Course sections

Core Routing, Lecture 1

Lesson 1: Interior Gateway Protocol

Describe, implement, and troubleshoot IS-IS

Integrated Intermediate System-to-Intermediate System (IS-IS), Internet Protocol Version 4 (IPv4), is a standards-based Interior Gateway Protocol (IGP). Cisco IOS XR software implements the IP routing capabilities described in International Organization for Standardization (ISO)/International Engineering Consortium (IEC) 10589 and RFC 1995, and adds the standard extensions for single topology and multi topology IS-IS for IP Version 6 (IPv6).

IS-IS requires configuration on both the router and the interface. An IS-IS process is created when you enable IS-IS on a router and define a specific tag to identify that routing process. Interfaces configured with a specific tag will be part of the corresponding router process. More than one IS-IS process can run on a router for Connectionless Network Service (CLNS), but only one IS-IS process can run for IP. Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas. The areas are connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.

Routers establish Level 1 adjacencies to perform routing within a local area (intra-area routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (inter-area routing). If the network administrator does not specify Level 1 or Level 2 routing for the routing process being configured, the default routing behavior for the routing process will be Level 1-2.

If Level 2 routing is configured on any process, additional processes are automatically configured as Level 1, with the exception of previously configured Level 2 process, which will remain Level 2. You can have only one Level-2 process. You can configure the Level-2 process to perform Level-1 routing at the same time. If Level-2 routing is not desired for a router instance, use the is-type command in router configuration mode to remove the Level-2 capability. You can also use the is-type command to configure a different router instance as a Level-2 router. Some networks use legacy equipment that supports only Level 1 routing. These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.

The idea behind the Designated Intermediate System (DIS) is similar to that behind the designated router in OSPF. The DIS creates a pseudo node (a virtual node), and all the routers on a LAN, including the DIS, form an adjacency with the pseudo node instead of forming n*(n-1) order adjacencies with each other in a full mesh.

On a LAN, one of the routers will elect itself the DIS based on interface priority (the default is 64). If all interface priorities are the same, the router with the highest subnetwork point of attachment (SNPA) is selected. MAC addresses are the SNPA on LANs. On Frame Relay networks, the local data-link connection identifier (DLCI) is the SNPA. If the SNPA is a DLCI and is the same at both sides of a link, the router with the higher system ID (in the NSAP address) will become the DIS.

A pseudo node LSP represents a LAN, including all ISs attached to that LAN, just as a non-pseudo node LSP represents a router, including all ISs and LANs connected with the router.

The DIS election is pre-emptive (unlike with OSPF). If a new router boots on the LAN with a higher interface priority, it becomes the DIS, purges the old pseudo node LSP, and a new set of LSPs will be flooded. The DIS sends CSNPs describing all the LSPs in the database every 3 seconds. If a router needs an LSP because it is older than the LSP advertised by the DIS in its CSNP or it is missing an LSP that is listed in the CSNP, it will send a PSNP to the DIS and receive the LSP in return. This mechanism can work both ways: If a router sees that it has a newer version of an LSP, or it has an LSP that the DIS does not advertise in its CSNP, the router will send the newer or missing LSP to the DIS. The Cisco IS-IS implementation offers an authentication mechanism to prevent unauthorized routers from forming adjacencies or injecting TLVs. Currently, only plain-text authentication is available where the configured password is transmitted inside the IS-IS PDUs unencrypted in plain text. As such, the password can be determined by sniffing the packets. Future Cisco IOS Software releases will also contain Hashed Message Authentication Codes with MD5 (HMAC-MD5) with encrypted passwords as specified in the corresponding IETF draft.

IS-IS authentication is configured independently for adjacency establishment (hello) and for LSP authentication.

Single area, Single topology

Small IS-IS networks are built as a single area that includes all the routers in the network. As the network grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 routers from all areas, which is in turn connected to local areas. Within a local area, routers know how to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone routers know how to reach other areas.

Routers establish Level 1 adjacencies to perform routing within a local area (intra-area routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (inter-area routing). Some networks use legacy equipment that supports only Level 1 routing. These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.


Step 1 Define areas, prepare an addressing plan for the routers (including defining the NETs), and determine interfaces that will run Integrated IS-IS.
Step 2 Enable IS-IS as an IP routing protocol on the routers and assign a tag to the process (if required).
Step 3 Configure the NETs on the routers. This identifies the routers for IS-IS.
Step 4 Enable Integrated IS-IS on the proper interfaces on the routers. Do not forget interfaces to stub IP networks, such as loopback interfaces (although there will not be any CLNS neighbors on these interfaces).


ISO has developed standards for two types of routing protocols:

  • ES-IS discovery protocol—ES-IS performs “routing” between End Systems and Intermediate Systems referred as Level 0 “routing.” ES-IS is analogous to the Address Resolution Protocol (ARP) in IP. Although it is not explicitly a routing protocol, ES-IS is included here because it is commonly used with routing protocols to provide end-to-end data movement through an internetwork.
  • IS-IS routing protocols—IS-IS performs hierarchical (Level 1, Level 2, and Level 3) routing between intermediate systems. Level 3 routing is done between separate domains. However, note that the IS-IS routing protocol is not itself capable of Level 3 routing.

There is no Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP) or Inter-domain Routing Protocol (IDRP) for CLNS, but End System-to-Intermediate System (ES-IS) Protocol provides the same kind of reporting functions for ISs and ESs.

Further Reading

Describe network types, levels and router types

NSAP addressing

NSAP is the network-layer address for CLNS packets. An NSAP describes an attachment to a particular service at the network layer of a node, similar to the combination of IP destination address and IP protocol number in an IP packet. NSAP encoding and format are specified by ISO 8348/Ad2.


