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ASR9000/XR: Load-balancing architecture and characteristics



In this document it is discussed how the ASR9000 decides how to take multiple paths when it can load-balance. This includes IPv4, IPv6 and both ECMP and Bundle/LAG/Etherchannel scenarios in both L2 and L3 environments

Core Issue

The load-balancing architecture of the ASR9000 might be a bit complex due to the 2 stage forwarding the platform has. In this article the various scenarios should explain how a load-balancing decision is made so you can architect your network around it.

In this document it is assumed that you are running XR 4.1 at minimum (the XR 3.9.X will not be discussed) and where applicable XR42 enhancements are alerted.

Load-balancing Architecture and Characteristics


ASR9000 has the following load-balancing characteristics:

  • ECMP:
  1. Non recursive or IGP paths : 32-way
  2. Recursive or BGP paths:
    1. 8-way for Trident
    2. 32 way for Typhoon
    3. 64 way Typhoon in XR 5.1+
    4. 64 way Tomahawk XR 5.3+ (Tomahawk only supported in XR 5.3.0 onwards)


  • Bundle:
  1. 64 members per bundle

The way they tie together is shown in this simplified L3 forwarding model:

Screen Shot 2012-08-28 at 12.12.25 PM.png

NRLDI = Non Recursive Load Distribution Index

RLDI = Recursive Load Distribution Index

ADJ = Adjancency (forwarding information)

LAG = Link Aggregation, eg Etherchannel or Bundle-Ether interface

OIF = Outgoing InterFace, eg a physical interface like G0/0/0/0 or Te0/1/0/3

What this picture shows you is that a Recursive BGP route can have 8 different paths, pointing to 32 potential IGP ways to get to that BGP next hop, and EACH of those 32 IGP paths can be a bundle which could consist of 64 members each!


The architecture of the ASR9000 load-balancing implementation surrounds around the fact that the load-balancing decision is made on the INGRESS linecard.

This ensures that we ONLY send the traffic to that LC, path or member that is actually going to forward the traffic.

The following picture shows that:

Screen Shot 2012-08-28 at 1.58.42 PM.png

In this diagram, let's assume there are 2 paths via the PATH-1 on LC2 and a second path via a Bundle with 2 members on different linecards.

(note this is a bit extraordinary considering that equal cost paths can't be mathematically created by a 2 member bundle and a single physical interface)

The Ingress NPU on the LC1 determines based on the hash computation that PATH1 is going to forward the traffic, then traffic is sent to LC2 only.

If the ingress NPU determines that PATH2 is to be chosen, the bundle-ether, then the LAG (link aggregation) selector points directly to the member and traffic is only sent to the NP on that linecard of that member that is going to forward the traffic.

Based on the forwarding achitecture you can see that the adj points to a bundle which can have multiple members.

Allowing this model, when there are lag table udpates (members appearing/disappearing) do NOT require a FIB update at all!!!

What is a HASH and how is it computed

In order to determine which path (ECMP) or member (LAG) to choose, the system computes a hash. Certain bits out of this hash are used to identify member or path to be taken.

  • Pre 4.0.x Trident used a folded XOR methodology resulting in an 8 bit hash from which bits were selected
  • Post 4.0.x Trident uses a checksum based calculation resulting in a 16 bit hash value
  • Post 4.2.x Trident uses a checksum based calculation resulting in a 32 bit hash value
  • Typhoon 4.2.0 uses a CRC based calculation of the L3/L4 info and computes a 32 bit hash

8-way recursive means that we are using 3 bits out of that hash result

32-way non recursive means that we are using 5 bits

64 members means that we are looking at 6 bits out of that hash result

It is system defined, by load-balancing type (recursive, non-recursive or  bundle member selection) which bits we are looking at for the  load-balancing decision.

