floating static routes

Answered Question
Mar 3rd, 2008
User Badges:

Hi,


I have two link between two routers one at site A and other at site B and I want to use link1 as primary and link 2 as secondary. I have configured two default route with the different AD at my site B so my link 1 will become primary and when it went down link 2 should come up in routing table. but when I shutdown the one of the interface manually on site A router then in site A's routing table i do have back up route in IP routing table but I didnt find the secondary route in routing table of Router B and the interface is also showing up up on router B, why?


so how can i have my link 2 be in active state? and why the secondary route come up in routing table of the router on which i did shutdown and not come up in routing table of other end? what is solution for that?

Correct Answer by Richard Burts about 9 years 2 months ago

Dan


Those conditions might include:

- if someone had previously configured cost on the interfaces that was different from what we want.

- if the switches calculate port cost differently - perhaps based on different models of switch or different versions of software.

- and finally and most importantly: when I am talking about something and the things I do not know about it are much larger than the things that I do know about it, then I always leave some wiggle room to accommodate unexpected results. Probably not likely to happen but I leave some room for it.


HTH


Rick

Correct Answer by Richard Burts about 9 years 2 months ago

Dan


You bring up a subtle and interesting question about how it works if the port on one switch is in VLAN 3 and the port it connects to on the other switch is in VLAN 1. In fact VLANs and VLAN IDs do not come into play until we have trunk ports or have some layer 3 device doing intervlan routing (layer 3 switch or router).


An access port is a member of a VLAN but that does not impact anything or change anything on the frames that it sends out its port or the frames that it receives on its port. These are just regular Ethernet frames.


If you think about it, we frequently define a VLAN by saying that it is a broadcast domain. And the broadcast domain that is VLAN 3 on 1 switch is connected to the broadcast domain that is VLAN 1 on the other switch. So the end stations in VLAN 1 in 1 switch are hearing broadcasts from VLAN 3 in the other switch.


Thanks for the ratings.


HTH


Rick

Correct Answer by Richard Burts about 9 years 2 months ago

Dan


I am glad that my previous response was helpful. Thanks for the rating.


If I am understanding this part of the question correctly there should be an easy solution. What I think I understand is that there are 2 physical connections between sites and that one is 100 MB and the other is 10 MB. And I think I understand that they are both access ports in VLAN 3. If they are both access ports and connect to the same remote then Spanning Tree should put one in blocking mode and only use it if the other port goes down. Spanning Tree uses port cost to determine which interface should go into blocking mode. It should block the 10 MB and if it does not you should be able to manipulate port cost on the interface to get Spanning Tree to block the one you want.


If I have understood something not correctly please clarify.


HTH


Rick

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (5 ratings)
Loading.
smothuku Mon, 03/03/2008 - 02:32
User Badges:
  • Silver, 250 points or more

Hi ,


Can you send us the config of the both the routers ? what type of media is used to connect Site A router to Site B router.


Hae you configured same enacpsulation on routers present at both the sites ?




Thanks,

Satish

Richard Burts Mon, 03/03/2008 - 03:44
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Dan


You only describe your environment in very general terms. It would be helpful if we had more specific information. But based on the symptoms I would guess that your links are LAN/Ethernet interfaces rather than being serial interfaces. It is an issue that an Ethernet interface can lose connectivity but will remain up. This causes a problem for floating static routes since the interface does not go down and therefore the floating route never gets into the routing table.


Cisco introduced a feature to deal with this. It is Reliable Static Route Backup using Object Tracking. This link will give you information to get started with this:

http://www.cisco.com/en/US/docs/ios/12_4/dial/configuration/guide/hdbackup.html


HTH


Rick

dangal.43 Mon, 03/03/2008 - 06:56
User Badges:

Rick,


You understand my question properly. And got my answer thank you very much.


Actually I have Site A switch with vlan 3 and have two ports as a part of that vlan and both the ports are connected to the remote site B and limitation i have is one port link is 100 MBPS and other port link is 10 MBPS and remote office port connected to Site A is not a trunk link or trunk port and I want to configure this scenario like I would like to use 100 MBPS as a primary link and when that link goes down it should switch over to 10MBPS backup link. as my both the ports are of different speed I cannot go for Etherchannel so what is the other alternative?

as i dont want to go for Layer 3 stuff. please help me for the same.......