ISO 8348/Ad2 uses the concept of hierarchical addressing domains. The global domain is the highest level. This global domain is subdivided into sub-domains, and each sub-domain is associated with an addressing authority that has a unique plan for constructing NSAP addresses.

An NSAP address has two major parts: the initial domain part (IDP) and the domain specific part (DSP). The IDP consists of a 1-byte authority and format identifier (AFI) and a variable-length initial domain identifier (IDI), and the DSP is a string of digits identifying a particular transport implementation of a specified AFI authority. Everything to the left of the system ID can be thought of as the area address of a network node.


The LSP identifier is derived from the system ID (along with the pseudonode ID and LSP number). Each IS is usually configured with one NET and in one area; each system ID within an area must be unique.

Further Reading

Point-to-point, broadcast

When a network consists of only two networking devices connected to broadcast media and uses the integrated IS-IS protocol, it is better for the system to handle the link as a point-to-point link instead of as a broadcast link. This feature introduces a new command to make IS-IS behave as a point-to-point link between the networking devices.


Router(config-if)# isis network point-to-point | broadcast

Describe operations

From a high level, IS-IS operates as follows:

  • Routers running IS-IS will send hello packets out all IS-IS-enabled interfaces to discover neighbors and establish adjacencies.
  • Routers sharing a common data link will become IS-IS neighbors if their hello packets contain information that meets the criteria for forming an adjacency. The criteria differ slightly depending on the type of media being used (p2p or broadcast). The main criteria are matching authentication, IS-type and MTU size.
  • Routers may build a link-state packet (LSP) based upon their local interfaces that are configured for IS-IS and prefixes learned from other adjacent routers.
  • Generally, routers flood LSPs to all adjacent neighbors except the neighbor from which they received the same LSP. However, there are different forms of flooding and also a number of scenarios in which the flooding operation may differ.
  • All routers will construct their link-state database from these LSPs.
  • A shortest-path tree (SPT) is calculated by each IS, and from this SPT the routing table is built.


Configuration Steps

On Cisco equipment there are some basic configuration parameters which are required to get IS-IS up and running. The commands to get IS-IS up and running on a router include:

  1. router isis – This command is used to enable the start of the IS-IS process on the router
  2. net address – This command is used to configure the NET address which will be used to identify this router
  3. ip router isis – This command is configured on each interface which will participate with IS-IS


By default, a Cisco router will operate in Level-1/Level-2 mode which supports both levels of routing. This makes IS-IS easier to configure but requires additional memory and processor on each device. If only Level-1 or Level-2 routing is required on a specific device you can enable only the level required on the device by using the is-type [level-1 | level-2] command while in IS-IS router configuration mode.


The sample configurations below configure all the routers in the above topology with these parameters:

  • Area 49.0001
  • Level 1 (L1) and Level 2 (L2) routers (this is the default unless otherwise specified)
  • No optional parameters
  • Running IS-IS for IP only
  • Loopback interfaces (loopbacks are advertised by IS-IS, not IS-IS enabled)
Router 1
interface Loopback0
ip address!— Creates loopback interface and assigns
!— IP address to interface Loopback0.!
interface Ethernet0
ip address
ip router isis!— Assigns IP address to interface Ethernet0
!— and enables IS-IS for IP on the interface.!
router isis
passive-interface Loopback0
net 49.0001.1720.1600.1001.00
!!— Enables the IS-IS process on the router,
!— makes loopback interface passive
!— (does not send IS-IS packets on interface),
!— and assigns area and system ID to router.


Router 2
interface Loopback0
ip address!— Creates loopback interface and assigns
!— IP address to interface Loopback0.!
Interface Ethernet0
ip address
ip router isis!— Assigns IP address to interface Ethernet0
!— and enables IS-IS for IP on the interface.!
Interface Serial0
ip address
ip router isis!— Assigns IP address to interface Serial0
!— and enables IS-IS for IP on the interface.

router isis
passive-interface Loopback0
net 49.0001.1720.1600.2002.00

!— Enables the IS-IS process on the router,
!— makes loopback interface passive
!— (does not send IS-IS packets on interface),
!— and assigns area and system ID to router.


Router 3
interface Loopback0
ip address!— Creates loopback interface
!— and assigns IP address to
!— interface Loopback0.!
Interface Serial0
ip address
ip router Isis!— Assigns IP address to
!— interface Serial0 and enables
!— IS-IS for IP on the interface.!
router isis
passive-interface Loopback0
net 49.0001.1234.1600.2231.00
!!— Enables the IS-IS process on the router,
!— makes loopback interface passive
!— (does not send IS-IS packets on interface),
!— and assigns area and system ID to router.


Describe optimization features

To minimize the number of adjacencies, LSDBs, and related SPF and PRC computations that are performed, it is recommended that you have configured all Level 1 routers as Level 1 by using the is-type command. We recommend that you use the metric-style wide command because some features, such as setting prefix tags and MPLS traffic engineering, require that routers that are running IS-IS generate the new-style TLVs that have wider metric fields. If you use the default narrow metric style for IS-IS, the router generates and accepts old-style type, length, and value objects (TLVs).

Metrics, wide metric

Cisco IOS Software allows for wide metrics with the support of a 24-bit metric field. Using the new metric style, link metrics now have a maximum value of 16777215 (224-1) with a total path metric of 4261412864 (254 x 224). Deploying IS-IS in the IP network with wide metrics is recommended to enable finer granularity and to support future applications such as Traffic Engineering.


