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diff --git a/doc/user/wecmp_linkbw.rst b/doc/user/wecmp_linkbw.rst new file mode 100644 index 0000000..4df6559 --- /dev/null +++ b/doc/user/wecmp_linkbw.rst @@ -0,0 +1,297 @@ +.. _wecmp_linkbw: + +Weighted ECMP using BGP link bandwidth +====================================== + +.. _features-of-wecmp-linkbw: + +Overview +-------- + +In normal equal cost multipath (ECMP), the route to a destination has +multiple next hops and traffic is expected to be equally distributed +across these next hops. In practice, flow-based hashing is used so that +all traffic associated with a particular flow uses the same next hop, +and by extension, the same path across the network. + +Weighted ECMP using BGP link bandwidth introduces support for network-wide +unequal cost multipathing (UCMP) to an IP destination. The unequal cost +load balancing is implemented by the forwarding plane based on the weights +associated with the next hops of the IP prefix. These weights are computed +based on the bandwidths of the corresponding multipaths which are encoded +in the ``BGP link bandwidth extended community`` as specified in +[Draft-IETF-idr-link-bandwidth]_. Exchange of an appropriate BGP link +bandwidth value for a prefix across the network results in network-wide +unequal cost multipathing. + +One of the primary use cases of this capability is in the data center when +a service (represented by its anycast IP) has an unequal set of resources +across the regions (e.g., PODs) of the data center and the network itself +provides the load balancing function instead of an external load balancer. +Refer to [Draft-IETF-mohanty-bess-ebgp-dmz]_ and :rfc:`7938` for details +on this use case. This use case is applicable in a pure L3 network as +well as in a EVPN network. + +The traditional use case for BGP link bandwidth to load balance traffic +to the exit routers in the AS based on the bandwidth of their external +eBGP peering links is also supported. + + +Design Principles +----------------- + +Next hop weight computation and usage +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +As described, in UCMP, there is a weight associated with each next hop of an +IP prefix, and traffic is expected to be distributed across the next hops in +proportion to their weight. The weight of a next hop is a simple factoring +of the bandwidth of the corresponding path against the total bandwidth of +all multipaths, mapped to the range 1 to 100. What happens if not all the +paths in the multipath set have link bandwidth associated with them? In such +a case, in adherence to [Draft-IETF-idr-link-bandwidth]_, the behavior +reverts to standard ECMP among all the multipaths, with the link bandwidth +being effectively ignored. + +Note that there is no change to either the BGP best path selection algorithm +or to the multipath computation algorithm; the mapping of link bandwidth to +weight happens at the time of installation of the route in the RIB. + +If data forwarding is implemented by means of the Linux kernel, the next hop’s +weight is used in the hash calculation. The kernel uses the Hash threshold +algorithm and use of the next hop weight is built into it; next hops need +not be expanded to achieve UCMP. UCMP for IPv4 is available in older Linux +kernels too, while UCMP for IPv6 is available from the 4.16 kernel onwards. + +If data forwarding is realized in hardware, common implementations expand +the next hops (i.e., they are repeated) in the ECMP container in proportion +to their weight. For example, if the weights associated with 3 next hops for +a particular route are 50, 25 and 25 and the ECMP container has a size of 16 +next hops, the first next hop will be repeated 8 times and the other 2 next +hops repeated 4 times each. Other implementations are also possible. + +Unequal cost multipath across a network +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +For the use cases listed above, it is not sufficient to support UCMP on just +one router (e.g., egress router), or individually, on multiple routers; UCMP +must be deployed across the entire network. This is achieved by employing the +BGP link-bandwidth extended community. + +At the router which originates the BGP link bandwidth, there has to be user +configuration to trigger it, which is described below. Receiving routers +would use the received link bandwidth from their downstream routers to +determine the next hop weight as described in the earlier section. Further, +if the received link bandwidth is a transitive attribute, it