPF calculation.

This command supersedes the timers spf command in previous FRR releases.

max-metric router-lsa [on-startup (5-86400)|on-shutdown (5-100)]
max-metric router-lsa administrative

This enables RFC 3137 support, where the OSPF process describes its transit links in its router-LSA as having infinite distance so that other routers will avoid calculating transit paths through the router while still being able to reach networks through the router.

This support may be enabled administratively (and indefinitely) or conditionally. Conditional enabling of max-metric router-lsas can be for a period of seconds after startup and/or for a period of seconds prior to shutdown.

Enabling this for a period after startup allows OSPF to converge fully first without affecting any existing routes used by other routers, while still allowing any connected stub links and/or redistributed routes to be reachable. Enabling this for a period of time in advance of shutdown allows the router to gracefully excuse itself from the OSPF domain.

Enabling this feature administratively allows for administrative intervention for whatever reason, for an indefinite period of time. Note that if the configuration is written to file, this administrative form of the stub-router command will also be written to file. If ospfd is restarted later, the command will then take effect until manually deconfigured.

Configured state of this feature as well as current status, such as the number of second remaining till on-startup or on-shutdown ends, can be viewed with the show ip ospf command.

auto-cost reference-bandwidth (1-4294967)

This sets the reference bandwidth for cost calculations, where this bandwidth is considered equivalent to an OSPF cost of 1, specified in Mbits/s. The default is 100Mbit/s (i.e. a link of bandwidth 100Mbit/s or higher will have a cost of 1. Cost of lower bandwidth links will be scaled with reference to this cost).

This configuration setting MUST be consistent across all routers within the OSPF domain.

network A.B.C.D/M area A.B.C.D
network A.B.C.D/M area (0-4294967295)

This command specifies the OSPF enabled interface(s). If the interface has an address from range 192.168.1.0/24 then the command below enables ospf on this interface so router can provide network information to the other ospf routers via this interface.

router ospf
network 192.168.1.0/24 area 0.0.0.0

Prefix length in interface must be equal or bigger (i.e. smaller network) than prefix length in network statement. For example statement above doesn’t enable ospf on interface with address 192.168.1.1/23, but it does on interface with address 192.168.1.129/25.

Note that the behavior when there is a peer address defined on an interface changed after release 0.99.7. Currently, if a peer prefix has been configured, then we test whether the prefix in the network command contains the destination prefix. Otherwise, we test whether the network command prefix contains the local address prefix of the interface.

It is also possible to enable OSPF on a per interface/subnet basis using the interface command (ip ospf area AREA [ADDR]). However, mixing both network commands (network) and interface commands (ip ospf) on the same router is not supported.

proactive-arp

This command enables or disables sending ARP requests to update neighbor table entries. It speeds up convergence for /32 networks on a P2P connection.

This feature is enabled by default.

clear ip ospf [(1-65535)] process

This command can be used to clear the ospf process data structures. This will clear the ospf neighborship as well and it will get re-established. This will clear the LSDB too. This will be helpful when there is a change in router-id and if user wants the router-id change to take effect.

clear ip ospf [(1-65535)] neighbor

This command can be used to clear the ospf neighbor data structures. This will clear the ospf neighborship and it will get re-established. This command can be used when the neighbor state get stuck at some state and this can be used to recover it from that state.

maximum-paths (1-64)

Use this command to control the maximum number of equal cost paths to reach a specific destination. The upper limit may differ if you change the value of MULTIPATH_NUM during compilation. The default is MULTIPATH_NUM (64).

write-multiplier (1-100)

Use this command to tune the amount of work done in the packet read and write threads before relinquishing control. The parameter is the number of packets to process before returning. The defult value of this parameter is 20.

socket buffer <send | recv | all> (1-4000000000)

This command controls the ospf instance’s socket buffer sizes. The ‘no’ form resets one or both values to the default.

no socket-per-interface

Ordinarily, ospfd uses a socket per interface for sending packets. This command disables those per-interface sockets, and causes ospfd to use a single socket per ospf instance for sending and receiving packets.