Running different metric styles within one network poses a serious problem: Link-state protocols calculate loop-free routes because all routers (within one area) calculate their routing table based on the same link-state database. This principle is violated if some routers look at old-style (narrow), and some at new-style (wider) TLVs. However, if the same interface cost is used for both the old- and new-style metrics, then the SPF will compute a loop-free topology.

Monitoring IS-IS Adjacencies

Use the show clns neighbor command to display the adjacencies for a specific router. This is the output of this command from Router 1 (R1) and Router 2 (R2):


R1# show clns neighbor
System Id   Interface  SNPA            State  Holdtime Type Protocol
R2          Et0        0000.0c47.b947  Up    24        L1L2 ISIS

R2# show clns neighbor
System Id  Interface  SNPA            State  Holdtime Type Protocol
R1         Et0        0000.0c09.9fea   Up    24        L1L2 ISIS
R3         Se0        *HDLC*           Up     28       L1L2 ISIS


In the above example, R1 recognizes R2 on its E0 interface with the adjacency type being L1L2. Because R1 and R2 are configured with default configurations, they send and receive both L1 and L2 hellos.


R2 recognizes R1 on its E0 interface, and Router 3 (R3) on its S0 interface. The same explanation as above is true for the adjacency type. Since R1 and R2 are on the same Ethernet interface, there is a DIS for both L1 and L2. You can verify this using the show clns interface <int> command on Router 1, as shown below:


R1# show clns interface ethernet 0
Ethernet0 is up, line protocol is up
Checksums enabled, MTU 1497, Encapsulation SAP
Routing Protocol: ISIS
Circuit Type: level-1-2
Interface number 0x0, local circuit ID 0x1
Level-1 Metric: 10, Priority: 64, Circuit ID: R2.01
Number of active level-1 adjacencies: 1
Level-2 Metric: 10, Priority: 64, Circuit ID: R2.01
Number of active level-2 adjacencies: 1
Next ISIS LAN Level-1 Hello in 5 seconds
Next ISIS LAN Level-2 Hello in 1 seconds


In the above output, R2 is the DIS. It is the R2 (DIS) that generates the pseudonode Link State Packet (LSP) and is denoted with a non-zero LSP-ID – R2.01 Since the Metric/Priority are the same for both routers in L1/L2, the tiebreaker for the DIS is the highest Subnetwork Points of Attachment (SNPA) address on the LAN segment. The SNPA address refers to the data link address, and in this case is the MAC address.


Notice that the DIS is elected for both levels, and that no backup DIS exists, as with Open Shortest Path First (OSPF), which has a backup Designated Router (DR). Some other points of interest from the above output include:

  • Circuit Type: L1L2
  • L1 and L2 metrics and priorities are at default values: 10 and 64
  • L1 and L2 adjacencies: 1 (from R1 perspective on the Ethernet interface – it is R2 only)
  • IS-IS LAN hellos for L1 and L2
  • Maximum Transmission Unit (MTU): 1497. This is because the Open Systems Interconnection (OSI) IS-IS header is encapsulated inside a 3-byte 802.2 header.

Monitoring the IS-IS Database

The show isis database (detail) command displays the contents of the IS-IS database. This is the output of this command when issued on R2. Since IS-IS is a link state protocol, the link state database should be the same for any router in the same area.


R2# show isis database
ISIS Level-1 Link State Database:
LSPID         LSP Seq Num  LSP Checksum LSP Holdtime    ATT/P/OL
R1.00-00      0x0000008B   0x6843       55              0/0/0
R2.00-00    * 0x00000083   0x276E       77              0/0/0
R2.01-00    * 0x00000004   0x34E1       57              0/0/0
R3.00-00      0x00000086   0xF30E       84              0/0/0
ISIS Level-2 Link State Database:
LSPID         LSP Seq Num  LSP Checksum LSP Holdtime    ATT/P/OL
R1.00-00      0x00000092   0x34B2       41              0/0/0
R2.00-00    * 0x0000008A   0x7A59       115             0/0/0
R2.01-00    * 0x00000004   0xC3DA       50              0/0/0
R3.00-00      0x0000008F   0x0766       112             0/0/0


There are a few things to notice in the above output. First, about the LSP-ID:

The LSP-ID, R1.00-00, can be broken down into three sections: R1/00/00

  • R1 = system ID
  • 00 = non-zero value for the pseudonode. Notice R2.01-00 is the pseudonode LSP.
  • 00 = fragment number. In this case, there are only fragment numbers of 00, which indicates that all the data fit into this LSP fragment, and there was no need to create more fragments. If there had been information that did not fit into the first LSP, IS-IS would have created more LSP fragments, such as 01, 02, and so on.


The * denotes the LSPs that were generated by this router, the router that the show command was issued on. Also, since this router is an L1 and L2 router, it contains an L1 and L2 database.

You can also look at a specific LSP and use the detail keyword to show more information. An example of this is shown here:


R2# show isis database R2.00-00 detail
ISIS Level-1 LSP R2.00-00
LSPID         LSP Seq Num  LSP Checksum LSP Holdtime  ATT/P/OL
R2.00-00    * 0x00000093   0x077E       71            0/0/0
Area Address: 49.0001
NLPID:        0xCC
Hostname: R2
IP Address:
Metric: 10         IP
Metric: 0          IP
Metric: 10         IP
Metric: 10         IS R2.01
Metric: 10         IS R3.00
ISIS Level-2 LSP R2.00-00
LSPID         LSP Seq Num  LSP Checksum LSP Holdtime  ATT/P/OL
R2.00-00    * 0x0000009A   0x5A69       103           0/0/0
Area Address: 49.0001
NLPID:        0xCC
Hostname: R2
IP Address:
Metric: 10         IS R2.01
Metric: 10         IS R3.00
Metric: 10         IP
Metric: 10         IP
Metric: 10         IP
Metric: 0          IP
Metric: 10         IP


The above output shows that the loopback address of this router is advertised with a value of 0. This is because the loopback is advertised with a passive-interface command under the router IS-IS process, and the loopback interface by itself is not enabled for IS-IS. All other IP prefixes have a value of 10, which is the default cost on the interfaces running IS-IS.