Fields used in ECMP HASH

What is fed into the HASH depends on the scenario:

Incoming Traffic Type Load-balancing Parameters

Source IP, Destination IP, Source port (TCP/UDP only), Destination port (TCP/UDP only), Router ID


Source IP, Destination IP, Source port (TCP/UDP only), Destination port (TCP/UDP only), Router ID

MPLS - IP Payload, with < 4 labels

Source IP, Destination IP, Source port (TCP/UDP only), Destination port (TCP/UDP only), Router ID

From 6.2.3 onwards, for Tomahawk + later ASR9K LCs:

MPLS - IP Payload, with < 8 labels

Source IP, Destination IP, Source port (TCP/UDP only), Destination port (TCP/UDP only), Router ID

Typhoon LCs retain the original behaviour of supporting IP hashing for only up to 4 labels.


MPLS - IP Payload, with > 9 labels

If 9 or more labels are present, MPLS hashing will be performed on labels 3, 4, and 5 (labels 7, 8, and 9 from 7.1.2 onwards). Typhoon LCs retain the original behaviour of supporting IP hashing for only up to 4 labels.

- IP Payload, with > 4 labels

4th MPLS Label (or Inner most) and Router ID

- Non-IP Payload

Inner most MPLS Label and Router ID

* Non IP Payload includes an Ethernet interworking, generally seen on Ethernet Attachment Circuits running VPLS/VPWS.

These have a construction of

EtherHeader-Mpls(next hop label)-Mpls(pseudowire label)-etherheader-InnerIP

In those scenarios the system will use the MPLS based case with non ip payload.

IP Payload in MPLS is a common case for IP based MPLS switching on LSR's whereby after the inner label an IP header is found directly.

Router ID

The router ID is a value taken from an interface address in the system in an order to attempt to provide some per node variation

This value is determined at boot time only and what the system is looking for is determined by:

sh arm router-ids




RP/0/RSP0/CPU0:A9K-BNG#show arm router-id

Tue Aug 28 11:51:50.291 EDT

Router-ID         Interface           Loopback0      


Bundle in L2 vs L3 scenarios

This section is specific to bundles. A bundle can either be an AC or attachment circuit, or it can be used to route over.

Depending on how the bundle ether is used, different hash field calculations may apply.

When the bundle ether interface has an IP address configured, then we follow the ECMP load-balancing scheme provided above.

When the bundle ether is used as an attachment circuit, that means it has the "l2transport" keyword associated with it and is used in an xconnect or bridge-domain configuration, by default L2 based balancing is used. That is Source and Destination MAC with Router ID.

If you have 2 routers on each end of the AC's, then the MAC's are not varying a lot, that is not at all, then you may want to revert to L3 based balancing which can be configured on the l2vpn configuration:




RP/0/RSP0/CPU0:A9K-BNG(config-l2vpn)#load-balancing flow ?

  src-dst-ip   Use source and destination IP addresses for hashing

  src-dst-mac  Use source and destination MAC addresses for hashing


Use case scenarios

Screen Shot 2012-08-28 at 1.11.11 PM.png

Case 1 Bundle Ether Attachment circuit (downstream)

In this case the bundle ether has a configuration similar to


interface bundle-ether 100.2 l2transport

  encap dot1q 2

  rewrite ingress tag pop 1 symmetric


And the associated L2VPN configuration such as:



  bridge group BG

  bridge-domain BD

   interface bundle-e100.2


In the downstream direction by default we are load-balancing with the L2 information, unless the load-balancing flow src-dest-ip is configured.

Case 2 Pseudowire over Bundle Ether interface (upstream)

The attachment circuit in this case doesn't really matter, whether it is bundle or single interface.

The associated configuration for this in the L2VPN is:



  bridge group BG

   bridge-domain BD

    interface bundle-e100.2

    vfi MY_VFI

    neighbor pw-id 2


interface bundle-ether 200

  ipv4 add


router static

  address-family ipv4 unicast


In this case neighbor is found via routing which appens to be egress out of our bundle Ethernet interface.

This is MPLS encapped (PW) and therefore we will use MPLS based load-balancing.

Case 3 Routing through a Bundle Ether interface

In this scenario we are just routing out the bundle Ethernet interface because our ADJ tells us so (as defined by the routing).


interface bundle-ether 200

ipv4 add


show route (OSPF inter area route)

O IA [110/2] via, 2w4d, Bundle-Ether200

Even if this bundle-ether is MPLS enabled and we assign a label to get to the next hop or do label swapping, in this case

the Ether header followed by MPLS header has Directly IP Behind it.