Areas

area A.B.C.D range A.B.C.D/M [advertise [cost (0-16777215)]]
area (0-4294967295) range A.B.C.D/M [advertise [cost (0-16777215)]]

Summarize intra area paths from specified area into one Type-3 summary-LSA announced to other areas. This command can be used only in ABR and ONLY router-LSAs (Type-1) and network-LSAs (Type-2) (i.e. LSAs with scope area) can be summarized. Type-5 AS-external-LSAs can’t be summarized - their scope is AS.

router ospf
 network 192.168.1.0/24 area 0.0.0.0
 network 10.0.0.0/8 area 0.0.0.10
 area 0.0.0.10 range 10.0.0.0/8

With configuration above one Type-3 Summary-LSA with routing info 10.0.0.0/8 is announced into backbone area if area 0.0.0.10 contains at least one intra-area network (i.e. described with router or network LSA) from this range.

area A.B.C.D range A.B.C.D/M not-advertise
area (0-4294967295) range A.B.C.D/M not-advertise

Instead of summarizing intra area paths filter them - i.e. intra area paths from this range are not advertised into other areas. This command makes sense in ABR only.

area A.B.C.D range A.B.C.D/M {substitute A.B.C.D/M|cost (0-16777215)}
area (0-4294967295) range A.B.C.D/M {substitute A.B.C.D/M|cost (0-16777215)}

Substitute summarized prefix with another prefix.

router ospf
 network 192.168.1.0/24 area 0.0.0.0
 network 10.0.0.0/8 area 0.0.0.10
 area 0.0.0.10 range 10.0.0.0/8 substitute 11.0.0.0/8

One Type-3 summary-LSA with routing info 11.0.0.0/8 is announced into backbone area if area 0.0.0.10 contains at least one intra-area network (i.e. described with router-LSA or network-LSA) from range 10.0.0.0/8.

By default, the metric of the summary route is calculated as the highest metric among the summarized routes. The cost option, however, can be used to set an explicit metric.

This command makes sense in ABR only.

area A.B.C.D shortcut
area (0-4294967295) shortcut

Configure the area as Shortcut capable. See RFC 3509. This requires that the ‘abr-type’ be set to ‘shortcut’.

area A.B.C.D stub
area (0-4294967295) stub

Configure the area to be a stub area. That is, an area where no router originates routes external to OSPF and hence an area where all external routes are via the ABR(s). Hence, ABRs for such an area do not need to pass AS-External LSAs (type-5s) or ASBR-Summary LSAs (type-4) into the area. They need only pass Network-Summary (type-3) LSAs into such an area, along with a default-route summary.

area A.B.C.D stub no-summary
area (0-4294967295) stub no-summary

Prevents an ospfd ABR from injecting inter-area summaries into the specified stub area.

area A.B.C.D nssa
area (0-4294967295) nssa

Configure the area to be a NSSA (Not-So-Stubby Area). This is an area that allows OSPF to import external routes into a stub area via a new LSA type (type 7). An NSSA autonomous system boundary router (ASBR) will generate this type of LSA. The area border router (ABR) translates the LSA type 7 into LSA type 5, which is propagated into the OSPF domain. NSSA areas are defined in RFC 3101.

area A.B.C.D nssa suppress-fa
area (0-4294967295) nssa suppress-fa

Configure the router to set the forwarding address to 0.0.0.0 in all LSA type 5 translated from LSA type 7. The router needs to be elected the translator of the area for this command to take effect. This feature causes routers that are configured not to advertise forwarding addresses into the backbone to direct forwarded traffic to the NSSA ABR translator.

area A.B.C.D nssa default-information-originate [metric-type (1-2)] [metric (0-16777214)]
area (0-4294967295) nssa default-information-originate [metric-type (1-2)] [metric (0-16777214)]
area A.B.C.D nssa range A.B.C.D/M [<not-advertise|cost (0-16777215)>]
area (0-4294967295) nssa range A.B.C.D/M [<not-advertise|cost (0-16777215)>]

Summarize a group of external subnets into a single Type-7 LSA, which is then translated to a Type-5 LSA and avertised to the backbone. This command can only be used at the area boundary (NSSA ABR router).

By default, the metric of the summary route is calculated as the highest metric among the summarized routes. The cost option, however, can be used to set an explicit metric.

The not-advertise option, when present, prevents the summary route from being advertised, effectively filtering the summarized routes.

area A.B.C.D default-cost (0-16777215)

Set the cost of default-summary LSAs announced to stubby areas.

area A.B.C.D export-list NAME
area (0-4294967295) export-list NAME

Filter Type-3 summary-LSAs announced to other areas originated from intra- area paths from specified area.

router ospf
 network 192.168.1.0/24 area 0.0.0.0
 network 10.0.0.0/8 area 0.0.0.10
 area 0.0.0.10 export-list foo
!
access-list foo permit 10.10.0.0/16
access-list foo deny any

With example above any intra-area paths from area 0.0.0.10 and from range 10.10.0.0/16 (for example 10.10.1.0/24 and 10.10.2.128/30) are announced into other areas as Type-3 summary-LSA’s, but any others (for example 10.11.0.0/16 or 10.128.30.16/30) aren’t.