would be +propagated to eBGP peers, with the additional change that if the next hop +is set to oneself, the cumulative link bandwidth of all downstream paths +is propagated to other routers. In this manner, the entire network will +know how to distribute traffic to an anycast service across the network. + +The BGP link-bandwidth extended community is encoded in bytes-per-second. +In the use case where UCMP must be based on the number of paths, a reference +bandwidth of 1 Mbps is used. So, for example, if there are 4 equal cost paths +to an anycast IP, the encoded bandwidth in the extended community will be +500,000. The actual value itself doesn’t matter as long as all routers +originating the link-bandwidth are doing it in the same way. + + +Configuration Guide +------------------- + +The configuration for weighted ECMP using BGP link bandwidth requires +one essential step - using a route-map to inject the link bandwidth +extended community. An additional option is provided to control the +processing of received link bandwidth. + +Injecting link bandwidth into the network +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +At the "entry point" router that is injecting the prefix to which weighted +load balancing must be performed, a route-map must be configured to +attach the link bandwidth extended community. + +For the use case of providing weighted load balancing for an anycast service, +this configuration will typically need to be applied at the TOR or Leaf +router that is connected to servers which provide the anycast service and +the bandwidth would be based on the number of multipaths for the destination. + +For the use case of load balancing to the exit router, the exit router should +be configured with the route map specifying the a bandwidth value that +corresponds to the bandwidth of the link connecting to its eBGP peer in the +adjoining AS. In addition, the link bandwidth extended community must be +explicitly configured to be non-transitive. + +The complete syntax of the route-map set command can be found at +:ref:`bgp-extended-communities-in-route-map` + +This route-map is supported only at two attachment points: +(a) the outbound route-map attached to a peer or peer-group, per address-family +(b) the EVPN advertise route-map used to inject IPv4 or IPv6 unicast routes +into EVPN as type-5 routes. + +Since the link bandwidth origination is done by using a route-map, it can +be constrained to certain prefixes (e.g., only for anycast services) or it +can be generated for all prefixes. Further, when the route-map is used in +the neighbor context, the link bandwidth usage can be constrained to certain +peers only. + +A sample configuration is shown below and illustrates link bandwidth +advertisement towards the "SPINE" peer-group for anycast IPs in the +range 192.168.x.x + +.. code-block:: frr + + ip prefix-list anycast_ip seq 10 permit 192.168.0.0/16 le 32 + route-map anycast_ip permit 10 + match ip address prefix-list anycast_ip + set extcommunity bandwidth num-multipaths + route-map anycast_ip permit 20 + ! + router bgp 65001 + neighbor SPINE peer-group + neighbor SPINE remote-as external + neighbor 172.16.35.1 peer-group SPINE + neighbor 172.16.36.1 peer-group SPINE + ! + address-family ipv4 unicast + network 110.0.0.1/32 + network 192.168.44.1/32 + neighbor SPINE route-map anycast_ip out + exit-address-family + ! + + +Controlling link bandwidth processing on the receiver +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +There is no configuration necessary to process received link bandwidth and +translate it into the weight associated with the corresponding next hop; +that happens by default. If some of the multipaths do not have the link +bandwidth extended community, the default behavior is to revert to normal +ECMP as recommended in [Draft-IETF-idr-link-bandwidth]_. + +The operator can change these behaviors with the following configuration: + +.. clicmd:: bgp bestpath bandwidth <ignore | skip-missing | default-weight-for-missing> + +The different options imply behavior as follows: + +- ignore: Ignore link bandwidth completely for route installation + (i.e., do regular ECMP, not weighted) +- skip-missing: Skip paths without link bandwidth and do UCMP among + the others (if at least some paths have link-bandwidth) +- default-weight-for-missing: Assign a low default weight (value 1) + to paths not having link bandwidth + +This configuration is per BGP instance similar to other BGP route-selection +controls; it operates on both IPv4-unicast and IPv6-unicast routes in that +instance. In