We will be able to do L3 load-balancing in that case as per chart above.

(Layer 3) Load-balancing in MPLS scenarios

As attempted to be highlighted throughout this technote the load-balacning in MPLS scenarios, whether that be based on MPLS label or IP is dependent on the inner encapsulation.

Depicted in the diagram below, we have an Ethernet frame with IP going into a pseudo wire switched through the LSR (P router) down to the remote PE.

Screen Shot 2012-08-28 at 1.22.26 PM.png

Pseudowires in this case are encapsulating the complete frame (with ether header) into mpls with an ether header for the next hop from the PE left router to the LSR in the middle.

Although the number of labels is LESS then 4. AND there is IP available, the system can't skip beyond the ether header and read the IP and therefore falls back to MPLS label based load-balancing.

How does system differentiate between an IP header after the inner most label vs non IP is explained here:

Just to recap, the MPLS header looks like this:

Screen Shot 2012-08-28 at 1.28.12 PM.png

Now the important part of this picture is that this shows MPLS-IP. In the VPLS/VPWS case this "GREEN" field is likely start with Ethernet headers.

Because hardware forwarding devices are limited in the number of PPS they can handle, and this is a direct equivalent to the number of instructions that are needed to process a packet, we want to make sure we can work with a packet in the LEAST number of instructions possible.

In order to comply with that thought process, we check the first nibble following the MPLS header and if that starts with a 4 (ipv4) or a 6 (ipv6) we ASSUME that this is an IP header and we'll interpret the data following as an IP header deriving the L3 source and destination.


Now this works great in the majority scenarios, because hey let's be honest, MAC addresses for the longest time started with 00-0......

in other words not a 4 or 6 and we'd default to MPLS based balancing, something that we wanted for VPLS/VPWS.

However, these days we see mac addresses that are not starting with zero's anymore and in fact 4's or 6's are seen!

This fools the system to believe that the inner packet is IP, while it is an Ether header in reality.

There is no good way to classify an ip header with a limited number of instruction cycles that would not affect performance.

In an ideal world you'd want to use an MD5 hash and all the checks possible to make the perfect decision.

Reality is different and no one wants to pay the price for it either what it would cost to design ASICS that can do high performance without affecting the PPS rate due to a very very comprehensive check of tests.

Bottom line is that if your DMAC starts with a 4 or 6 you have a situation.


Use the MPLS control word.

Control word is negotiated end to end and inserts a special 4 bytes with zero's especially to accommodate this purpose.

The system will now read a 0 instead of a 4 or 6 and default to MPLS based balancing.


to enable control word use the follow template:



pw-class CW

  encapsulation mpls




xconnect group TEST

  p2p TEST_PW

   interface GigabitEthernet0/0/0/0

   neighbor pw-id 100

    pw-class CW





Alternative solutions: Fat Pseudowire

Since you might have little control over the inner label, the PW label, and you probably want to ensure some sort of load-balancing, especially on P routers that have no knowledge over the offered service or mpls packets it transports another solution is available known as FAT Pseudowire.

FAT PW inserts a "flow label" whereby the label has a value that is computed like a hash to provide some hop by hop variation and more granular load-balancing. Special care is taken into consideration that there is variation (based on the l2vpn command, see below) and that no reserved values are generated and also don't collide with allocated label values.

Fat PW is supported starting XR 4.2.1 on both Trident and Typhoon based linecards.

One note to make is that with the relatively newer functionality of PseudoWire Headend (aka PWHE), FAT labels cannot be used. You can configure it, but the forwarding, especially downstream, entering the pwhe interface will break.

PWHE requires a pindown list to identify the possible ingress/egress interfaces for the pw to come in on (and all features are programmed there). The system will loadbalance based on pwid, on both the a9k startign the pwhe as well as then obviously also interim P nodes. The reason for that is that the egress interface selection is already made at the time the pw encap is made. It isnt a simple change in the current forwarding architecture to "rehash" based on something else, without incurring heavy pps reduction (and possible recirculation).