This command is only relevant if the router is an ABR for the specified area.

area A.B.C.D import-list NAME
area (0-4294967295) import-list NAME

Same as export-list, but it applies to paths announced into specified area as Type-3 summary-LSAs.

area A.B.C.D filter-list prefix NAME in
area A.B.C.D filter-list prefix NAME out
area (0-4294967295) filter-list prefix NAME in
area (0-4294967295) filter-list prefix NAME out

Filtering Type-3 summary-LSAs to/from area using prefix lists. This command makes sense in ABR only.

area A.B.C.D authentication
area (0-4294967295) authentication

Specify that simple password authentication should be used for the given area.

area A.B.C.D authentication message-digest
area (0-4294967295) authentication message-digest

Specify that OSPF packets must be authenticated with MD5 HMACs within the given area. Keying material must also be configured on a per-interface basis (ip ospf message-digest-key).

MD5 authentication may also be configured on a per-interface basis (ip ospf authentication message-digest). Such per-interface settings will override any per-area authentication setting.

Interfaces

ip ospf area AREA [ADDR]

Enable OSPF on the interface, optionally restricted to just the IP address given by ADDR, putting it in the AREA area. If you have a lot of interfaces, and/or a lot of subnets, then enabling OSPF via this command instead of (network A.B.C.D/M area A.B.C.D) may result in a slight performance improvement.

Notice that, mixing both network commands (network) and interface commands (ip ospf) on the same router is not supported. If (ip ospf) is present, (network) commands will fail.

ip ospf authentication-key AUTH_KEY

Set OSPF authentication key to a simple password. After setting AUTH_KEY, all OSPF packets are authenticated. AUTH_KEY has length up to 8 chars.

Simple text password authentication is insecure and deprecated in favour of MD5 HMAC authentication.

ip ospf authentication message-digest

Specify that MD5 HMAC authentication must be used on this interface. MD5 keying material must also be configured. Overrides any authentication enabled on a per-area basis (area A.B.C.D authentication message-digest)

Note that OSPF MD5 authentication requires that time never go backwards (correct time is NOT important, only that it never goes backwards), even across resets, if ospfd is to be able to promptly reestablish adjacencies with its neighbours after restarts/reboots. The host should have system time be set at boot from an external or non-volatile source (e.g. battery backed clock, NTP, etc.) or else the system clock should be periodically saved to non-volatile storage and restored at boot if MD5 authentication is to be expected to work reliably.

ip ospf message-digest-key KEYID md5 KEY

Set OSPF authentication key to a cryptographic password. The cryptographic algorithm is MD5.

KEYID identifies secret key used to create the message digest. This ID is part of the protocol and must be consistent across routers on a link.

KEY is the actual message digest key, of up to 16 chars (larger strings will be truncated), and is associated with the given KEYID.

ip ospf authentication key-chain KEYCHAIN

Specify that HMAC cryptographic authentication must be used on this interface using a key chain. Overrides any authentication enabled on a per-area basis (area A.B.C.D authentication message-digest)

  • KEYCHAIN: Specifies the name of the key chain that contains the authentication

key(s) and cryptographic algorithms to be used for OSPF authentication. The key chain is a logical container that holds one or more authentication keys, allowing for key rotation and management.

Note that OSPF HMAC cryptographic authentication requires that time never go backwards (correct time is NOT important, only that it never goes backwards), even across resets, if ospfd is to be able to promptly reestablish adjacencies with its neighbours after restarts/reboots. The host should have system time be set at boot from an external or non-volatile source (e.g. battery backed clock, NTP, etc.) or else the system clock should be periodically saved to non-volatile storage and restored at boot if HMAC cryptographic authentication is to be expected to work reliably.

Example:

r1(config)#key chain temp
r1(config-keychain)#key 13
r1(config-keychain-key)#key-string ospf
r1(config-keychain-key)#cryptographic-algorithm hmac-sha-256
r1(config)#int eth0
r1(config-if)#ip ospf authentication key-chain temp
r1(config-if)#ip ospf area 0
ip ospf cost (1-65535)

Set link cost for the specified interface. The cost value is set to router-LSA’s metric field and used for SPF calculation.

ip ospf dead-interval (1-65535)
ip ospf dead-interval minimal hello-multiplier (2-20)

Set number of seconds for RouterDeadInterval timer value used for Wait Timer and Inactivity Timer. This value must be the same for all routers attached to a common network. The default value is 40 seconds.

