Question on BGP round robin

Unanswered Question
Oct 23rd, 2007

Hi all,

If a router receives an eBGP route from two of its iBGP peers (with similar weight/local pref/metric/as path)

Is it possible for the router to put both next hop routers in the routing table or can only one "BEST" route be chosen between the two?

Note: Both paths do show up in "show ip bgp"

Looking forward to comments. Thanks in advance.

I have this problem too.
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guruprasadr Tue, 10/23/2007 - 02:47

HI,

In my understading, BGP will select the "BEST" path to the destination.

In the above scenario, if 2" paths with similar weight/local pref/metric/as path received means,

the PATH which is FIRST injected in the routing table will be used.

PLS RATE if HELPS

Best Regards,

Guru Prasad R

bangkydotnet Tue, 10/23/2007 - 02:56

Hi,

Upon further reading (http://www.cisco.com/warp/public/459/25.shtml), it seems that indeed a BEST path will be selected and that is the path that will be advertised to neighbours.

However, there is still the issue of BGP multipath. I don't seem to be able to get it working. maximum-paths has already been set to 2.

Pavel Bykov Tue, 10/23/2007 - 04:13

There will ALWAYS be only one best path, even in case of path load balancing.

To find out if a route is balanced or not, type "show ip bgp n.n.n.n" where n.n.n.n is a destination network.

Example:

BGP routing table entry for 100:1:10.22.22.0/24, version 50

Paths:(6 available, best #1)

Multipath:iBGP

Advertised to non peer-group peers:

200.1.12.12

22

10.22.7.8 (metric 11) from 10.11.3.4 (100.0.0.8)

Origin IGP, metric 0, localpref 100, valid, internal, multipath, best

Extended Community:RT:100:1

Originator:100.0.0.8, Cluster list:100.1.1.44

In this example with MPLS the multipath keyword denotes that this path is used with another path to propagate data.

http://cisco.com/en/US/partner/products/ps6350/products_configuration_guide_chapter09186a0080455702.html#wp1023954

Hope this helps.

guruprasadr Tue, 10/23/2007 - 04:31

HI, [PLS RATE if HELPS]

Issue #show ip route command

You can see 2" paths learned and by default load balancing should occur per-packet basis and neverthless issue #ip route-cache command over the interfaces connected to ISP to do load balancing via per-destination basis.

Hope the below explanation will fix your problem:

By default BGP chooses one best path among the possible equal-cost paths. However you can change the maximum number of parallel equal-cost paths allowed.

To make this change include:#maximum-paths paths command under the BGP Config.

User the number between 1 to 6 for the paths.

Sample Config:

!

router bgp

neighbor remote-as

neighbor remote-as

network

maximum-paths 2

!

!!This Command allows max number of paths to particular destination to install in the routing table.

Use: #show ip route and #show ip bgp

for verification.

Hope i am Informative.

PLS RATE if HELPS

Best Regards,

Guru Prasad R

gaurav_thapar79 Tue, 10/23/2007 - 08:09

Hi,

In addition to below said feedback, in case everything is equal like local pref, Weight,etc. then it will always consider lower router-id as best path to exit that router for outbound traffic.

bangkydotnet Tue, 10/23/2007 - 18:07

Hi all, thanks for the comments. You are all right.

Problem is identified, silly me, I used command "maximum-paths 2" instead of "maximum-paths ibgp 2".

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