GNS3 - VitualBox Part 5: Creating OSPF Virtual Links

Adding a Cisco 3745 WAN router connected to five Cisco 2691 branch office routers by point-to-point T-1.  This article starts using the topology configured in Part 4 of the series.

Introduction

The illustration above depicts what will be configured:  a Cisco 3745 router equipped with a Fast Ethernet adapter and six T-1 adapters connected by point-to-point T-1 to five Cisco 2691 routers equipped with a Fast Ethernet adapter and one T-1 adapter.  The Cisco 3745 router is located in the Philadelphia Data Center and its Fast Ethernet adapter is in OSPF Area 10.64.0.0; its T-1 adapters are in the new OSPF Area 10.72.0.0.  The five Cisco 2691 routers' T-1 and Fast Ethernet interfaces are all in OSPF Area 10.72.0.0.

The 10.72.0.0 - 10.72.255.255 address range is reserved for T-1 point-to-point links (e.g. networks 10.72.0.0/255.255.255.252, 10.72.0.4/255.255.255.252, 10.72.0.8/255.255.255.252, etc.).  Each branch office router's Fast Ethernet interface services an address range (e.g. 10.73.0.0/255.255.0.0, 10.74.0.0/255.255.0.0, 10.75.0.0/255.255.0.0, etc.).

Role of the OSPF Backbone Area

The OSPF Backbone Area 0.0.0.0 is the logical core of and OSPF Domain; all other area must be connected to it.  The Backbone Area is responsible for distributing routing information between non-Backbone Areas (i.e. Areas with non-zero numbers).  The Backbone Area must also be logically contiguous; it may be physically discontiguous if connected by virtual links.

Backbone Routers

Backbone Routers (BRs) have at least one interface connected to OSPF Area 0.0.0.0.  In the design above, the four Debian Linux Gateway Routers and four Cisco 7206 Routers have interfaces in Area 0.0.0.0 and are Backbone Routers.

Area Border Routers

Area Border Routers (ABRs) have interfaces in more than one OSPF Area.  In the design above, the four Debian Linux Gateway Routers and four Cisco 7206 Routers, and the new Cisco 3745 WAN Router are ABRs

Internal Routers

Internal Routers (IRs) have interfaces connected to the same OSPF Area.  In the design above, the Debian Linux Layer 3 Switches, Philadelphia Data Center Cisco 3745 and five new Cisco 2691 Branch Office Routers are IRs.

Autonomous System Border Routers

Autonomous System Border Routers (ASBRs) have at least one interface connected to systems outside of the OSPF Domain.  These typically run an exterior routing protocol (such as Border Gateway Protocol, BGP) or have static routes.  In the design above, the four Debian Linux Gateway Routers (with DHCP addresses and a static default route assigned by the Wireless Router) are ASBRs.

Physical and Logical OSPF Area Topology

The OSPF Backbone Area 0.0.0.0 is physically and logically contiguous; all routers with interfaces in the Backbone Area 0.0.0.0 have at least one physical connection to another router in Backbone Area 0.0.0.0.

Area 10.64.0.0 (range 10.64.0.0/255.248.0.0) is connected to OSPF Backbone Area 0.0.0.0 by the Backbone BR/ABR PHL_7206.  Thus, Area 10.64.0.0 receives updates about all other non-Backbone Areas directly connected to the Backbone (i.e. 10.0.0.0, 10.128.0.0 and 10.192.0.0).  Consider the example of an Area 10.64.0.0 IR such as the Philadelphia Data Center 3745.  This router receives route summaries that define networks and gateways for the listed networks:

10.0.0.0/13 via 10.64.0.1 FastEthernet0/0
10.128.0.0/13 via 10.64.0.1 FastEthernet0/0 
10.192.0.0/13 via 10.64.0.1 FastEthernet0/0

These routing table entries specify the summarized networks, default gateway (10.64.0.1 on PHL_7206) and local interface to use (FastEthernet0/0).

Area 10.72.0.0 is not contiguous with the OSPF Backbone Area 0.0.0.0.  It consists of an ABR -- PHL_WAN_3745 -- connected to Areas 10.64.0.0 and 10.72.0.0 and five IRs -- the Cisco 2691 Branch Office routers -- connected only to Area 10.72.0.0.  ABR PHL_WAN_3745, by virtue of membership in Area 10.64.0.0, will receive updates from the Backbone Area from PHL_7206.  The five Cisco 2691 Branch Office routers will not and are only aware of Area 10.72.0.0 routes; they also lack default gateway definitions and can not route outside of Area 10.72.0.0.

OSPF Virtual Links

An OSPF Virtual Link is a logical link to OSPF Backbone Area 0.0.0.0.  The discontiguous Area (in this case 10.72.0.0) must use a Transit Area (in this case 10.64.0.0) to establish a connection with the OSPF Backbone Area.  A Virtual Link must provide a connection between a BR (PHL_7206) and the discontiguous Area's ABR (PHL_WAN_3745).  The Transit Area (in this case 10.64.0.0) may not be in any way stubby because it must pass all OSPF LSA types.  Stub Areas will be the subject of Part 6 in the series.

The process is quite simple.  The format of the Router OSPF command is:

area <transit area number a.b.c.d> virtual-link <destination a.b.c.d IP address>

On the Backbone Router PHL_7206:

area 10.64.0.0 virtual-link 10.64.0.3

On the Area Border Router PHL_WAN_3745:

area 10.64.0.0 virtual-link 10.64.0.1

The status of the Virtual Link may be checked with the command:

show ip ospf virtual-links

on either the PHL_7206 or PHL_WAN_3745 routers.

The Cisco 2691 Branch Office routers will now receive updates from Backbone Area 0.0.0.0, including Inter Area route summaries (e.g. 10.0.0.0/13, 10.128.0.0/13 and 10.192.0.0/13).  They will also have default gateways assigned (the corresponding PHL_WAN_3745 serial T-1 interface).  One additional check is the command:

show ip ospf broder-routers

This command will return the Area 10.72.0.0 ABR (PHL_WAN_3745) and the OSPF Domain's ASBRs (the Debian Linux Gateway Routers).

The video below illustrates adding the six routers for Area 10.72.0.0, configuring OSPF routing and adding the Virtual Link between PHL_7206 and PHL_WAN_3745.