Packet transformation with a Flow Label

Screen Shot 2012-08-29 at 1.19.42 PM.png

Configuration of FAT Pseudowire

The following is configuration example :



load-balancing flow src-dst-ip

pw-class test

encapsulation mpls


   flow-label both static




You can also affect the way that the flow label is computed:

Under L2VPN configuration, use the “load-balancing flow” configuration command to determine how the flow label is generated:


    load-balancing flow src-dst-mac

This is the default configuration, and will cause the NP to build the flow label from the source and destination MAC addresses in each frame.


    load-balancing flow src-dst-ip


This is the recommended configuration, and will cause the NP to build the flow label from the source and destination IP addresses in each frame.

• Note that IPv6 hashing is not supported in the first release.
FAT Pseudowire TLV

Flow Aware Label (FAT) PW signalled sub-tlv id is currently carrying value 0x11 as specified originally in draft draft-ietf-pwe3-fat-pw. This value has been recently corrected in the draft and should be 0x17. Value 0x17 is the flow label sub-TLV identifier assigned by IANA.

When Inter operating between XR versions 4.3.1 and earlier, with XR version 4.3.2 and later. All XR releases 4.3.1 and prior that support FAT

PW will default to value 0x11. All XR releases 4.3.2 and later default to value 0x17.


Use the following config on XR version 4.3.2 and later to configure the sub-tlv id

pw-class <pw-name>

   encapsulation mpls


   flow-label both

  flow-label code 17

NOTE: Got a lot of questions regarding the confusion about the statement of 0x11 to 0x17 change (as driven by IANA) and the config requirement for number 17 in this example.

The crux is that the flow label code is configured DECIMAL, and the IANA/DRAFT numbers mentioned are HEX.

So 0x11, the old value is 17 decimal, which indeed is very similar to 0x17 which is the new IANA assigned number. Very annoying, thank IANA

(or we could have made the knob in hex I guess )


Loadbalancing and priority configurations

In the case of VPWS or VPLS, at the ingress PE side, it’s possible to change the load-balance upstream to MPLS Core in three different ways:


1. At the L2VPN sub-configuration mode with “load-balancing flow” command with the following options:


RP/0/RSP1/CPU0:ASR9000(config-l2vpn)# load-balancing flow ?


  src-dst-mac [default]


2. At the pw-class sub-configuration mode with “load-balancing” command with the following options:



  flow-label [see FAT Pseudowire section]

  pw-label [per-VC load balance]


3. At the Bundle interface sub-configuration mode with “bundle load-balancing hash” command with the following options:


RP/0/RSP1/CPU0:ASR9000(config-if)#bundle load-balancing hash ? [For default, see previous sections]




It’s important to not only understand these commands but also that: 1 is weaker than 2 which is weaker than 3.




load-balancing flow src-dst-ip 

pw-class FAT


encapsulation mpls



transport-mode ethernet




flow-label both static

interface Bundle-Ether1


bundle load-balancing hash dst-ip


Because of the priorities, on the egress side of the ingress PE (to the MPLS Core), we will do per-dst-ip load-balance (3).

If the bundle-specific configuration is removed, we will do per-VC load-balance (2).

If the pw-class load-balance configuration is removed, we will do per-src-dst-ip load-balance (1).

with thanks to Bruno Oliveira for this priority section

P2MP MPLS TE Tunnels

Only one bundle member will be selected to forward traffic on the P2MP MPLS TE mid-point node.

Possible alternatives that would achieve better load balancing are: a) increase the number of tunnels or b) switch to mLDP.


Pre 4.2.0 releases, for the ipv6 hash calculation we only use the last 64 bits of the address to fold and feed that into the hash, this including the regular routerID and L4 info.

In 4.2.0 we made some further enhancements that the full IPv6 Addr is taken into consideration with L4 and router ID.

Determining load-balancing

You can determine the load-balancing on the router by using the following commands


For IP :

RP/0/RSP0/CPU0:A9K-BNG#show cef exact-route protocol udp ?

  source-port  Set source port

You have the ability to only specify L3 info, or include L4 info by protocol with source and destination ports.