Further Reading

Describe, implement, and troubleshoot OSPFv2 and OSPFv3

LSA types (1, 2, 3, 4, 5, 7, 9)

Table 1-1, shows LSA types and their meaning

LSA Type Description
1 Router Link advertisements. Generated by each router for each area it belongs to. They describe the states of the router’s link to the area. These are only flooded within a particular area.
2 Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular network. Flooded in the area that contains the network.
3 or 4 Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes. Type 3 describes routes to networks, also used for aggregating routes. Type 4 describes routes to ASBR.
5 AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas.
7 Routers in a Not-so-stubby-area (NSSA) do not receive external LSAs from Area Border Routers, but are allowed to send external routing information for redistribution. They use type 7 LSAs to tell the ABRs about these external routes, which the Area Border Router then translates to type 5 external LSAs and floods as normal to the rest of the OSPF network.
9 a link-local “opaque” LSA (defined by RFC2370) in OSPFv2 and the Intra-Area-Prefix LSA in OSPFv3. It is the OSPFv3 LSA that contains prefixes for stub and transit networks in the link-state ID.


A designated router (DR) is the router interface elected among all routers on a particular multi-access network segment, generally assumed to be broadcast multi-access. The basic neighbor discovery process (Hello), flooding (, DR election (priority, RID). Special techniques, often vendor-dependent, may be needed to support the DR function on non-broadcast multi-access (NBMA) media. It is usually wise to configure the individual virtual circuits of a NBMA subnet as individual point-to-point lines. DRs exist for the purpose of reducing network traffic by providing a source for routing updates. The DR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in a multi-access network segment will form a slave/master relationship with the DR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address The DR will then send the update out to all other routers in the area, to the multicast address This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load. DRs and BDRs are always setup/elected on OSPF broadcast networks. DR’s can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay. DRs or BDRs are not elected on point-to-point links (such as a point-to-point WAN connection) because the two routers on either sides of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.

Table 1-2, shows network types and recommended traffic type for use

Network Type Traffic Type

p2mp broadcast




loopback Stub

Route types (N1, N2, E1, E2)

There are several types of OSPF routes:

  • Intra-Area—In a multi-area OSPF network, routes, originated within an area, are known by the routers in the same area as Intra-Area routes. These routes are flagged as O in the show ip route command output.
  • Inter-Area—When a route crosses an OSPF Area Border Router (ABR), the route is known as an OSPF Inter-Area route. These routes are flagged as O IA in the show ip route command output.
  • Both Intra and Inter-Area routes are also called OSPF Internal routes, as they are generated by OSPF itself, when an interface is covered with the OSPF network command.
  • External Type-2 or External Type-1—Routes which were redistributed into OSPF, such as Connected, Static, or other Routing Protocol, are known as External Type-2 or External Type-1. These routes are flagged as O E2 or O E1 in the show ip route command output.
  • NSSA external type 2 or NSSA external type 1—When an area is configured as a Not-So-Stub Area (NSSA), and routes are redistributed into OSPF, the routes are known as NSSA external type 2 or NSSA external type 1. These routes are flagged as O N2 or O N1 in the show ip route command output.

Implement and troubleshoot neighbor relationship

You can use the show ip ospf neighbor command to determine the state of the OSPF neighbor or neighbors.

If the show ip ospf neighbor command reveals nothing at all—or reveals nothing about the particular neighbor you are analyzing—then this router has not seen any “valid” OSPF HELLOs from that neighbor. This means that OSPF either did not receive any HELLO packets from the neighbor or received HELLO packets that failed very basic sanity checks.

  • state = down

A neighbor that is discovered dynamically through reception of HELLO packets can fall back to a down state if it is being deleted, for example when OSPF does not receive HELLO packets from the neighbor for period of time longer than the Dead timer interval. Therefore, the down state is transient for such neighbors; they will either advance to higher states or be completely deleted from the table of known neighbors. This is known as being “forgotten”.

Usually, neighbors that are seen in the down state were manually configured with the neighbor command. Manually configured neighbors are always present in the OSPF neighbor table. If OSPF has never received HELLO packet from the manually configured neighbor, or if no HELLO packets were heard from the neighbor during the previous Dead timer interval, then the manually configured neighbor will be listed as down.

  • state = init

The init state indicates that a router sees HELLO packets from the neighbor, but two-way communication has not been established. A Cisco router includes the Router IDs of all neighbors in the init (or higher) state in the Neighbor field of its HELLO packets. For two-way communication to be established with a neighbor, a router also must see its own Router ID in the Neighbor field of the neighbor’s HELLO packets.

  • state = exstart / exchange

OSPF neighbors that are in exstart or exchange state are trying to exchange DBD packets. The router and its neighbor form a master and slave relationship. The adjacency should continue past this state. If it does not, there is a problem with the DBD exchange, such as a maximum transmission unit (MTU) mismatch or the receipt of an unexpected DBD sequence number

  • state = 2-way

The 2-way state indicates that the router has seen its own Router ID in the Neighbor field of the neighbor’s HELLO packet. Receiving a Database Descriptor (DBD) packet from a neighbor in the init state will also a cause a transition to 2-way state. The OSPF neighbor 2-way state is not a cause for concern (e.g. for broadcast network type).

