hsrp and correct routing path

[fabio.marino]

Hi All,

I have a question based on the following topology.

In brief, router A and B can reach the network A via static route

A#show ip route | i 2.2.2.0
S       2.2.2.0/24 [1/0] via 1.1.1.1

B#show ip route | i 2.2.2.0
S       2.2.2.0/24 [1/0] via 1.1.1.1

where 1.1.1.1 is the virtual hsrp ip configured between router C and D.

My question is: as soon the router B will receive a packet to be routed towards the network 2.2.2.0 which is the path that will follow ?

Option 1 : Router B -> Router A -> Router C -> network 2.2.2.2
Option 2 : Router B -> Router D -> Router C -> network 2.2.2.2

Why this question? I am wondering what will happen in case I will shutdown the link between A and C (of course assuming that I track this interface in order to have then Router D as active.)

Note:
I tried to follow the virtual mac address 0000.0c9f.f0ae but the mac address table does not show any entry on Router A and B.
Assume that the network 1.1.1.0/29 is not redistribuited, for example in ospf, so router A does not have any entry for the 1.1.1.0/29 in the ospf database.

Thanks.

[Richard Burts]

You have showed us that A and B have a route to 2.2.2.0 with 1.1.1.1 as the next hop. To answer your question about how B will forward you also need to show us what is in the routing table for A and B for 1.1.1.1. Without knowing this information we do not have enough information to answer your question.

HTH

Rick

[fabio.marino]

Hi Richard,

Thanks for your time.

Here

A#show ip route 1.1.1.1
Routing entry for 1.1.1.0/29
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via GigabitEthernet1/1.101
      Route metric is 0, traffic share count is 1

B#show ip route 1.1.1.1
Routing entry for 1.1.1.0/29
  Known via "connected", distance 0, metric 0 (connected, via interface)
  Routing Descriptor Blocks:
  * directly connected, via FastEthernet1/1.101
      Route metric is 0, traffic share count is 1

In any case I think the following: 

• As soon as the link between Router A and Router C will be down, we have that Router A does not know how to reach the next hop 1.1.1.1 (virtual hsrp ip between C and D)
• Router A then will delete the  following static route and in a few words will not be able to reach the 2.2.2.0/24 anymore. Do you agree? 

 A#show ip route | i 2.2.2.0

S       2.2.2.0/24 [1/0] via 1.1.1.1

• ​Last point even if I create an ospf process between all routers in order to force router A to redistribute the static route, as soon as the next-hop 1.1.1.1 is not longer reacheable (due to the shutdown of the interface) again the static route will disappear from the routing table of router A and then will not be redistributed.
• The only solution I think will be to create an ospf process between the routers in order to redistribute the 1.1.1.0/29. In this condition Router A will have an alternative path to reach the network 2.2.2.0/24
• In a few words I think that a topology like this is not not redundant. But again, this is my though so happy to read here comments on this topic.

Thanks!!!

[Richard Burts]

There are inconsistencies between what your drawing shows and what is in the content of your show commands. The drawing shows that A is connected to B and to C and has no connection to D. And that B is connected to a and to D and has no connection to B. But in the show commands both A and B show 1.1.1.1 as a connected route which would seem to indicate that they are connected in a switched/vlan connection. Which means that A really does have connectivity to D and that B does have connectivity to C. If that is the case then it is a quite different situation from what you seem to be presenting in your question. You need to give us a clear explanation of your environment and how the connections between these devices are made. Only when we clearly understand what we are dealing with will we be able to give you intelligent answers to your questions.

HTH

Rick

[fabio.marino]

Hi Rick,

Connection is the one already posted.

Please refer to the update topology where you can see the ip addresses of all interfaces now.

Thanks.

[Richard Burts]

The updated topology does have the IP addresses and that is at least somewhat helpful. It does make it clear that all 4 routers are in subnet 1.1.1.0. And that implies that each router does have connectivity to the other 3 routers. Is that the case?

Perhaps another way of clarifying the question is that when devices are in a common subnet there is an assumption that they are in a common broadcast domain. Your diagram (and perhaps your question) suggests that they are not in a common broadcast domain (all in a common vlan). Can you clarify the layer 2 relationship between these routers?

HSRP is designed to provide redundancy when you are coming from outside the subnet to a destination in the subnet. Since all 4 routers are already in the subnet I am not sure what HSRP does for you.

HTH

Rick

[fabio.marino]

Hi Richard,

What you see is the current connection of the devices. The HSRP here is used for redundancy purpose in order to allow router A and B to reach the networks connected behind C and D.

No swicthes between A,B,C,D.

Thanks.

[Richard Burts]

Am I correct in understanding that it is an Ethernet cable between A and C and an Ethernet cable between B and D? If that is the case then HSRP does not work.

HTH

Rick

[fabio.marino]

Hi Richard,

I did not get you.

The cable is an Ethernet cable yes and the above configuration is working.

In any case I was able to trace the virtual mac address and I can see that

as soon the router B will receive a packet to be routed towards the network 2.2.2.0  the path that will follow  is the

Option 2 : Router B -> Router D -> Router C -> network 2.2.2.2

I configured hsrp version 1 but I was looking for the mac address (0000.0C9F.FXXX) that is valid for version2!!! My bad.

I wait for your feedback Richard.