OSPF ABR and ASBR: Differences Insight

As we have already discussed about difference between switch and router hope you understood the meaning and the difference between them. Today we are going to discuss about difference between the two main Routers i.e. Area border router (ABR) and Autonomous system border router (ASBR).

Area border router (ABR)

Area border router (ABR): As we already know a router connects one or more areas to the OSPF backbone. When a router is located near the border between one or more Open Shortest Path First (OSPF) areas, such type of router is known as Area border router (ABR).

It stores and operate different routing information related to the topology of the area to which it is attached or to the backbone, as ABR is the member of both the particular areas it is connected and to the main backbone. To make a connection between the OSPF areas and backbone networks, ABR is used. You can define the area ID as a dotted decimal number (i.e., 0.0.0.0) or a standard number (i.e., 0).

To show the path to all networks the ABR will only send one router i.e. the default gateway route outside the area. ABR don’t send full route updates to other routers which will be time taking and complex process.

Given below is an example to check whether a router is area border router or not Command: show ip protocols

Ex. As it is clear from the above diagram Router R2 is ABR

R2(config)#do show ip protocols

***IP Routing is NSF aware***

Routing Protocol is “OSPF”

Outing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 2.2.2.2

It is an area border router

Number of areas in this router is 2.2 normal 0 stub 0 nssa

Maximum path: 4

Routing for Network:

12.0.0.0 0.0.0.255 area 0

23.0.0.0 0.0.0.255 area 1

Routing Information Sources:

Gateway           Distance           Last update

1.1.1.1                110                   00:01:30

Distance: (default is 110)

Autonomous system border router (ASBR)

Autonomous system border router (ASBR): A router which is usually connected through an exterior routing protocol like BGP to one or more logical entities (AS).

The major work of ABRs is to brief sub networks found throughout the OSPF system. It stores many copies of its link-state database and provide the path to the router.

ASBR is a router that is attached to other OSPF areas, followed by other routing protocols like IS-IS, IGRP, BGP,EIGRP, RIP, Static.

Given below is an example to check whether a router is area border router or not Command: show ip protocols

Ex. As it is clear from the above diagram Router R3 is ASBR

R3(config)#router ospf 3

R3(config-router)#redistribute eigrp 1 subnets

R3(config-router)#ex

R3(config)#do show ip protocols

***IP Routing is NSF aware***

Routing Protocol is “ospf 3”

Outing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 3.3.3.3

It is an autonomous system boundary router

Redistributing External Routers from,

eigrp 1, includes subnets in redistribution

Number of areas in this router is 1.1 normal 0 stub 0 nssa

Maximum path: 4

Routing for Network:

23.0.0.0 0.0.0.255 area 1

Routing Information Sources:

Gateway           Distance           Last update

2.2.2.2                110                   00:09:52

Distance: (default is 110)