Correct Answer
Richard Burts Mon, 03/03/2008 - 12:13
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Dan


I am glad that my previous response was helpful. Thanks for the rating.


If I am understanding this part of the question correctly there should be an easy solution. What I think I understand is that there are 2 physical connections between sites and that one is 100 MB and the other is 10 MB. And I think I understand that they are both access ports in VLAN 3. If they are both access ports and connect to the same remote then Spanning Tree should put one in blocking mode and only use it if the other port goes down. Spanning Tree uses port cost to determine which interface should go into blocking mode. It should block the 10 MB and if it does not you should be able to manipulate port cost on the interface to get Spanning Tree to block the one you want.


If I have understood something not correctly please clarify.


HTH


Rick

dangal.43 Mon, 03/03/2008 - 12:22
User Badges:

Yes Rick you understood me correctly. I was also thinking the same as the remote switch is purly layer 2 connectivity and have access port then STP should take care of it, thanks for your reply.


Now i have one more question is as Site A switch has both the port in VLAN 3 as well as access port and site B we dont have any vlan

created on the switch so by default all port are in VLAN 1 so how switch at site A and Switch at site B will communicate as on Site A both the port connected to site B are in VLAN3 and on site B port connecting to site A are in VLAN1 so how they will communicate? (this is what I am thinking is as we configure both the port connecting to each other are configured as access port even though they are in different VLAN at two different site)???


hope this will be my last question! ;-)

dangal.43 Mon, 03/03/2008 - 12:23
User Badges:

one more thing is you said "It should block the 10 MB and if it does not you should be able to manipulate port cost on the interface to get Spanning Tree to block the one you want. " so what are the condition where this can happen?



Correct Answer
Richard Burts Mon, 03/03/2008 - 12:37
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Dan


You bring up a subtle and interesting question about how it works if the port on one switch is in VLAN 3 and the port it connects to on the other switch is in VLAN 1. In fact VLANs and VLAN IDs do not come into play until we have trunk ports or have some layer 3 device doing intervlan routing (layer 3 switch or router).


An access port is a member of a VLAN but that does not impact anything or change anything on the frames that it sends out its port or the frames that it receives on its port. These are just regular Ethernet frames.


If you think about it, we frequently define a VLAN by saying that it is a broadcast domain. And the broadcast domain that is VLAN 3 on 1 switch is connected to the broadcast domain that is VLAN 1 on the other switch. So the end stations in VLAN 1 in 1 switch are hearing broadcasts from VLAN 3 in the other switch.


Thanks for the ratings.


HTH


Rick

dangal.43 Mon, 03/03/2008 - 12:49
User Badges:

Rick,


broadcast is not a issue for me but I am concerning about the backup connection. so in this case will the backup link work as normal STP? I mean backup link in blocking state and 100 MBPS link will be my primary link?

Richard Burts Mon, 03/03/2008 - 12:55
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Dan


I would certainly expect that it would work that way so that normally traffic was carried on the 100 MB link and the 10 would be used as backup. So long as there is not some other aspect of the environment that we do not yet know about I would certainly expect that Spanning Tree would put the 10 MB into blocking while the 100 MB was operational.


when the switches are running in normal mode you should be able to look and see which interfaces are forwarding and which interfaces are blocking. If what you find is not what you expect then we need to investigate and find what is different in the environment.


HTH


Rick

dangal.43 Mon, 03/03/2008 - 12:58
User Badges:

Rick,


thanks once again, but again what are those condition in which we will not have desire output of operation as per STP?



Correct Answer
Richard Burts Mon, 03/03/2008 - 13:10
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Dan


Those conditions might include:

- if someone had previously configured cost on the interfaces that was different from what we want.

- if the switches calculate port cost differently - perhaps based on different models of switch or different versions of software.

- and finally and most importantly: when I am talking about something and the things I do not know about it are much larger than the things that I do know about it, then I always leave some wiggle room to accommodate unexpected results. Probably not likely to happen but I leave some room for it.


HTH


Rick

dangal.43 Mon, 03/03/2008 - 13:12
User Badges:

Ok sir thanks for your help, I will come back to you if I will have any queries or concern about the same, will it be ok?

Richard Burts Mon, 03/03/2008 - 13:29
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN

Dan


If other queries or concerns come up it will certainly be ok to come back to me for follow up.


HTH


Rick

Actions

This Discussion