Router Descriptor Blocks

iamtheone12345 · ‎01-24-2012

Hi guys,

I am have a network running OSPF in packet tracer.

The network diagram is attached below.

When i use the command show ip route in all 3 routers it gives me routes to networks in my diagram.

However when i try to show ip route <A paritcular ip address on one of the networks> i see something called router descriptor blocks.

What is router descriptor block ?I think i will need a precise explanation for this question as i have looked for documents in google which wasnt much of a help.

can you help guys??

For CBTrouter

=====================

CBTrouter#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 68.110.171.97 to network 0.0.0.0

     68.0.0.0/27 is subnetted, 1 subnets
C       68.110.171.96 is directly connected, Serial1/0
     172.16.0.0/24 is subnetted, 4 subnets
O IA    172.16.0.0 [110/783] via 192.168.1.2, 00:00:23, FastEthernet0/0
O IA    172.16.1.0 [110/783] via 192.168.1.2, 00:00:23, FastEthernet0/0
O IA    172.16.2.0 [110/783] via 192.168.1.2, 00:00:23, FastEthernet0/0
O IA    172.16.8.0 [110/846] via 192.168.1.2, 00:00:23, FastEthernet0/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
O    192.168.2.0/24 [110/782] via 192.168.1.2, 00:00:23, FastEthernet0/0
O    192.168.3.0/24 [110/783] via 192.168.1.2, 00:00:23, FastEthernet0/0
S*   0.0.0.0/0 [1/0] via 68.110.171.97

CBTrouter#show ip route 192.168.1.2

Routing entry for 192.168.1.0/24
Known via "connected", distance 0, metric 0 (connected, via interface)
Routing Descriptor Blocks:
* directly connected, via FastEthernet0/0
Route metric is 0, traffic share count is 1

CBTrouter#show ip route 192.168.2.1
Routing entry for 192.168.2.0/24
Known via "ospf 1", distance 110, metric 782, type intra area
Last update from 192.168.1.2 on FastEthernet0/0, 00:00:40 ago
Routing Descriptor Blocks:
* 192.168.1.2, from 192.168.2.1, 00:00:40 ago, via FastEthernet0/0
Route metric is 782, traffic share count is 1

CBTrouter#show ip route 192.168.3.1
Routing entry for 192.168.3.0/24
Known via "ospf 1", distance 110, metric 783, type intra area
Last update from 192.168.1.2 on FastEthernet0/0, 00:00:44 ago
Routing Descriptor Blocks:
* 192.168.1.2, from 192.168.3.1, 00:00:44 ago, via FastEthernet0/0
Route metric is 783, traffic share count is 1

CBTrouter#show ip route 172.16.0.1

Routing entry for 172.16.0.0/24

Known via "ospf 1", distance 110, metric 783, type intra area

Last update from 192.168.1.2 on FastEthernet0/0, 00:05:34 ago

Routing Descriptor Blocks:

* 192.168.1.2, from 0.0.0.0, 00:05:34 ago, via FastEthernet0/0

Route metric is 783, traffic share count is 1

FOR R1

==============

R1#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 192.168.1.1 to network 0.0.0.0

     172.16.0.0/24 is subnetted, 4 subnets
O IA    172.16.0.0 [110/782] via 192.168.2.2, 00:07:14, Serial1/0
O IA    172.16.1.0 [110/782] via 192.168.2.2, 00:07:14, Serial1/0
O IA    172.16.2.0 [110/782] via 192.168.2.2, 00:07:14, Serial1/0
O IA    172.16.8.0 [110/845] via 192.168.2.2, 00:07:14, Serial1/0
C    192.168.1.0/24 is directly connected, FastEthernet0/0
C    192.168.2.0/24 is directly connected, Serial1/0
O    192.168.3.0/24 [110/782] via 192.168.2.2, 00:07:24, Serial1/0
S*   0.0.0.0/0 [1/0] via 192.168.1.1

R1#show ip route 192.168.1.1

Routing entry for 192.168.1.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Routing Descriptor Blocks:

* directly connected, via FastEthernet0/0

Route metric is 0, traffic share count is 1

R1#show ip route 192.168.2.2

Routing entry for 192.168.2.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Routing Descriptor Blocks:

* directly connected, via Serial1/0

Route metric is 0, traffic share count is 1

R1#show ip route 192.168.3.1

Routing entry for 192.168.3.0/24

Known via "ospf 1", distance 110, metric 782, type intra area

Last update from 192.168.2.2 on Serial1/0, 00:09:01 ago

Routing Descriptor Blocks:

* 192.168.2.2, from 192.168.3.1, 00:09:01 ago, via Serial1/0

Route metric is 782, traffic share count is 1

R1#show ip route 172.16.0.1

Routing entry for 172.16.0.0/24

Known via "ospf 1", distance 110, metric 782, type intra area

Last update from 192.168.2.2 on Serial1/0, 00:09:09 ago

Routing Descriptor Blocks:

* 192.168.2.2, from 0.0.0.0, 00:09:09 ago, via Serial1/0

Route metric is 782, traffic share count is 1

FOR R2

===============

R2#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 192.168.2.1 to network 0.0.0.0

     172.16.0.0/24 is subnetted, 4 subnets
C       172.16.0.0 is directly connected, FastEthernet0/1
C       172.16.1.0 is directly connected, FastEthernet1/0
C       172.16.2.0 is directly connected, FastEthernet1/1
C       172.16.8.0 is directly connected, Serial1/1/1
O    192.168.1.0/24 [110/65] via 192.168.2.1, 00:17:56, Serial1/1/0
C    192.168.2.0/24 is directly connected, Serial1/1/0
C    192.168.3.0/24 is directly connected, FastEthernet0/0
S*   0.0.0.0/0 [1/0] via 192.168.2.1

R2#show ip route 192.168.1.1

Routing entry for 192.168.1.0/24

Known via "ospf 1", distance 110, metric 65, type intra area

Last update from 192.168.2.1 on Serial1/1/0, 00:11:19 ago

Routing Descriptor Blocks:

* 192.168.2.1, from 192.168.2.1, 00:11:19 ago, via Serial1/1/0

Route metric is 65, traffic share count is 1

R2#show ip route 192.168.2.1

Routing entry for 192.168.2.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Routing Descriptor Blocks:

* directly connected, via Serial1/1/0

Route metric is 0, traffic share count is 1

R2#show ip route 192.168.3.1

Routing entry for 192.168.3.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Routing Descriptor Blocks:

* directly connected, via FastEthernet0/0

Route metric is 0, traffic share count is 1

R2#show ip route 172.16.0.1

Routing entry for 172.16.0.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Routing Descriptor Blocks:

* directly connected, via FastEthernet0/1

Route metric is 0, traffic share count is 1

Richard Burts · ‎01-28-2012

It is a fairly simple issue. The Routing Descriptor Block is a Block (several lines) of information that Describe (how was it learned, what is the connected interface or next hop, what is the metric, what is the traffic share count) for this Route.

HTH

Rick

HTH

Rick

iamtheone12345 · ‎01-28-2012

Rick,

I did think it was simple.But what baffles me is the output.It doesnot tally with what i know about OSPF.

Like for example:

CBTrouter#show ip route 172.16.0.1

Routing entry for 172.16.0.0/24

Known via "ospf 1", distance 110, metric 783, type intra area

Last update from 192.168.1.2 on FastEthernet0/0, 00:05:34 ago

Routing Descriptor Blocks:

* 192.168.1.2, from 0.0.0.0, 00:05:34 ago, via FastEthernet0/0********************what is this ?

Route metric is 783, traffic share count is 1

Can you explain to me what does 0.0.0.0 signify ?You can say its static route that i configured on R2.

But why is it showing for only 172.16.0.0/24 network only and not 192.168.3.0/24 network and 192.168.2.0 network.

Because i have configured static route in all 3 routers.

Please have a look at the [acket tracer diagram that i attached.

Richard Burts · ‎01-28-2012

There are several things that do not seem to be normal and I wonder if there is some thing that packet tracer is doing not quite right. You suggest that it might be a static route, but R2 clearly shows that 172.16.0.0 is a connected interface.

Also network 172.16.0.0 shows up in the routing table of R1 (and CBTrouter) as an inter area route

O IA 172.16.0.0 [110/782] via 192.168.2.2, 00:07:14, Serial1/0

but the routing descriptor identifies it as intra area.

My first guess is that the 0.0.0.0 in the Routing Descriptor Block was related to the fact that it was an inter area route. But given uncertainty about what packet tracer is doing I am not sure about that.

HTH

Rick

HTH

Rick