If ‘minimal’ is specified instead, then the dead-interval is set to 1 second and one must specify a hello-multiplier. The hello-multiplier specifies how many Hellos to send per second, from 2 (every 500ms) to 20 (every 50ms). Thus one can have 1s convergence time for OSPF. If this form is specified, then the hello-interval advertised in Hello packets is set to 0 and the hello-interval on received Hello packets is not checked, thus the hello-multiplier need NOT be the same across multiple routers on a common link.

ip ospf hello-interval (1-65535)

Set number of seconds for HelloInterval timer value. Setting this value, Hello packet will be sent every timer value seconds on the specified interface. This value must be the same for all routers attached to a common network. The default value is 10 seconds.

This command has no effect if ip ospf dead-interval minimal hello-multiplier (2-20) is also specified for the interface.

ip ospf graceful-restart hello-delay (1-1800)

Set the length of time during which Grace-LSAs are sent at 1-second intervals while coming back up after an unplanned outage. During this time, no hello packets are sent.

A higher hello delay will increase the chance that all neighbors are notified about the ongoing graceful restart before receiving a hello packet (which is crucial for the graceful restart to succeed). The hello delay shouldn’t be set too high, however, otherwise the adjacencies might time out. As a best practice, it’s recommended to set the hello delay and hello interval with the same values. The default value is 10 seconds.

ip ospf network (broadcast|non-broadcast|point-to-multipoint [delay-reflood]|point-to-point)
ip ospf priority (0-255)

Set RouterPriority integer value. The router with the highest priority will be more eligible to become Designated Router. Setting the value to 0, makes the router ineligible to become Designated Router. The default value is 1.

ip ospf retransmit-interval (1-65535)

Set number of seconds for RxmtInterval timer value. This value is used when retransmitting Database Description and Link State Request packets. The default value is 5 seconds.

ip ospf transmit-delay (1-65535) [A.B.C.D]

Set number of seconds for InfTransDelay value. LSAs’ age should be incremented by this value when transmitting. The default value is 1 second.

ip ospf passive [A.B.C.D]

Do not speak OSPF on the interface, but do advertise the interface as a stub link in the router-LSA for this router. This allows one to advertise addresses on such connected interfaces without having to originate AS-External/Type-5 LSAs (which have global flooding scope) - as would occur if connected addresses were redistributed into OSPF (Redistribution). This is the only way to advertise non-OSPF links into stub areas.

ip ospf area (A.B.C.D|(0-4294967295))

Enable ospf on an interface and set associated area.

OSPF route-map

Usage of ospfd’s route-map support.

set metric [+|-](0-4294967295)

Set a metric for matched route when sending announcement. Use plus (+) sign to add a metric value to an existing metric. Use minus (-) sign to substract a metric value from an existing metric.

Redistribution

redistribute <bgp|connected|isis|kernel|ospf|rip|static|table> [metric-type (1-2)] [metric (0-16777214)] [route-map WORD]
default-information originate
default-information originate metric (0-16777214)
default-information originate metric (0-16777214) metric-type (1|2)
default-information originate metric (0-16777214) metric-type (1|2) route-map WORD
default-information originate always
default-information originate always metric (0-16777214)
default-information originate always metric (0-16777214) metric-type (1|2)
default-information originate always metric (0-16777214) metric-type (1|2) route-map WORD

Originate an AS-External (type-5) LSA describing a default route into all external-routing capable areas, of the specified metric and metric type. If the ‘always’ keyword is given then the default is always advertised, even when there is no default present in the routing table.

distribute-list NAME out <kernel|connected|static|rip|isis|bgp|table>
default-metric (0-16777214)
distance (1-255)
distance ospf (intra-area|inter-area|external) (1-255)

Graceful Restart

graceful-restart [grace-period (1-1800)]

Configure Graceful Restart (RFC 3623) restarting support. When enabled, the default grace period is 120 seconds.

To perform a graceful shutdown, the “graceful-restart prepare ip ospf” EXEC-level command needs to be issued before restarting the ospfd daemon.

