Hi Aaron, Xander,

Shunta Tsuchida · ‎05-10-2016

ASR 9010 (IOS XR 4.3.4), I want to make sure what's BGP "multipath" config implications here.

router bgp 17511
|
neighbor 61.122.240.149
use neighbor-group INTERNAL
address-family ipv4 unicast <<<<< we can set "multipath" here

For BGP "multipaths" setting, it's configuration below would be more of popular,
--------
router bgp XXXXX
address-family ipv4 unicast
maximum-paths ebgp 8
!
address-family ipv6 unicast
maximum-paths ebgp 8
------

so I'm wondering what lies the difference between "multipath" and "maximum-paths" syntax.

AARON WEINTRAUB · ‎05-11-2016

on XR, 'multipath' under the global bgp configuration is actually a different knob:

-----

router(config-bgp)#bgp multipath as-path ?
ignore Ignore as-path related check for multipath selection

-----

In this case, it means that you would install multipaths even when the next-hop as was different, for example if you learned the same path from AS1 and AS2, they could be candidate multipaths.

You would want maximum-paths under the AFI to actually enable multiple paths.

xthuijs · ‎05-11-2016

yeah by default mpath only is for those prefixes with the exact same AS-length and the exact same AS-PATH also. This is standards based.

You can override this behavior of the as-path needing to be same and the as-length also being the same with the as-path ignore and relax (respectively).

xander

xthuijs · ‎05-13-2016

Aaron correct, and point taken ;) there is a bug doc open for this to describe teh functionality better.

Here is a write up:

This feature may be called Selective multipath.

=================

Configuration:

The feature is enabled by configuring the keyword "selective" in the corresponding "maximum-paths" command. The "maximum-paths" command is under the VRF address-family level.

Next, the user needs to select the neighbors whose path would be eligible for being a multipath. This is done by adding the keyword "multipath" under the neighbor or neighbor-group or af-group address-family.

=================

Functionality: Once the "selective" option is turned on, paths from the neighbors that are configured with "multipath" are eligible for being multipath, but paths from the neighbors that are not configured with "multipath" are not eligible for being multipath.

There is no change in the bestpath calculations. A bestpath is in effect always in the set of multipaths.

For VPN prefixes, the PE paths are always eligible for being multipath.
================

Example: For default VRF

router bgp 1
  address-family ipv4 unicast
     maximum-paths ibgp 4 selective
     maximum-paths ebgp 5 selective
 
  neighbor 1.1.1.1
    address-family ipv4 unicast
       multipath

  neighbor 2.2.2.2
    address-family ipv4 unicast

  neighbor 3.3.3.3
    address-family ipv4 unicast
       multipath

In the above example, for ipv4 unicast prefixes, paths from 1.1.1.1 and 3.3.3.3 are eligible for being a multipath, but paths from 2.2.2.2 are not eligible for being multipath even if it satisfies all other criteria

Shunta Tsuchida · ‎05-15-2016

Hi Xander,

Thank you for your comment.

Can you tell me the case number for that doc bug ?

Or you try to open now ?

xthuijs · ‎05-16-2016

the documentation request is filed in a different system for this one Shunta, but it is in the works. the detail will be the same as what I provided above. So there is no need to file/request a separate documentation request.

cheers!

xander

Shunta Tsuchida · ‎05-16-2016

Hi Xander,

Thank you for your update.

So can we assume recently we will be able to catch your updated information in Configuration File / Command Reference on CCO ?

If there available documentations on CCO, I hope you to update to me.

Regards,

Shunta Tsuchida

Shunta Tsuchida · ‎05-18-2016

Hi Xander, Aaron,

Thank you for your comment.

>So can we assume recently we will be able to catch your updated information in >Configuration File / Command Reference on CCO ?

>If there available documentations on CCO, I hope you to update to me.

Xander, I hope you to make sure above point.

Regards,

Shunta Tsuchida

Shunta Tsuchida · ‎05-11-2016

Hi Aaron, Xander,

Thank you for your comment.

Now customer CLI the entire syntax option there goes as below.

--------

9010A(config-bgp-nbr-af)#?

aigp Enable AIGP for this neighbor

allowas-in Allow as-path with my AS present in it

apply-group Apply configuration from a group

as-override Override matching AS-number while sending update

bestpath Change default route selection criteria

capability Advertise capability to the peer

clear Clear the uncommitted configuration

commit Commit the configuration changes to running

default-originate Originate default route to this neighbor

describe Describe a command without taking real actions

do Run an exec command

exit Exit from this submode

maximum-prefix Maximum number of prefixes to accept from this peer

multipath Paths from this neighbor is eligible for multipath <<<<<<<<<<<<<<<<<<<<<<<<<<<<<

next-hop-self Disable the next hop calculation for this neighbor

next-hop-unchanged Do not overwrite next hop before advertising to eBGP peers

no Negate a command or set its defaults

orf Specify ORF and inbound filtering criteria

origin-as BGP origin-AS knobs

pwd Commands used to reach current submode

remove-private-AS Remove private AS number from outbound updates

root Exit to the global configuration mode

route-policy Apply route policy to neighbor

route-reflector-client Configure a neighbor as Route Reflector client

send-community-ebgp Send community attribute to this external neighbor

send-extended-community-ebgp Send extended community attribute to this external neighbor

show Show contents of configuration

soft-reconfiguration Per neighbor soft reconfiguration

use Inherit configuration for this address-family from a group

weight Set default weight for routes from this neighbor

9010A(config-bgp-nbr-af)#

9010A(config-bgp-nbr-af)#multipath ?

<cr> <<<<<<<<<<<<<<<<<<<<<<<<<<<<<

9010A(config-bgp-nbr-af)#

-------------------

So can we assume basically this command would be the same function for "maximum-paths" to provide more relaxed selection rule to store BGP best path prefixes in RIB (neighbor based), right ?

xthuijs · ‎05-11-2016

oh I see what you're saying Shunta, here is the difference between these 2:

maximum-paths is how many paths from different neighbors we will be willing to consider.

noting that for a path to be mpath elibigible the as length, path needs to be the same, and then also noting that by default only eBGP paths are considered. There are overrides for each of these clauses as mentioned in the previous comment (ignore, relax and eiBGP respectively).

Now you may have neighbors XYZ all advertising the same prefix. If you have max-paths 2 configured, we only take the first two learnt. now let's say that your neighbor Z is merely a backup path and you don't want to use it all the time or ever. you can then cofnigure multipath on enighbors X and Y, so that Z is not considered for multipath selection.

xander

Shunta Tsuchida · ‎05-11-2016

>Now you may have neighbors XYZ all advertising the same prefix. If you have max-paths 2 >configured, we only take the first two learnt. now let's say that your neighbor Z is merely a >backup path and you don't want to use it all the time or ever. you can then cofnigure multipath >on enighbors X and Y, so that Z is not considered for multipath selection.

So can we assume that "(config-bgp-nbr-af)#multipath" provides "neighbor based" path selection (in your case, XY or XZ or YZ) with global "max-paths" enabled ?

AARON WEINTRAUB · ‎05-11-2016

I have a strong feeling (but will defer to someone with source code access) that 'neighbor 1.2.3.4 address-family ipv4 unicast multipath' is the default and that you could selectively make a neighbor NOT eligible for mpath routes via 'no neighbor 1.2.3.4 address-family ipv4 unicast multipath '.Unfortunately I could not find an exact reference for multipath used under neighbor configuration for the 5.3.X pages (hint to cisco)

ASR 9010 (IOS XR 4.3.4) BGP "multipath" config