16

on Cisco (this command is hidden for some reason)

#bgp bestpath as-path multipath-relax

The default BGP behavior only installs only routes with exact the same AS_PATH into RIB. With multipath-relax, the AS_PATH only needs to be of the same length.

What problems can it potentially cause? Why isn't is used more often?

As a transit provider, does this feature complicate troubleshooting (I am thinking about end-user complaints about network performance)? Does it make it more difficult to know the path specific traffic took at a given time? Is there something else to that can assist troubleshooting. I am not sure about scalability and cost for NetFlow in SP network.

Improve this question
3
  • 1
    I've instructed several customers to use it, I've not heard of any issues. draft-lapukhov-bgp-routing-large-dc-05 heavily relies on this feature
    – ytti
    Jul 17, 2013 at 11:35
  • I'm looking for a similar feature in Juniper JunOS... Is there any? networkengineering.stackexchange.com/questions/6735/…
    – Everton
    Mar 10, 2014 at 20:43
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin ♦
    Jan 3, 2021 at 2:26

2 Answers 2

Reset to default
16

bgp bestpath as-path multipath-relax was introduced by CSCea19918. Normally eBGP load-balancing requires the candidate routes to be equal-cost paths; i.e. identical BGP attributes:

  • same Weight
  • same Local-Pref
  • same AS-Path (both the AS numbers, and the AS path length)
  • same Origin
  • same MED
  • different next-hop

As you mentioned, this command relaxes the same AS-Path requirement so any candidate eBGP AS-Path with the same AS-path length could be used for eBGP load-balancing (this will not load-balance between eBGP and iBGP paths). If you run BGP between multiple ISPs and you are looking for better egress load-balancing between your upstream connections this may help you out.

What problems can it potentially cause?

There isn't much danger as long as you're an enterprise customer that doesn't give transit service to another ASN; for a transit provider it might be perfectly safe, but I can't be sure there aren't routing loops if a transit ASN uses this feature. At first, I thought there would easily be a loop in transit ASN cases, on more reflection I can't find a real problem.

Why will it is rarely used?

Good question, it's been around since at least 2005.

Improve this answer
1
  • 1
    Could you please explain how it can cause loops with #bgp bestpath as-path multipath-relax (not #bgp bestpath as-path ignore)?
    – sdaffa23fdsf
    Jul 18, 2013 at 3:10
0 
The   basic  issue   is   that  the   BGP   speaker  configured   with
"multipath-relax"   gets  into   a  control   plane  <->   data  plane
inconsistency; i.e.  it advertises  only the  best path,  but installs
multiple paths in the forwarding  that have different ASPATHs than the
best. This breaks the basic tool BGP has to detect loops - ASPATH loop
check.

A (distorted) scenario below. I am sure you can come up with
a better example with a bit more time at hand.

            ...............
            : R4  AS1 (10/8)
           /:..............
     ..../......
     :  R5 AS2
     :....\.....
       /   \  ...............
     /      --:--R1
  R6 AS4      :    \    AS3
     \--------:--- R2
              :    /
              : R3 (10/8)
              :..............

In this example,
- R3 in AS3 and R4 in AS1 announce a prefix 10/8. R5 in AS2 receives
  the prefix from R1(AS3) and R4(AS1).
- AS2 is configured with 'multipath-relax' and chooses both paths
  for multipath forwarding, though it selects AS1's path as best.
- R5 advertises the prefix with AS_PATH "2 1" to R6, and R6 in turn
   to R2.
- Because of some specific policy, it is possible that R2(AS3)
  chooses R6's path as best. If it happens, there is a loop.
  Note that R1-R2-R3 represents the physical connectivity of
  the routers in AS3.
Improve this answer
4
  • Thanks for the example. Do you mean R3 selects R4 as the best path for 10/8, R5 forwards part of the traffic to 10/8 back to AS3 at R1? Why is this caused by multipath-relax? Without multipath-relax, the R2 - R6 - R5 -R1 loop can still exist if there isn't proper outbound filtering( or simple 10/8 to null0 at the originating AS). It appears to me the issue here is BGP hijacking. Am I understanding it wrong?
    – sdaffa23fdsf
    Jul 22, 2013 at 6:40
  • No. What I mean is: (a) R3 has its own external best path, (b) R1 chooses R3 as the best, but because of the physical connectivity, has to send traffic towards R2 to reach R3, (c) R2 selects the external path (received from R6) as best. This is related to multipath-relax, since R5 chooses a path from AS3 for multipath forwarding that it does not disclose. Thus ASPATH loop prevention technique fails in AS3.
    – Pradosh Mohapatra
    Jul 22, 2013 at 17:41
  • I still don't get how this could be prevented without multipath-relax. R1 sends traffic to 10/8 via R2 and R2 chooses R6 as exit (weight), R5 can choose R1 as best for 10/8 even without multipath-relax and cause a loop. ASPATH loop prevent cannot prevent loops caused by preferring an external exit point for internal network. If R1 picks R5, R2 picks R6 as best for 10/8, loop is formed regardless, no?
    – sdaffa23fdsf
    Jul 28, 2013 at 7:05
  • Not sure how that would be a loop. R1 picks R5 as best => implies that R5 picks R4 as the exit for traffic (otherwise, an AS_PATH loop would have been detected). Thus, traffic flow paths will be: R1->R5->R4 or R2->R6->R5->R4 or R5->R4 or R3.
    – Pradosh Mohapatra
    Jul 29, 2013 at 5:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.