When Graceful Restart is enabled and the ospfd daemon crashes or is killed abruptely (e.g. SIGKILL), it will attempt an unplanned Graceful Restart once it restarts.

graceful-restart helper enable [A.B.C.D]

Configure Graceful Restart (RFC 3623) helper support. By default, helper support is disabled for all neighbours. This config enables/disables helper support on this router for all neighbours. To enable/disable helper support for a specific neighbour, the router-id (A.B.C.D) has to be specified.

graceful-restart helper strict-lsa-checking

If ‘strict-lsa-checking’ is configured then the helper will abort the Graceful Restart when a LSA change occurs which affects the restarting router. By default ‘strict-lsa-checking’ is enabled”

graceful-restart helper supported-grace-time

Supports as HELPER for configured grace period.

graceful-restart helper planned-only

It helps to support as HELPER only for planned restarts. By default, it supports both planned and unplanned outages.

graceful-restart prepare ip ospf

Initiate a graceful restart for all OSPF instances configured with the “graceful-restart” command. The ospfd daemon should be restarted during the instance-specific grace period, otherwise the graceful restart will fail.

This is an EXEC-level command.

Showing Information

show ip ospf [vrf <NAME|all>] [json]

Show information on a variety of general OSPF and area state and configuration information.

show ip ospf interface [INTERFACE] [json]

Show state and configuration of OSPF the specified interface, or all interfaces if no interface is given.

show ip ospf neighbor [json]
show ip ospf [vrf <NAME|all>] neighbor INTERFACE [json]
show ip ospf [vrf <NAME|all>] neighbor A.B.C.D [detail] [json]
show ip ospf [vrf <NAME|all>] neighbor INTERFACE detail [json]

Display lsa information of LSDB. Json o/p of this command covers base route information i.e all LSAs except opaque lsa info.

show ip ospf [vrf <NAME|all>] database [self-originate] [json]

Show the OSPF database summary.

show ip ospf [vrf <NAME|all>] database max-age [json]

Show all MaxAge LSAs present in the OSPF link-state database.

show ip ospf route [json]

Show the OSPF routing table, as determined by the most recent SPF calculation.

show ip ospf [vrf <NAME|all>] border-routers [json]

Show the list of ABR and ASBR border routers summary learnt via OSPFv2 Type-3 (Summary LSA) and Type-4 (Summary ASBR LSA). User can get that information as JSON format when json keyword at the end of cli is presented.

show ip ospf graceful-restart helper [detail] [json]

Displays the Grcaeful Restart Helper details including helper config changes.

Debugging OSPF

debug ospf [(1-65535)] bfd

Enable or disable debugging for BFD events. This will show BFD integration library messages and OSPF BFD integration messages that are mostly state transitions and validation problems.

debug ospf [(1-65535)] default-information

Show debug information of default information

debug ospf [(1-65535)] packet (hello|dd|ls-request|ls-update|ls-ack|all) (send|recv) [detail]
debug ospf [(1-65535)] ism [status|events|timers]

Show debug information of Interface State Machine

debug ospf [(1-65535)] nsm [status|events|timers]

Show debug information of Network State Machine

debug ospf [(1-65535)] event

Show debug information of OSPF event

debug ospf [(1-65535)] nssa

Show debug information about Not So Stub Area

debug ospf [(1-65535)] ldp-sync

Show debug information about LDP-Sync

debug ospf [(1-65535)] lsa [aggregate|flooding|generate|install|refresh]
debug ospf [(1-65535)] zebra [interface|redistribute]

Show debug information of ZEBRA API

debug ospf [(1-65535)] graceful-restart

Enable/disable debug information for OSPF Graceful Restart Helper

show debugging ospf

OSPF Configuration Examples

A simple example, with MD5 authentication enabled:

!
interface ge0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 ABCDEFGHIJK
!
router ospf
 network 192.168.0.0/16 area 0.0.0.1
 area 0.0.0.1 authentication message-digest

An ABR router, with MD5 authentication and performing summarisation of networks between the areas:

!
log syslog
!
interface ge0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 ABCDEFGHIJK
!
interface ge1
 ip ospf passive
!
interface ge2
 ip ospf authentication message-digest
 ip ospf message-digest-key 2 md5 XYZ12345
!
router ospf
 ospf router-id 192.168.0.1
 redistribute connected
 network 192.168.0.0/24 area 0.0.0.0
 network 10.0.0.0/16 area 0.0.0.0
 network 192.168.1.0/24 area 0.0.0.1
 area 0.0.0.0 authentication message-digest
 area 0.0.0.0 range 10.0.0.0/16
 area 0.0.0.0 range 192.168.0.0/24
 area 0.0.0.1 authentication message-digest
 area 0.0.0.1 range 10.2.0.0/16
!