an EVPN network, this configuration (if required) should be +implemented in the tenant VRF and is again applicable for IPv4-unicast and +IPv6-unicast, including the ones sourced from EVPN type-5 routes. + +A sample snippet of FRR configuration on a receiver to skip paths without +link bandwidth and do weighted ECMP among the other paths (if some of them +have link bandwidth) is as shown below. + +.. code-block:: frr + + router bgp 65021 + bgp bestpath as-path multipath-relax + bgp bestpath bandwidth skip-missing + neighbor LEAF peer-group + neighbor LEAF remote-as external + neighbor 172.16.35.2 peer-group LEAF + neighbor 172.16.36.2 peer-group LEAF + ! + address-family ipv4 unicast + network 130.0.0.1/32 + exit-address-family + ! + + +Stopping the propagation of the link bandwidth outside a domain +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The link bandwidth extended community will get automatically propagated +with the prefix to EBGP peers, if it is encoded as a transitive attribute +by the originator. If this propagation has to be stopped outside of a +particular domain (e.g., stopped from being propagated to routers outside +of the data center core network), the mechanism available is to disable +the advertisement of all BGP extended communities on the specific peering/s. +In other words, the propagation cannot be blocked just for the link bandwidth +extended community. The configuration to disable all extended communities +can be applied to a peer or peer-group (per address-family). + +Of course, the other common way to stop the propagation of the link bandwidth +outside the domain is to block the prefixes themselves from being advertised +and possibly, announce only an aggregate route. This would be quite common +in a EVPN network. + +BGP link bandwidth and UCMP monitoring & troubleshooting +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Existing operational commands to display the BGP routing table for a specific +prefix will show the link bandwidth extended community also, if present. + +An example of an IPv4-unicast route received with the link bandwidth +attribute from two peers is shown below: + +.. code-block:: frr + + CLI# show bgp ipv4 unicast 192.168.10.1/32 + BGP routing table entry for 192.168.10.1/32 + Paths: (2 available, best #2, table default) + Advertised to non peer-group peers: + l1(swp1) l2(swp2) l3(swp3) l4(swp4) + 65002 + fe80::202:ff:fe00:1b from l2(swp2) (110.0.0.2) + (fe80::202:ff:fe00:1b) (used) + Origin IGP, metric 0, valid, external, multipath, bestpath-from-AS 65002 + Extended Community: LB:65002:125000000 (1000.000 Mbps) + Last update: Thu Feb 20 18:34:16 2020 + + 65001 + fe80::202:ff:fe00:15 from l1(swp1) (110.0.0.1) + (fe80::202:ff:fe00:15) (used) + Origin IGP, metric 0, valid, external, multipath, bestpath-from-AS 65001, best (Older Path) + Extended Community: LB:65001:62500000 (500.000 Mbps) + Last update: Thu Feb 20 18:22:34 2020 + +The weights associated with the next hops of a route can be seen by querying +the RIB for a specific route. + +For example, the next hop weights corresponding to the link bandwidths in the +above example is illustrated below: + +.. code-block:: frr + + spine1# show ip route 192.168.10.1/32 + Routing entry for 192.168.10.1/32 + Known via "bgp", distance 20, metric 0, best + Last update 00:00:32 ago + * fe80::202:ff:fe00:1b, via swp2, weight 66 + * fe80::202:ff:fe00:15, via swp1, weight 33 + +For troubleshooting, existing debug logs ``debug bgp updates``, +``debug bgp bestpath <prefix>``, ``debug bgp zebra`` and +``debug zebra kernel`` can be used. + +A debug log snippet when ``debug bgp zebra`` is enabled and a route is +installed by BGP in the RIB with next hop weights is shown below: + +.. code-block:: frr + + 2020-02-29T06:26:19.927754+00:00 leaf1 bgpd[5459]: bgp_zebra_announce: p=192.168.150.1/32, bgp_is_valid_label: 0 + 2020-02-29T06:26:19.928096+00:00 leaf1 bgpd[5459]: Tx route add VRF 33 192.168.150.1/32 metric 0 tag 0 count 2 + 2020-02-29T06:26:19.928289+00:00 leaf1 bgpd[5459]: nhop [1]: 110.0.0.6 if 35 VRF 33 wt 50 RMAC 0a:11:2f:7d:35:20 + 2020-02-29T06:26:19.928479+00:00 leaf1 bgpd[5459]: nhop [2]: 110.0.0.5 if 35 VRF 33 wt 50 RMAC 32:1e:32:a3:6c:bf + 2020-02-29T06:26:19.928668+00:00 leaf1 bgpd[5459]: bgp_zebra_announce: 192.168.150.1/32: announcing to zebra (recursion NOT set) + + +References +---------- + +.. [Draft-IETF-idr-link-bandwidth] <https://tools.ietf.org/html/draft-ietf-idr-link-bandwidth> +.. [Draft-IETF-mohanty-bess-ebgp-dmz] <https://tools.ietf.org/html/draft-mohanty-bess-ebgp-dmz> + |