MaxPath

Unanswered Question
Jul 10th, 2010

Hi All,

We have Four links running with BGP (Each of these links are running as Primary and also running as backup for other links). Recently we have owned TWO more links that will be configured under same BGP with same AS no. . These Two new Links will be Primary and also configure as backup of each other only  with as path prepend command (i.e not for Old Four Links, They have seperate route MAP, Prefix List ....)

Currently , For FOUR links we have command

maximum-paths 4

We are bit confused about the maximum-path command when we'll add TWO new links under BGP .

Should it be same i.e " maximum-paths 4"  or " maximum-paths 6" or something else.

Txns

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
Peter Paluch Sat, 07/10/2010 - 09:58

Hello,

The command maximum-paths 4 in essence means that the routing protocol is allowed to install at most 4 different routes to the same destination network into the routing table. Obviously, these routes must differ in their next-hop but otherwise must be evaluated by that routing protocol as being the best paths available.

The relation of this command to the number of uplinks your router has is only indirect. Even if your router has many uplinks, they may not provide equivalent paths to a particular destination so having this command set to 4 or 6 (or even higher in recent IOSes) does not make a difference. On the other hand, if the uplinks do provide equally good paths to a particular destination and this command is set too low then you obviously do not make use of all the links - the BGP or OSPF or whatever routing protocol will communicate over these links and will know for itself that the particular network is reachable through all of them. However, it will not be allowed to install so many equal cost paths to that network into the routing table.

To sum this up - you may increase the value of this command as you see fit. Your BGP peerings will not be impaired. Whether it makes sense to have this command set to 6, that depends on other circumstances - whether it is actually possible that your BGP will see 6 best paths to a particular destination. This does not depend on the maximum-paths command but rather on the BGP configuration and the attributes of the networks discovered via BGP.

Please feel free to ask further.

Best regards,

Peter

netbeginner Sat, 07/10/2010 - 10:47

Thanks Peter for qick response.

So you mean in our scenrio we don't need to configure any max path command. or stay with samer command " maximum-paths 4".

This will not impact BGP working ...Right dear...?

Peter Paluch Sat, 07/10/2010 - 11:01

Hello,

Exactly as you wrote: the maximum-paths value will not have any impact on the BGP alone. It will only influence the number of equal-cost routes to the same particular destination that can be inserted into routing table by BGP.

There is no harm in increasing the value from 4 to 6, and there is no harm in leaving it on the current value. Right now, do you see 4 equal-cost best routes to a destination (whichever) in your routing table? If not then there is probably nothing to improve by setting this parameter to 6. It all depends on whether the BGP can actually see so many routes to a single destination. If the BGP does not know about so many routes, it obviously cannot install them into the routing table.

Best regards,

Peter

Actions

This Discussion