  • state = loading

In the loading state, routers send link-state request packets. During the adjacency, if a router receives an outdated or missing link-state advertisement (LSA), it requests that LSA by sending a link-state request packet. Neighbors that do not transition beyond this state are most likely exchanging corrupted LSAs. This problem is usually accompanied by a %OSPF-4-BADLSA console message.


A common problem when using Open Shortest Path First (OSPF) is routes in the database don’t appear in the routing table. In most cases OSPF finds a discrepancy in the database so it doesn’t install the route in the routing table. Often, you can see the Adv Router is not-reachable message (which means that the router advertising the LSA is not reachable through OSPF) on top of the link-state advertisement (LSA) in the database when this problem occurs. Here is an example:


Router# show ip ospf database router

Adv Router is not-reachable

LS age: 418

Options: (No TOS-capability, DC)

LS Type: Router Links

Link State ID:

Advertising Router:

LS Seq Number: 80000002

Checksum: 0xFA63

Length: 60

Number of Links: 3


There are several reasons for this problem, most of which deal with mis-configuration or a broken topology. When the configuration is corrected the OSPF database discrepancy goes away and the routes appear in the routing table.

Further Reading

Implement and troubleshoot OSPFv3 address-family support

Configuration Steps

  1. enable
  2. configure terminal
  3. router ospfv3 [process-id]
  4. area area-ID [default-cost | nssa | stub]
  5. auto-cost reference-bandwidth Mbps
  6. bfd all-interfaces
  7. default {area area-ID[range ipv6-prefix | virtual-link router-id]} [default-information originate [always | metric | metric-type | route-map] | distance | distribute-list prefix-list prefix-list-name {in | out} [interface] | maximum-paths paths | redistribute protocol | summary-prefix ipv6-prefix]
  8. ignore lsa mospf
  9. interface-id snmp-if-index
  10. log-adjacency-changes [detail]
  11. passive-interface [default | interface-type interface-number]
  12. queue-depth {hello | update} {queue-size | unlimited}
  13. router-id {router-id}


To filter outgoing link-state advertisements (LSAs) to an Open Shortest Path First (OSPF) interface, use the ip ospf database-filter all out command in interface or virtual network interface configuration modes. To restore the forwarding of LSAs to the interface, use the no form of this command.

Router(config-if)#ipv6 ospf neighbor 2002:234::2

OSPFv3:Neighbor address needs to be a link-local address


You need to add the additional static frame-relay DLCI mapping statement to neighbor’s link local address before OSPF adjacency will form.

ip ospf database-filter all out [disable]


Verification Steps

  • show ospfv3 [process-id] border-routers
  • show ospfv3 [process-id [area-id]] database [database-summary | internal | external[ipv6-prefix ] [link-state-id] | grace | inter-area prefix [ipv6-prefix | link-state-id] | inter-area router [destination-router-id | link-state-id] | link [interface interface-name |link-state-id] | network [link-state-id] | nssa-external [ipv6-prefix] [link-state-id] | prefix [ref-lsa {router | network} | link-state-id] | promiscuous | router [link-state-id] | unknown [{area | as | link} [link-state-id]] [adv-router router-id] [self-originate]
  • show ospfv3 [process-id] events [generic | interface | lsa | neighbor | reverse | rib | spf]
  • show ospfv3 [process-id] [area-id] flood-list interface-type interface-number
  • show ospfv3 [process-id] graceful-restart
  • show ospfv3 [process-id] [area-id] interface[type number] [brief]
  • show ospfv3 [process-id] [area-id] neighbor[interface type interface-number] [neighbor-id] [detail]
  • show ospfv3 [process-id] [area-id] request-list[neighbor] [interface] [interface neighbor]
  • show ospfv3 [process-id] [area-id] retransmission-list [neighbor] [interface] [interface neighbor]
  • show ospfv3 [process-id] statistic[detail]
  • show ospfv3 [process-id] summary-prefix
  • show ospfv3 [process-id] timers rate-limit
  • show ospfv3 [process-id] traffic[interface-type interface-number]
  • show ospfv3 [process-id] virtual-links

Further Reading

IPv4/v6 address-family

The OSPFv3 address families feature enables both IPv4 and IPv6 unicast traffic to be supported. With this feature, users may have two router processes per interface, but only one process per AF. If the IPv4 AF is used, an IPv4 address must first be configured on the interface, but IPv6 must be enabled on the interface. A single IPv4 or IPv6 OSPFv3 process running multiple instances on the same interface is not supported.


Users with an IPv6 network that uses OSPFv3 as its IGP may want to use the same IGP to help carry and install IPv4 routes. All routers on this network have an IPv6 forwarding stack. Some (or all) of the links on this network may be allowed to do IPv4 forwarding and be configured with IPv4 addresses. Pockets of IPv4-only routers exist around the edges running an IPv4 static or dynamic routing protocol. In this scenario, users need the ability to forward IPv4 traffic between these pockets without tunneling overhead, which means that any IPv4 transit router has both IPv4 and IPv6 forwarding stacks (e.g., is dual stack). This feature allows a separate (possibly incongruent) topology to be constructed for the IPv4 AF. It installs IPv4 routes in IPv4 RIB, and then the forwarding occurs natively. The OSPFv3 process fully supports an IPv4 AF topology and can redistribute routes from and into any other IPv4 routing protocol.

An OSPFv3 process can be configured to be either IPv4 or IPv6. The address-family command is used to determine which AF will run in the OSPFv3 process, and only one address family can be configured per instance however multiple instances of OSPFv3 can be enabled on single interface. Once the AF is selected, users can enable multiple instances on a link and enable address-family-specific commands.


