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I would like to use monitoring software installed in an Ubuntu VM to manage the network emulated into GNS3, which is also installed in the Ubuntu VM.

Interfaces in my VM are:

docker0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255
        ether 02:42:01:73:bd:8e  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

ens33: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.221.143  netmask 255.255.255.0  broadcast 192.168.221.255
        inet6 fe80::5f6a:96d3:65b9:4fe0  prefixlen 64  scopeid 0x20<link>
        ether 00:0c:29:96:7f:80  txqueuelen 1000  (Ethernet)
        RX packets 408  bytes 440051 (440.0 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 303  bytes 35261 (35.2 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 166  bytes 16050 (16.0 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 166  bytes 16050 (16.0 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

virbr0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 192.168.122.1  netmask 255.255.255.0  broadcast 192.168.122.255
        ether 52:54:00:f8:73:7f  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

The network in GNS3 has two routers with this topology:

enter image description here

I am using the virtual bridge interface, virbr0:192.168.122.1, to connect the two routers R1 and R2 with the VM Ubuntu. The cloud in the topology represents the connection with the Ubuntu VM host.

I am using the OSPF routing protocol in R1 and R2.

The routing table of R1:

R1#sh ip route
Codes: C - connected, S - static, 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
       i - IS-IS, su - IS-IS summary, 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 192.168.122.1 to network 0.0.0.0

C    192.168.122.0/24 is directly connected, GigabitEthernet0/0
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.0.0.2/32 [110/2] via 10.1.1.2, 00:11:04, GigabitEthernet1/0
C       10.1.1.0/24 is directly connected, GigabitEthernet1/0
C       10.0.0.1/32 is directly connected, Loopback0
S*   0.0.0.0/0 [1/0] via 192.168.122.1

The routing table of R2:

R2#sh ip route
Codes: C - connected, S - static, 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
       i - IS-IS, su - IS-IS summary, 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 10.1.1.1 to network 0.0.0.0

S    192.168.122.0/24 [1/0] via 10.1.1.1
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C       10.0.0.2/32 is directly connected, Loopback0
C       10.1.1.0/24 is directly connected, GigabitEthernet0/0
O       10.0.0.1/32 [110/2] via 10.1.1.1, 00:32:36, GigabitEthernet0/0
O*E2 0.0.0.0/0 [110/1] via 10.1.1.1, 00:32:36, GigabitEthernet0/0

Communication between R1 and R2 is working. For example, they can PING each other's loopback interfaces.

R1 can PING to the virbr0 interface, 192.168.122.1. R2 can PING to g0/0 of R1 with IP 192.168.122.2, but it can not PING to virbr0 IP 192.168.122.1.

In the Ubuntu VM host, the routes are:

$ ip route
default via 192.168.221.2 dev ens33 proto dhcp metric 100 
169.254.0.0/16 dev ens33 scope link metric 1000 
172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown 
192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1 
192.168.221.0/24 dev ens33 proto kernel scope link src 192.168.221.143 metric 100

From the Ubuntu VM I can PING loopback interfaces of R1 and R2 but only if I set the output interface:

$ ping -I virbr0 10.0.0.2
PING 10.0.0.2 (10.0.0.2) from 192.168.122.1 virbr0: 56(84) bytes of data.
64 bytes from 10.0.0.2: icmp_seq=1 ttl=254 time=50.6 ms
64 bytes from 10.0.0.2: icmp_seq=2 ttl=254 time=29.2 ms

It looks like the Ubuntu VM receives the PING requests from router R2 (Lo: 10.0.0.2) but it does not know which interface to use to return the PING replies.

Besides, the PING requests from the Ubuntu VM also have the same problem. I have to indicate the interface to use un the PING requests.

I can add a static route through virbr0:

IP route add 10.0.0.0/24 via 192.168.122.1 dev virbr0

But this is a solution that can only work in this particular network and not in a general scenario with different IP assignation. Also, with this solution in this particular network, I would have to PING from R2 indicating the source IP 10.0.0.2, which is not desired.

I would like to integrate the best possible Ubuntu VM in the GNS3 network so that the VM could behave as directly connected to interface g0/0 of R1.

I do not know if it is possible to use the OSPF routing protocol also in the Ubuntu VM, and it will solve the communication problem between the VM and the routers.

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1 Answer 1

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As you mention, this:

From the Ubuntu VM I can PING loopback interfaces of R1 and R2 but only if I set the output interface

is true because:

it (VM) does not know which interface to use to return the PING replies.

because there is no specific route for 10.x.x.x IPs on the VM, so it sends these packets to the default gateway "default via 192.168.221.2 dev ens33"

A route should exist in the VM so that it knows where to send the packages for 10.x.x.x and that is why everything works when you add:

IP route add 10.0.0.0/24 via 192.168.122.1 dev virbr0

You could also add a "wider" range on this route like 10.0.0.0/16 if this helps to overcome the "particular network" issue that you mention (and assuming that the whole 10.x.x.x network is not used anywhere else)

To avoid adding routes manually I can think of 2 possible solutions (I have not done any of them so I cannot provide detailed info, but I am mentioning them in case they help you)

  • You could try as you mention to enable OSPF also in the Linux VM maybe using Quagga. Some info about this are here

  • On the DHCP service enabled on g0/0 of R1 you could add an option (33 or 121) to send a route to the DHCP clients (IF this ability is available in the DHCP server)

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