ospf_manual.pdf

OSPF

Document revision 1.3 (Mon Sep 06 04:56:42 GMT 2004)

This document applies to MikroTik RouterOS V2.8

Table of Contents

Summary

Specifications

Related Documents

•

Package Management

•

IP Addresses and ARP

•

Routes, Equal Cost Multipath Routing, Policy Routing

•

Log Management

Description

Open Shortest Path First protocol is a link-state routing protocol. It's uses a link-state algorithm to

build and calculate the shortest path to all known destinations. The shortest path is calculated using

the Dijkstra algorithm. OSPF distributes routing information between the routers belonging to a

single autonomous system (AS). An AS is a group of routers exchanging routing information via a

common routing protocol.

In order to deploy the OSPF all routers it will be running on should be configured in a coordinated

manner (note that it also means that the routers should have the same MTU for all the networks

advertized by OSPF protocol).

The OSPF protocol is started after you will add a record to the OSPF network list. The routes

learned by the OSPF protocol are installed in the routes table list with the distance of 110.

General Setup

Home menu level: /routing ospf

Description

In this section you will learn how to configure basic OSPF settings.

Property Description

distribute-default ( never | if-installed-as-type-1 | if-installed-as-type-2 | always-as-type-1 |

always-as-type-2 ; default: never ) - specifies how to distribute default route. Should be used for

ABR (Area Border router) or ASBR (Autonomous System boundary router) settings

• never - do not send own default route to other routers

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• if-installed-as-type-1 - send the default route with type 1 metric only if it has been installed (a

static default route, or route added by DHCP, PPP, etc.)

• if-installed-as-type-2 - send the default route with type 2 metric only if it has been installed (a

static default route, or route added by DHCP, PPP, etc.)

• always-as-type-1 - always send the default route with type 1 metric

• always-as-type-2 - always send the default route with type 2 metric

metric-bgp ( integer ; default: 20 ) - specifies the cost of the routes learned from BGP protocol

metric-connected ( integer ; default: 20 ) - specifies the cost of the routes to directly connected

networks

metric-default ( integer ; default: 1 ) - specifies the cost of the default route

metric-rip ( integer ; default: 20 ) - specifies the cost of the routes learned from RIP protocol

metric-static ( integer ; default: 20 ) - specifies the cost of the static routes

redistribute-bgp ( as-type-1 | as-type-2 | no ; default: no ) - with this setting enabled the router will

redistribute the information about all routes learned by the BGP protocol

redistribute-connected ( as-type-1 | as-type-2 | no ; default: no ) - if set, the router will redistribute

the information about all connected routes, i.e., routes to directly reachable networks

redistribute-rip ( as-type-1 | as-type-2 | no ; default: no ) - with this setting enabled the router will

redistribute the information about all routes learned by the RIP protocol

redistribute-static ( as-type-1 | as-type-2 | no ; default: no ) - if set, the router will redistribute the

information about all static routes added to its routing database, i.e., routes that have been created

using the /ip route add command

router-id ( IP address ; default: 0.0.0.0 ) - OSPF Router ID. If not specified, OSPF uses the largest

IP address configured on the interfaces as its router ID

Notes

Within one area, only the router that is connected to another area (i.e. Area border router) or to

another AS (i.e. Autonomous System boundary router) should have the propagation of the default

route enabled.

OSPF protocol will try to use the shortest path (path with the smallest total cost) if available.

OSPF protocol supports two types of metrics:

• type1 - external metrics are expressed in the same units as OSPF interface cost. In other words

the router expects the cost of a link to a network which is external to AS to be the same order of

magnitude as the cost of the internal links.

• type2 - external metrics are an order of magnitude larger; any type2 metric is considered

greater than the cost of any path internal to the AS. Use of type2 external metric assumes that

routing between AS is the major cost of routing a packet, and climinates the need conversion of

external costs to internal link state metrics.

Both Type 1 and Type 2 external metrics can be used in the AS at the same time. In that event,

Type 1 external metrics always take precedence.

In /ip route you can see routes with Io status. Because router receives routers from itself.

The metric cost can be calculated from line speed by using the formula 10e+8/line speed. The table

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contains some examples:

network type

cost

ethernet

64kb/s

1562

Example

To enable the OSPF protocol redisrtibute routes to the connected networks as type1 metrics with

the cost of 1 , you need do the following:

[admin@MikroTik] routing ospf> set redistribute-connected=as-type-1 \

\... metric-connected=1

[admin@MikroTik] routing ospf> print

router-id: 0.0.0.0

distribute-default: never

redistribute-connected: as-type-1

redistribute-static: no

redistribute-rip: no

redistribute-bgp: no

metric-default: 1

metric-connected: 1

metric-static: 20

metric-rip: 20

metric-bgp: 20

[admin@MikroTik] routing ospf>

Areas

Home menu level: /routing ospf area

Description

OSPF allows collections of routers to be grouped together. Such group is called an area. Each area

runs a separate copy of the basic link-state routing algorithm. This means that each area has its own

link-state database and corresponding graph

The structure of an area is invisible from the outside of the area. This isolation of knowledge

enables the protocol to effect a marked reduction in routing traffic as compared to treating the entire

Autonomous System as a single link-state domain

60-80 routers have to be the maximum in one area

Property Description

area-id ( IP address ; default: 0.0.0.0 ) - OSPF area identifier. Default area-id=0.0.0.0 is the

backbone area. The OSPF backbone always contains all area border routers. The backbone is

responsible for distributing routing information between non-backbone areas. The backbone must

be contiguous. However, areas do not need to be physical connected to backbone. It can be done

with virtual link. The name and area-id for this area can not be changed

authetication ( none | simple | md5 ; default: none ) - specifies authentication method for OSPF

protocol messages

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• none - do not use authentication

• simple - plain text authentication

• md5 - keyed Message Digest 5 authentication

default-cost ( integer ; default: 1 ) - specifies the default cost used for stub areas. Applicable only to

area boundary routers

name ( name ; default: "" ) - OSPF area's name

stub (yes | no; default: no ) - a stub area is an area which is out from part with no routers or areas

beyond it. A stub area is configured to avoid AS External Link Advertisements being flooded into

the Stub area. One of the reason to configure a Stub area is that the size of the link state database is

reduced along with the routing table and less CPU cycles are used to process. Any router which is

trying access to a network outside the area sends the packets to the default route

Example

To define additional OSPF area named local_10 with area-id=0.0.10.5 , do the following:

[admin@WiFi] routing ospf area> add area-id=0.0.10.5 name=local_10

[admin@WiFi] routing ospf area> print

Flags: X - disabled, I - invalid

# NAME

AREA-ID

STUB DEFAULT-COST AUTHENTICATION

0 backbone

0.0.0.0

none

1 local_10

0.0.10.5

no 1

none

[admin@WiFi] routing ospf area>

Networks

Home menu level: /routing ospf network

Description

There can be Point-to-Point networks or Multi-Access networks. Multi-Access network can be a

broadcast network (a single message can be sent to all routers)

To start the OSPF protocol, you have to define the networks on which it will run and the area ID for

each of those networks

Property Description

area ( name ; default: backbone ) - the OSPF area to be associated with the specified address range

network ( IP address/mask ; default: 20 ) - the network associated with the area. The network

argument allows defining one or multiple interfaces to be associated with a specific OSPF area.

Only directly connected networks of the router may be specified

Notes

You should set the network address exactly the same as the remote point IP address for

point-to-point links. The right netmask in this case is /32 .

Example

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