Different instance ID ranges are used for each AF (it is carried inside packet header as instance ID field and that’s unique to IPv6). Each AF establishes different adjacencies, has a different link state database, and computes a different shortest path tree. The AF then installs the routes in AF-specific RIB. LSAs that carry IPv6 unicast prefixes are used without any modification in different instances to carry each AFs’ prefixes.

The IPv4 subnets configured on OSPFv3-enabled interfaces are advertised through intra-area prefix LSAs, just as any IPv6 prefixes. External LSAs are used to advertise IPv4 routes redistributed from any IPv4 routing protocol, including connected and static. The IPv4 OSPFv3 process runs the SPF calculations and finds the shortest path to those IPv4 destinations. These computed routes are then inserted in the IPv4 RIB (computed routes are inserted into an IPv6 RIB for an IPv6 AF).


Because the IPv4 OSPFv3 process allocates a unique pdbindex in the IPv4 RIB, all other IPv4 routing protocols can redistribute routes from it. The parse chain for all protocols is same, so the ospfv3 keyword added to the list of IPv4 routing protocols causes OSPFv3 to appear in the redistribute command from any IPv4 routing protocol. With the ospfv3 keyword, IPv4 OSPFv3 routes can be redistributed into any other IPv4 routing protocol as defined in the redistribute ospfv3 command. In case of using OSPF for IP VPNs as PE-CE protocol, the down bit helps prevent routing loops between MP-BGP and OSPF, but not when external routes are announced, such as when redistribution between multiple OSPF domains or when external routes are injected in an area that is dual-homed to the provider network. The PE router redistributes an OSPF route from a different OSPF domain into an OSPF domain as an external route. The down bit is not set because LSA Type 5 does not support the down bit. The redistributed route is propagated across the OSPF domain.

Implement and troubleshoot network types, area types and router types

Point-to-point, multipoint, broadcast, non-broadcast

The command used to set the network type of an OSPF interface is:

ip ospf network {broadcast | non-broadcast | point-to-multipoint}

Point-to-Point Sub-interfaces

A sub-interface is a logical way of defining an interface. The same physical interface can be split into multiple logical interfaces, with each sub-interface being defined as point-to-point. This was originally created in order to better handle issues caused by split horizon over NBMA and vector-based routing protocols. A point-to-point sub-interface has the properties of any physical point-to-point interface. As far as OSPF is concerned, an adjacency is always formed over a point-to-point sub-interface with no DR or BDR election.


Point-to-Multipoint Interfaces

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. This concept takes the previously discussed point-to-point concept one step further. Administrators do not have to worry about having multiple subnets for each point-to-point link. The cloud is configured as one subnet. This should work well for people who are migrating into the point-to-point concept with no change in IP addressing on the cloud. Also, they would not have to worry about DRs and neighbor statements. OSPF point-to-multipoint works by exchanging additional link-state updates that contain a number of information elements that describe connectivity to the neighboring routers.


Broadcast Interfaces

In case of broadcast, the interface will be logically set to broadcast (using ip ospf network broadcast command) and will behave as if the router were connected to a LAN. DR and BDR election will still be performed so special care should be taken to assure either a full mesh topology or a static selection of the DR based on the interface priority.

LSA types, area type: backbone, normal, transit, stub, NSSA, totally stub

Table 1-3 shows the differences between the types of the OSPF areas.

Area Type Description
Stub No Type 5 AS-external LSA allowed
Totally Stub No Type 3, 4 or 5 LSAs allowed except the default summary route
NSSA No Type 5 AS-external LSAs allowed, but Type 7 LSAs that convert to Type 5 at the NSSA ABR can traverse
NSSA Totally Stub No Type 3, 4 or 5 LSAs except the default summary route, but Type 7 LSAs that convert to Type 5 at the NSSA ABR are allowed

Internal router, ABR, ASBR

OSPF uses flooding to exchange link-state updates between routers. Any change in routing information is flooded to all routers in the network. Areas are introduced to put a boundary on the explosion of link-state updates. Flooding and calculation of the Dijkstra algorithm on a router is limited to changes within an area. All routers within an area have the exact link-state database. Routers that belong to multiple areas and connect these areas to the backbone area are called area border routers (ABR). ABRs must therefore maintain information describing the backbone areas and other attached areas.


An area is interface specific. A router that has all of its interfaces within the same area is called an internal router (IR). A router that has interfaces in multiple areas is called an area border router (ABR). Routers that act as gateways (redistribution)between OSPF and other routing protocols (IGRP, EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called autonomous system boundary router (ASBR). Any router can be an ABR or an ASBR. The P-bit is used in order to tell the NSSA ABR whether to translate type 7 into type 5.

Virtual link

Area 0 has to be at the center of all other areas. In some rare case where it is impossible to have an area physically connected to the backbone, a virtual link is used. The virtual link will provide the disconnected area a logical path to the backbone. The virtual link has to be established between two ABRs that have a common area, with one ABR connected to the backbone.

Packets sent on a virtual link with IPsec must use predetermined source and destination addresses. The first local area address found in the router’s intra-area-prefix LSA for the area is used as the source address. This source address is saved in the area data structure and used when secure sockets are opened, and packets sent over the virtual link. The virtual link will not transition to the point-to-point state until a source address is selected. Also, when the source or destination address changes, the previous secure sockets must be closed, and new secure sockets opened.

Implement and troubleshoot path preference

The order of preference for OSPF routes is:

  • intra-area routes, O
  • inter-area routes, O IA
  • external routes type 1, O E1
  • external routes type 2, O E2


This rule of preference cannot be changed. However, it applies only within a single OSPF process. If a router is running more than one OSPF process, route comparison occurs. With route comparison, the metrics and administrative distances (if they have been changed) of the OSPF processes are compared. Route types are disregarded when routes supplied by two different OSPF processes are compared.

