Router link redundancy

corradomapelli · ‎04-06-2016

Hello everyone.

I'm new to the forum, so i'm sorry if this is a frequent topic or an altrady answered one, but i browsed a little and all i found was similar problems but no sulutions.

I'm going to attach an image to help clarify my issue.

So, this is the simplest case i could explain: i have the two VLANS - VLAN 10 and VLAN 20, that must be able to forward data to the Internet.

A router-on-a-stick solution could do the job very well, but i wanted to add this "feature": if a trunk or a switch fails, there should be another link leading to the router and deprting from another device that will be able to forward data.

The part i'm missing is how to implement the solution. Since I know there can't be two interfaces in the same subnet, I imagined I should be able to use a virtual IP address shared by two interfaces on the same router, but I don't know if this is actually possible - actually I don't know if the entire lab is a possible thing but i've seen some images where two switches in the same lan are both connected to a single router.

I'm studying for the CCNA now, but in the FHRP / Router redundancy protocols i haven't found a suitable solution.

Looking forward for your help.

Thank you, Corrado

dukenuk96 · ‎04-06-2016

Hi

if you could use L3 switches instead of L2 for Switch0 and Switch1, you would configure dynamic routing int this triangle network. playing with costs would allow you to load-balance traffic the way you want.

Another option, if your router supports this feature - configure backup interface, it is already discussed here https://supportforums.cisco.com/discussion/11661931/redundant-interface-configuration-cisco-router

Also you can try IP SLA and EEM, which are rather difficult.

I would recommend to configure dynamic routing.

corradomapelli · ‎04-06-2016

Hello and thank you for the quick answer.

Since this is a small laboratory, i don't have access to L3 switches for my purpose.

About my routing options, i was planning to use OSPF area 0, or to set a default static route (0.0.0.0/0 forwarded to S0/0/0), or maybe both.

I will now check the solution on the other discussion. Thank you.

corradomapelli · ‎04-06-2016

Little update from the previous image:

- The "GigabitEthernet" interfaces are actually FastEthernet.

- The "Gi 0/1.20 - Gateway" in the top left router interface description is a mistake - that would obviously be the "Gi 0/0.20 - Gateway"

#########

So, this is what i tried so far: (To save space, only VLAN 10 configuration is shown. Mirror that to VLAN20)

Router(config)# int fa0/0.10

Router(config-subif)# enc dot1q 10

Router(config-subif)# ip add 192.168.10.1 255.255.255.0

Router(config)# int fa0/1.10

Router(config-subif)# enc dot1q 10

Router(config-subif)# ip add 192.168.10.1 255.255.255.0

% 192.168.10.0 overlaps with FastEthernet0/0.10

## as expected, it won't let me do it

Router(config-subif)#standby ip 192.168.1.1

% Warning: address is not within a subnet on this interface

%HSRP-6-STATECHANGE: FastEthernet0/1.10 Grp 0 state Speak -> Standby

%HSRP-6-STATECHANGE: FastEthernet0/1.10 Grp 0 state Standby -> Active

## i'm not sure what happened - can someone please explain?

Router# show run

!

interface FastEthernet0/0

no ip address

duplex auto

speed auto

!

interface FastEthernet0/0.10

encapsulation dot1Q 10

ip address 192.168.10.1 255.255.255.0

!

interface FastEthernet0/0.20

encapsulation dot1Q 20

no ip address

standby version 2

standby 0 ip 192.168.20.1

!

interface FastEthernet0/1

no ip address

duplex auto

speed auto

!

interface FastEthernet0/1.10

encapsulation dot1Q 10

no ip address

standby version 2

standby 0 ip 192.168.10.1

!

interface FastEthernet0/1.20

encapsulation dot1Q 20

ip address 192.168.20.1 255.255.255.0

dukenuk96 · ‎04-06-2016

Router(config-subif)#standby ip 192.168.1.1

% Warning: address is not within a subnet on this interface

%HSRP-6-STATECHANGE: FastEthernet0/1.10 Grp 0 state Speak -> Standby

%HSRP-6-STATECHANGE: FastEthernet0/1.10 Grp 0 state Standby -> Active

## i'm not sure what happened - can someone please explain?

You are trying to configure HSRP on an interface without IP address, so IOS tells you about this in first message. Second and third messages tell about state change in HSRP engine. Now only FastEthernet0/1.10 will play role in HSRP.

But why 192.168.1.1 since in your running config you are using 192.168.10.1?

Anyway, if you are trying to configure HSRP/VRRP/GLBP on one router on different interfaces and place them into the same network - it will not work.

corradomapelli · ‎04-06-2016

I think the 192.168.1.1 was a mistake, or maybe i copied the wrong line. In the "show running-config" you can see it has actually the correct IP.

Thank you for the explaination - i was just wondering if/how that could work.

I'm going to try something else now.

Joseph W. Doherty · ‎04-06-2016

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I'm guessing your two switches are not stackable. If they were, you could Etherchannel between the router and the stack members.

If your router supports IRB (http://www.cisco.com/c/en/us/tech/lan-switching/integrated-routing-bridging-irb/index.html) on its routed ports, you can interconnect your router to two different switches. (If the router supports L2 VLAN ports, you can interconnect using them too.)

corradomapelli · ‎04-06-2016

Thank you Joseph for the answer.

I'm using two Cisco 2950 switches, and a Cisco 1841 router.

The switches are not stackable as you correctly guessed. Still, i like the idea of the etherchannel - as i never ever thought about it.

Unluckily on the router IRB is not supported, and the "switchport" command on interfaces is absent.

Thank you for your support. I really appreciated.

Joseph W. Doherty · ‎04-06-2016

http://www.cisco.com/c/en/us/products/collateral/interfaces-modules/high-speed-wan-interface-cards/product_data_sheet0900aecd8016bf0b.html

corradomapelli · ‎04-06-2016

http://www.cisco.com/c/en/us/products/collateral/interfaces-modules/high-speed-wan-interface-cards/product_data_sheet0900aecd8016bf0b.html

Checked. Thank you. Actually, the module HWIC-ESW is installed.

(If the router supports L2 VLAN ports, you can interconnect using them too.)

How to?

Joseph W. Doherty · ‎04-06-2016

Can you define such ports as trunks and define SVI interfaces?

corradomapelli · ‎04-11-2016

Hello Joseph - Sorry for this late reply but i've been busy recently.

Well, yes, I can define so. I also know well how to route between VLANs, but that's not my goal actually.

I wanted to find a way - as the discussion title says - to do some kind of Gateway redundancy, so that if the IP set as the "default" fails for some reason (link fail, router port shut down, or other reasons) the same device will still be reachable using another link, better if this link departs from another switch.

The Ether Channel is actually an interesting solution - it just misses the final "requirement" of the link departing from Switch2. (I'm sorry if this is/was not clear but english is not my mother language).

Thank you for the support (: if the solution I need is somewhat impossible to configure (even since I saw more than a network tree with two-three switches from the same LAN connected to one single gateway..) please let me know.