It is important to understand that the 9k does FLOW based hashing, that is, all packets belonging to the same flow will take the same path.

If one flow is more active or requires more bandwidth then another flow, path utilization may not be a perfect equal spread.

UNLESS you provide enough variation in L3/L4 randomness, this problem can't be alleviated and is generally seen in lab tests due the limited number of flows.

For MPLS based hashing :

RP/0/RSP0/CPU0:A9K-BNG#sh mpls forwarding exact-route label 1234 bottom-label 16000 ... location 0/1/cpu0

This command gives us the output interface chosen as a result of hashing with mpls label 16000. The bottom-label (in this case '16000') is either the VC label (in case of PW L2 traffic) or the bottom label of mpls stack (in case of mpls encapped L3 traffic with more than 4 labels). Please note that for regular mpls packets (with <= 4 labels) encapsulating an L3 packet, only IP based hashing is performed on the underlying IP packet.

Also note that the mpls hash algorithm is different for trident and typhoon. The varied the label is the better is the distribution. However, in case of trident there is a known behavior of mpls hash on bundle interfaces. If a bundle interface has an even number of member links, the mpls hash would cause only half of these links to be utlized. To get around this, you may have to configure "cef load-balancing adjust 3" command on the router. Or use odd number of member links within the bundle interface. Note that this limitation applies only to trident line cards and not typhoon.

Bundle member selection

RP/0/RSP0/CPU0:A9K-BNG#bundle-hash bundle-e 100 loc 0/0/cPU0

Calculate Bundle-Hash for L2 or L3 or sub-int based: 2/3/4 [3]: 3

Enter traffic type (1.IPv4-inbound, 2.MPLS-inbound, 3:IPv6-inbound): [1]: 1

Single SA/DA pair or range: S/R [S]:

Enter source IPv4 address []:

Enter destination IPv4 address []:

Compute destination address set for all members? [y/n]: y

Enter subnet prefix for destination address set: [32]:

Enter bundle IPv4 address []:

Enter L4 protocol ID. (Enter 0 to skip L4 data) [0]:

Invalid protocol. L4 data skipped.

Link hashed [hash_val:1] to is GigabitEthernet0/0/0/19 LON 1 ifh 0x4000580

The hash type L2 or L3 depends on whether you are using the bundle Ethernet interface as an Attachment Circuit in a Bridgedomain or VPWS crossconnect, or whether the bundle ether is used to route over (eg has an IP address configured).


Polarization pertains mostly to ECMP scenarios and is the effect of routers in a chain making the same load-balancing decision.

The following picture tries to explain that.

Screen Shot 2012-08-28 at 11.11.32 AM.png

In this scenario we assume 2 bucket, 1 bit on a 7 bit hash result. Let's say that in this case we only look at bit-0. So it becomes an "EVEN" or "ODD" type decision. The routers in the chain have access to the same L3 and L4 fields, the only varying factor between them is the routerID.

In the case that we have RID's that are similar or close (which is not uncommon), the system may not provide enough variation in the hash result which eventually leads to subsequent routers to compute the same hash and therefor polarize to a "Southern" (in this example above) or "Northern" path.

In XR 4.2.1 via a SMU or in XR 4.2.3 in the baseline code, we provide a knob that allows for shifting the hash result. By choosing a different "shift" value per node, we can make the system look at a different bit (for this example), or bits.

Screen Shot 2012-08-28 at 11.57.20 AM.png

In this example the first line shifts the hash by 1, the second one shifts it by 2.

Considering that we have more buckets in the real implementation and more bits that we look at, the member or path selection can alter significantly based on the same hash but with the shifting, which is what we ultimately want.