Further Reading

General operations

The most common reasons for OSPF neighborship to not form are:

  • MTU mismatch
  • Area type mismatch
  • Network type mismatch

Further Reading

Graceful shutdown

The Graceful Shutdown for OSPFv3 feature provides the ability to temporarily shut down the OSPFv3 protocol in the least disruptive manner and to notify its neighbors that it is going away. All traffic that has another path through the network will be directed to that alternate path. A graceful shutdown of the OSPFv3 protocol can be initiated using the shutdown command in router configuration mode or in address family configuration mode.

This feature also provides the ability to shut down OSPFv3 on a specific interface. In this case, OSPFv3 will not advertise the interface or form adjacencies over it; however, all of the OSPFv3 interface configuration will be retained. To initiate a graceful shutdown of an interface, use the ipv6 ospf shutdown or the ospfv3 shutdown command in interface configuration mode.

Generic TTL Security Mechanism (GTSM)

OSPF is a link state protocol that requires networking devices to detect topological changes in the network, flood Link State Advertisement (LSA) updates to neighbors, and quickly converge on a new view of the topology. However, during the act of receiving LSAs from neighbors, network attacks can occur, because there are no checks that unicast, or multicast packets are originating from a neighbor that is one hop away or multiple hops away over virtual links.

For virtual links, OSPF packets travel multiple hops across the network; hence, the TTL value can be decremented several times. For these types of links, a minimum TTL value must be allowed and accepted for multiple-hop packets.


To filter network attacks originating from invalid sources traveling over multiple hops, the Generalized TTL Security Mechanism (GTSM), RFC 3682, is used to prevent the attacks. GTSM filters link-local addresses and allows for only one-hop neighbor adjacencies through the configuration of TTL value 255. The TTL value in the IP header is set to 255 when OSPF packets are originated and checked on the received OSPF packets against the default GTSM TTL value 255 or the user configured GTSM TTL value, blocking unauthorized OSPF packets originated from TTL hops away.

Further Reading


OSPF convergence is extremely fast when compared to other protocols. To keep this desirable behavior fully functional in your network, you need to consider the three components that determine how long it takes for OSPF to converge:


  • The length of time it takes OSPF to detect a link or interface failure
  • The length of time it takes the routers to exchange routing information via LSAs, rerun the Shortest Path First algorithm, and build a new routing table
  • A built-in SPF delay time of five seconds (default value)


Thus, the average time for OSPF to propagate LSAs and rerun the SPF algorithm is approximately 1 second. Then the SPF delay timer of five seconds must elapse. Therefor OSPF convergence can be a anything from 6 to 46 seconds, depending upon the type of failure, SPF timer settings, size of the network, and size of the LSA database. The worst-case scenario is when a link fails but the destination is still reachable via an alternate route, because the 40 second default dead timer will need to expire before the SPF is rerun. If OSPF interface costs are auto-calculated based on interface bandwidth then the OSPF reference bandwidth on a router should be at least twice the highest interface bandwidth configured on any of the router’s interfaces. OSPF link cost is an integer value calculated by dividing the reference bandwidth by the interface’s bandwidth value. If interface bandwidth values are large and the reference bandwidth is too small, this calculation will result in interfaces with different bandwidths being assigned a metric of 1. To avoid such issues, you can use auto-cost reference-bandwidth command.

LSA throttling, SPF tuning, fast hello

The OSPF Link-State Advertisement (LSA) Throttling feature provides a dynamic mechanism to slow down link-state advertisement (LSA) updates in OSPF during times of network instability. It also allows faster Open Shortest Path First (OSPF) convergence by providing LSA rate limiting in milliseconds.


The use of SPF throttle timer tuning can aid in improving the convergence of the campus network to within the sub-second threshold but is not sufficient to ensure optimal convergence times. Two factors impact the ability of OSPF to converge: the time waiting for an SPF calculation, and the time waiting for an LSA to be received indicating a network topology change. Before the 12.2S release, Cisco IOS implemented two internal timers affecting the generation of LSAs. The first was an internal delay timer that throttled the generation of router (type-1) and network (type-2) LSAs for 500 msec after a network interface change. A second timer throttled the generation of any specific updated LSA for at least five seconds after having sent the same LSA. These two timers could impact the speed at which the network was able to converge. On the detection of any interface change, OSPF would not generate an LSA indicating the link status change for 500 msec, thus preventing the SPF process from responding to the link failure for at least 500 additional msec. After this occurred, any additional change such as link restoration was throttled for a further five seconds, also potentially impacting recovery. The presence of these delay timers, like the SPF timers, was based on a need to ensure the stability of the network and mitigate against OSPF thrashing in the event of a flapping link or other network problem. The same design and physical factors that allow for SPF tuning in the campus environment also make it amenable to tuning of the LSA timers. The use of routed point-to-point interfaces in the campus removes the need to consider the loss of multiple logical links in the event of a single interface failure (as is the case in a multi-point WAN environment). The use of direct fiber connections between devices also reduces the probability for link loss and ensures a higher degree of accurate link status detection (no LMI or other soft WAN-like failures need to be considered). Interface-specific features such as debounce timers and IP event dampening also lessen the probability of false or flapping interface conditions. The combination of these factors serves to mitigate the factors with which the LSA timers were initially designed to address.


Tuning LSA throttle timers uses an approach similar to that described above for SPF. Three configuration values are used: an initial delay timer, a hold timer, and a maximum hold timer. Using a similar approach to that discussed above results in the use of the same timer values for the LSA configuration as for the SPF configuration.


