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skbluecollars
Beginner

How to find Alternate Paths in a routing table

Hi,

 

I have a destination IP and a VRF Name using this am able to collect the primary path of a destination IP. Now i have to collect all the alternate paths towards the destination IP.

 

Whether  we have to collect all the routing table from all devices ??

 

Please guide me on how to trace alternate paths.

 

BR

Sujith

 

9 REPLIES 9
balaji.bandi
VIP Master

show ip route - or show ip route vrf XXXX (XXXX VRF name)  should give you all the information.

 

again depends on the routing table and VRF you need to use specific commands 

 

example :

show IP OSPF route or show IP OSPF database gives more information.

 

Whether  we have to collect all the routing table from all devices ??

For this, you need to get network topology and you know well depends on links connected how many paths to reach the destination. ( from the specific source)

 

 

 

BB

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Yes, I do have the network topology and based on links for single destination more than 10 Paths i can find manually using show route vrf command. Whether this a normal way to run thee commands and collect data.

 

I am trying to automate this process using best possible method.

 

Please suggest

 single destination more than 10 Paths

I am more interested see this?  do you see this in the routing table? (or are we considering here Physical connection ?)

 

You need to get individually from each Device since the source is changed from every device right?

 

BB

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Hi Balaji,

 

In Telecom network, Radio tower will be connected to a router in the mesh network. I have to find what are all the alternate paths available and cost of each paths and capacity of the alternate paths to be identified.

 

So for the telecom network, It is possible for having n no.of alternate paths.

paul driver
VIP Mentor

Hello

sh ip cef vrf xxxx
sh ip cef vrf xxxx | in receive  < these are address on the rtrs interface

sh ip cef vrf xxxx | in attached  < these are address attached to the rtrs interface
sh ip route vrf xxxx



kind regards
Paul

Please rate and mark posts accordingly if you have found any of the information provided useful.
It will hopefully assist others with similar issues in the future

Hi Paul,

 

Thanks for the reply, Given commands provides very minimal no of route paths, Also it branches the route paths whenever i go to next hops.

ngkin2010
Rising star

Hi all,

 

@skbluecollars is asking about the alternative path to a given destination IP. But unfortunately, routing table only will install the best routes to a given destination prefix, which mean you may no find this information from either 'show ip route' or 'show ip cef'. 

 

For example, you have got two static routes. Two routes with the same prefix; only one of it will be installed to routing table. 

 

ip route 10.20.0.0 255.255.0.0 192.168.1.1 name primary
ip route 10.20.0.0 255.255.0.0 192.168.2.1 100 name backup

 

 

However, here are some exceptions:

 

1. If your backup route(s) prefix length is different from the primary route, you could see both routes on routing table. For example:

ip route 10.20.0.0 255.255.0.0 192.168.1.1 name primary
ip route 10.0.0.0 255.0.0.0 192.168.2.1 100 name backup

2. If you are running OSPF, and the backup route(s) is external route. Then you can view all the external routes (LSA-Type 5) by 'show ip ospf database external'

 

Hi ngkin2010,

 

You have understood my requirement. I have attached a sample connectivity which i have manually traced up to a level based on physical connections and routes on each device. I want to automate this process.

 

Ex:- If a router goes down or a port goes down then what are all the alternate paths available for the destination and predict what is the current capacity and what if additional traffic diverts to those alternate paths, will the packetloss occur or congestion in any of the ports / links to be captured.

 

So Please guide me with your suggestions.

Hello @skbluecollars ,

in your initial post you say that the prefix belongs to a vrf .

 

Possible alternate paths in MP BGP can be found if :

each PE is using its own RD value so that best path selection is not performed by RR servers and they will treat VPNv4 prefixes as different on remote PE . From remote PE point of view it can see more then one path in this way if the VRF Site where the prefix is learned is multihomed connected to multiple PE nodes.

 

Then, you speak of finding alternate paths in your backbone explaining you have a RF mesh and a PE/P router on each radio tower.

This is a different matter in this case you need to look for all alternate paths to the Loopback address of the PE that is advertising the prefix in MP BGP as VPNv4 prefix.

RF links are not so reliable when compared to fiber optic links and can be influenced by weather conditions.

 

You need to find out what devices have "direct sight" of each other in RF terms and this should be reflected in OSPF neighbor states.

But the picture you can draw this way can vary over time.

 

I would suggest to use

show ip ospf neighbor detail

that provides also the uptime of each neighbor. This can be considered a first sign of a stable connection ( an high uptime in OSPF neighborship).

 

You should do this on every node multiple times per day  ( 4 to 6) for two weeks.

This will allow you to create a baseline and to classify OSPF neighbors in reliable and not so reliable.

 

Hope to help

Giuseppe