HASH result Shifting

  • Trident allows for a shift of maximum of 4 (performance reasons)
  • Typhoon allows for a shift of maximum of 32.


cef load-balancing algorithm adjust <value>

The command allows for values larger then 4 on Trident, if you configure values large then 4 for Trident, you will effectively use a modulo, resulting in the fact that shift of 1 is the same as a shift of 5

Fragmentation and Load-balancing

When the system detects fragmented packets, it will no longer use L4 information. The reason for that is that if L4 info were to be used, and subsequent fragments don't contain the L4 info anymore (have L3 header only!) the initial fragment and subsequent fragments produce a different hash result and potentially can take different paths resulting in out of order.

Regardless of release, regardless of hardware (ASR9K or CRS), when fragmentation is detected we only use L3 information for the hash computation.

Hashing updates

- Starting release 6.4.2, when an layer 2 interface (EFP) receives mpls encapped ip packets, the hashing algorithm if configured for src-dest-ip will pick up ip from ingress packet to create a hash. Before 6.4.2 the Hash would be based on MAC.

- Starting XR 6.5, layer 2 interfaces receiving GTP encapsulated packets will automatically pick up the TEID to generate a hash when src-dest-ip is configured.

Related Information

Xander Thuijs, CCIE #6775

Sr Tech Lead ASR9000


So now you've fooled the P into thinking it's forwarding an IPv4 packet, if you want to have it forward over multiple links you'll need to create some entropy in the fields that it considers to be the fundaments of an IP packet.  In this scenario obviously the frame merely has to have entropy at the offsets typical of an IP packet.

Cisco Employee

correct, just letting the mac start with a 4 or 6 by itself doesn't do it 100% of course.

you need to offset that with the location where the destination ip address or source ip address is located and map that to where that would meet in the ether header.

the next thing is also that it could totally be that eventhough you varied some of the fields that they still take the same link since though different hashes, still mapping to the same member or path.

To increase the chances of taking a different path, add some more vlans between the PE and P router. and make sure the T-LDP destination is found over those paths also obviously.


Community Member

Hi Alexander, 

is Pseudo-Wire fragmentation supported on ASR9000? We don't know what would happen to a frame  if MTU on the core facing interface is lower than MTU on the customer facing interface ? I have not found any documents relating to that for this platform and not sure how to check it when bringing up a PW for a Customer.



Cisco Employee

hey meh-d!

there is no frag supported in hw at all. only in the sw path. for bng only we have spp based (that is interrupt based, 10kpps) frag support for subscribers (pppoe).

for any other application, like pw-ether it is processed based (netio) with low performance and no features.

summary, in general taht is also, always prevent frag.

happy easter!



Community Member

Thanks Xander for your prompt response. Does your response  mean ,if a PW frame is larger than MTU size defined on Core facing interface ,the frame will be silently discarded ?




Cisco Employee

oh I meant to say that if a packet requires fragmentation, and that is always detected on the egress LC then the packet gets punted to the egress LC CPU, where netio will frag the pak and reinject the frags back in, but egress features will not get applied (eg out-qos).

frag needed punts on the egress LC is by default limited to 1000pps by LPTS.




Hi Alexander,

I have two phisycal links, balancing is working fine, no problem.

Capacity Planning says we need to add one more link. New pseudowires will see 3 ways

What about pws currently established, how could I force a redistribution... rebalancing... clearing ldp neigh?... or shuting down interfaces?... clear xconnect command ?

Which one takes the less time.. less impact to service??






Cisco Employee

hi david,

when you add a new link it will result in that the buckets are being redistributed over the new number of members.

so a PW today that takes member1 *may* take member 2 or 3 later on.

assuming you have enough variation in in the pw labels, a good 33/33/33 distribution will be achieved.

you can also consider fAT labels to provide some per flow distribution.

so you dont need to clear labels or peerings when you add new members, the new distribution will automatically occur.





Thanks Xander for the quick answer,


So, if we do nothing eventually we'll get 33/33/33, but if it is a not very dynamic network... even with FAT.


How could I force, as soon as we have new link, to redistribute pseudowires?




Cisco Employee

hey david, there is nothing that you have to configure to achieve that redistribution.

when the new member is added the 256 buckets (8bits) from the CRC-32 hash are now redistributed over 3 members instead of 2. That all goes automatically.