The recommended values are as follows:

lsa-start: 10 msec

lsa-hold: 100 to 500 msec

lsa-max-wait: 5 seconds


router ospf 100



auto-cost reference-bandwidth 10000

area 120 stub no-summary

timers throttle spf 10 100 5000

timers throttle lsa all 10 100 5000

network area 120


Using this configuration for both LSA and SPF throttle timers produces a further reduction in network convergence from 0.72 to 0.24 seconds. The combination of tuning LSA and SPF timers from their defaults values down to the values shown above provides an overall improvement in convergence time from 5.68 seconds to 0.24 seconds. In tuning the throttle timer controlling the generation of LSAs, it is necessary to make a similar configuration to the throttle timer controlling the receipt of LSAs. The “lsa arrival” timer controls the rate at which a switch accepts a second LSA with the same LSA ID. If distribution switch A is configured to generate LSAs with a hold time of 100 msec, it is necessary for the adjacent switches, such as distribution switch B for example, to be configured to accept LSAs at a rate at least equal to that with which they are generated. It is considered best practice to tune the arrival rate at some value less than the generated rate to accommodate for any buffering or internal process timer scheduling delays. Using a hold time of 100 msec, an LSA arrival value of 80 msec is considered sufficient.


router ospf 100



auto-cost reference-bandwidth 10000

area 120 stub no-summary

timers throttle spf 10 100 5000

timers throttle lsa all 10 100 5000

timers lsa arrival 80

network area 120


The timers throttle lsa all command controls the generation (sending) of LSAs. The first LSA is always generated immediately upon an OSPF topology change, and the next LSA generated is controlled by the minimum start interval. The subsequent LSAs generated for the same LSA are rate-limited until the maximum interval is reached. The “same LSA” is defined as an LSA instance that contains the same LSA ID number, LSA type, and advertising router ID.

The timers lsa arrival command controls the minimum interval for accepting the same LSA. If an instance of the same LSA arrives sooner than the interval that is set, the LSA is dropped. It is recommended that the arrival interval be less than or equal to the hold-time interval of the timers throttle lsa all command.

LSA propagation control (area types, ISPF)

By default, OSPF LSA propagation is controlled by three parameters:

  • OSPF_LSA_DELAY_INTERVAL: Controls the length of time that the router should wait before generating a type 1 router LSA or type 2 network LSA. By default, this parameter is set at 500 ms.
  • MinLSInterval: Defines the minimum time between distinct originations of any particular LSA. The value of MinLSInterval is set to 5 seconds.
  • MinLSArrival: The minimum time that must elapse between reception of new LSA instances during flooding for any particular LSA. LSA instances received at higher frequencies are discarded. The value of MinLSArrival is set to 1 second.

IP FRR/fast reroute (single and multi-hop)

The OSPFv2 Loop-Free Alternate Fast Reroute feature uses a pre-computed alternate next hop to reduce failure reaction time when the primary next hop fails. It lets you configure a per-prefix loop-free alternate (LFA) path that redirects traffic to a next hop other than the primary neighbor. The forwarding decision is made, and service is restored without other routers’ knowledge of the failure.

Further Reading

OSPFv3 prefix suppression

Open Shortest Path First version 3 (OSPFv3) can hide the IPv4 and IPv6 prefixes of connected networks from link-state advertisements (LSAs). When OSPFv3 is deployed in large networks, limiting the number of IPv4 and IPv6 prefixes that are carried in the OSPFv3 LSAs can speed up OSPFv3 convergence. The OSPFv3 prefix suppression feature allows you to hide IPv4 and IPv6 prefixes that are configured on interfaces running OSPFv3.

In OSPFv3, addressing semantics have been removed from the OSPF protocol packets and the main LSA types, leaving a network-protocol-independent core. This means that Router-LSAs and network-LSAs no longer contain network addresses, but simply express topology information. The process of hiding prefixes is simpler in OSPFv3 and suppressed prefixes are simply removed from the intra-area-prefix-LSA. Prefixes are also propagated in OSPFv3 via link LSAs.

The OSPFv3 Prefix Suppression feature provides a number of benefits. The exclusion of certain prefixes from advertisements means that there is more memory available for LSA storage, bandwidth and buffers for LSA flooding, and CPU cycles for origination and flooding of LSAs and for SPF computation. Prefixes are also filtered from link LSAs. A device only filters locally configured prefixes, not prefixes learnt via link LSAs. In addition, security has been improved by reducing the possibility of remote attack with the hiding of transit-only networks.

Further Reading

Describe and optimize IGP scale and performance

Your ability to scale an OSPF internetwork depends on your overall network structure and addressing scheme. As outlined in the preceding discussions concerning network topology and route summarization, adopting a hierarchical addressing environment and a structured address assignment will be the most important factors in determining the scalability of your internetwork.

Network scalability is affected by operational and technical considerations:

  • Operationally, OSPF networks should be designed so that areas do not need to be split to accommodate growth. Address space should be reserved to permit the addition of new areas.
  • Technically, scaling is determined by the utilization of three resources: memory, CPU, and bandwidth.


An OSPF router stores all of the link states for all of the areas that it is in. In addition, it can store summaries and externals. Careful use of summarization and stub areas can reduce memory use substantially.


An OSPF router uses CPU cycles whenever a link-state change occurs. Keeping areas small and using summarization dramatically reduces CPU use and creates a more stable environment for OSPF.


OSPF sends partial updates when a link-state change occurs. The updates are flooded to all routers in the area. In a quiet network, OSPF is a quiet protocol. In a network with substantial topology changes, OSPF minimizes the amount of bandwidth used.