LAGing backbone links

Shane Amante shane at
Tue Apr 5 15:30:47 UTC 2011


On Apr 4, 2011, at 18:17 MDT, Payam Chychi wrote:
> Hello All,
> I was wondering if anyone had any thoughts as to the best practices of
> running multiple backbone links between 2 routers.  In the past we've added
> additional links as needed, then simply enabled IS-IS when they were good to
> go.  I'd then let IS-IS handle load balancing the traffic over the two
> links.  But I know that others out there would setup a LAG once they had
> more than one link between two routers.  Is there a best practice?  Does it
> matter?  Any implications to a MPLS setup?

In general, if you're using relatively modern, medium- to higher-end equipment, it should "just work".  Some things to watch out for in order of importance:
1)  Be mindful of the number of component-links you can put into a single LAG.  This varies by platform.  In general, for higher-end routers/switches the minimum number of component-links in a single LAG is 16.  More recently, in the last couple of years, several vendors are shipping equipment and/or software that will take this up to 64x component-links in a single LAG.  (Depending on platform, LAG's may allow you to build larger virtual-links between adjacent devices compared to ECMP which may be limited to 8x component-links in a single ECMP ... but, again, that all depends on the platform type).
2)  The distribution of flows across the component-links in a single LAG could vary, dramatically, depending on the type of traffic you're pushing.  Specifically, for /Internet/ (IPv4 or IPv6) over MPLS traffic, you will most likely very get good load distribution given the pseudo-randomness of IP addresses and Layer-4 port information, (in particular source port's from a client toward a server).  OTOH, if you have traffic in [very large] PW's, then typically LSR's/switches/routers can't look past the MPLS labels and inner Layer-2 encapsulation to find granular input keys used for the load-balancing hash.  Thus, the load-balancing hash result will cause all traffic for a single PW VC to non-deterministically be placed on a single component-link in the LAG.  The good news is that there is hope on the horizon in the form of:
... which, in short, expects the ingress PE to [try to] find granular input keys from the incoming traffic, (e.g.: find input keys from an IP header contained within an Ethernet frame that will be transported as a PW VC over your MPLS core), and create a hash of that that will get placed into a "FAT PW" label that sits below the PW VC label.  The idea is that Core LSR's would still load-balance based on the bottom-most to top-most MPLS labels, which should result in more even load-distribution of PW VC flows over component-links in a LAG.  This feature is just starting to appear in one vendor's equipment and will hopefully show up in others soon, as well.  (Please bug your vendors for this!  ;-)
3)  Depending on the vendor, you may specifically have to configure the device to do load-balancing over LAG's or ECMP paths, (e.g.: Juniper & Brocade, possibly others).  Generally, you have to configure the device what input keys to look for and/or what # of MPLS labels to look past for those input-keys, e.g.: in Juniper you configure forwarding-options -> hash-key -> family mpls -> labels-1, label-2, payload -> ip, etc.

Some other things to look out for:
4)  Some vendor's may use different hash algorithms for LAG vs. ECMP, so you may get "better" load-balancing from one compared to the other.  Ask your vendor for details as this may not be obvious from Lab testing.
5)  Some vendors may have a limit, of the maximum number of MPLS labels that they can look past to find, say, an IP payload that can be used as input-keys for the load hashing algorithm.  This used to be a concern several years ago, but in general most medium- to high-end equipment can look past /at least/ 3 MPLS labels, which should cover you in the more common cases where either:
   a)  You have IP/LDP/RSVP/RSVP-FRR, where the outermost label is a RSVP Bypass Label when you're [briefly] running on a Bypass; or,
   b)  You have VPN-label/LDP/RSVP, where you're moving IPVPN or 6PE, etc. traffic and using LDP over RSVP tunneling.

Anyway, HTH,


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