If you see uneven loadbalancing on your PW's the only control you have there are:

1) on PE's you can configure loadbalancing shift to try to get better spread over your core mpls enabled bundle interface

2) on P routers, leverage FAT labels imposed on the PE's

3) on P routers, use cef shift to loadbalance some of the traffic if ther eis polarization.

check also the updates deck from cisco live san diego 2015 with more details on precisely this in relation to LB. also sanfran 2014 had some extra detail there.



Community Member

Hi Xander,


Thanks for covering this subject, it's very informative and I have learned a lot from it. We have large 240 Gig bundle (24xTenGig) and same interfaces are transmitting more traffic than others (40% of interfaces are 85% utilization while the remaining is only at 60%). Initially it was a mix of Trident/Typhoon line cards but we rearranged to make it all Trident however the spread has not improved (still 20% usage gap between interface members). 

We are running software version is 4.2.3, we have different type of traffic going across the bundle MPLS, IPv4 unicast and Multicast (only 2%) and issue is for transmitting bandwidth.  My goal is to reduce the spread as much as possible and maybe you could suggest how to go about this... The only option remaining would to apply the 'cef load-balancing adjust 3' but I'm not sure if this would solve my issue. 


Cisco Employee

hi samatar,

a 24 interface bundle! impressive! :)

so with 24 members and a 256 buckets (8 bits) out of a 32bit hash so you have a load of 10 buckets per member. That is not a lot, which only means that you need to have a LOT of variation to produce enough spread on the hash/buckets to achieve a more equal load.

Generally skewed or uneven balancing occurs due to that and that suspicion can be substantiated by the fact if there is a single (set) of destinations or originating from a single (set of) sources.

If that is the case then indeed the cef shift will help trying to produce a more even spread. but the value for that will be trial and error.

typhoon/trident in that regard doesnt matter.

also if there is mpls in play, if that is mpls/ip then no issue we'll do L3/L4 still, but if there are PW's then the LSR's/P routers will resort back to label based balancing which always will skew the LB results especially in case of this high number of members.



Community Member

Thanks for the prompt reply Xander. 

How did you determine only 8 bits were used for hashing? We are running software version 4.2.3 and as per your article for Trident based line cards hash value should be 32 "Post 4.2.x Trident uses a checksum based calculation resulting in a 32 bit hash value"

And since it's 24 members shouldn't use 5 bits? And 98% of traffic going across this link is IPv4 unicast destine for internet (remaining 2% is for multicast) that should provide large sets of variations for L3/L4 destination.  

20% spread on 10Gig interfaces multiply for about 15 members leaves about 30 Gig available bandwidth. I'll add one more member to make an ODD number of interfaces and use the cef load-balance command to see if that would yield to better load balancing results. 

Cisco Employee

how I determine that 8 bits are used out of the crc32? well, if you write the code one can only hope that you know what you wrote right? :)

so out of that CRC32, 32 bits, 8 bits are taken to for bundle, a different 8 bits are used for ECMP.

with the 8 bits taht we take, we can identify 256 buckets.

these buckets are then distributed over the members (or paths).

so with 24 members you have 16 members that have 11 buckets and 8 members that have 10 buckets.

with 24 members (240G) each bucket is worth 0.93G. Which means that some members, 8 of them wont be loaded 100% (930M tops) so you can expect some uneveness here, but not to the extent you're seeing it.

What you have is just that there is polarization going whereby certain buckets are favored more. and that is the direct result of the traffic variation.

and that you can influence by different hash calc schemes, like with the shfit command, or resorting to L3 dest only or 3 tuple or using 5 tuple etc. cef shift is easiest to try with limited impact.

Trident can only shift up to 4 positions.

another option is to use 2 bundles of 12 members, this way you have ecmp and bundle hashing so using more bits out of the crc32. but that is an operational challenge to implement, so first less mess with shfiting.


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You wrote the code eh... that means I am talking to the right person :) but thanks again Xander, I only asked to clear confusion and this blog really helps with future planning regarding bundles.

I'll propose the cef shifting for better load balancing and I will let you know the outcome once it's implemented (hopefully better results) Thanks again for shedding light on this.