Route load balancing

Unicast Load balancing

This cisco document has a good discussion on load balancing. In summary:

The number of paths used is limited by the number of entries the routing protocol puts in the routing table. Four entries is the default in IOS for most IP routing protocols with the exception of Border Gateway Protocol (BGP), where one entry is the default. Six different paths configured is the maximum number.

The IGRP and EIGRP routing processes also support unequal cost load-balancing.

show ip route command output to a particular subnet that has multiple routes. Notice there are two routing descriptor blocks. Each block is one route. There is also an asterisk (*) next to one of the block entries. This corresponds to the active route that is used for new traffic. The term ‘new traffic’ corresponds to a single packet or an entire flow to a destination, depending on the type of switching configured.

  • For process-switching—load balancing is on a per-packet basis and the asterisk (*) points to the interface over which the next packet is sent.
  • For fast-switching—load balancing is on a per-destination basis and the asterisk (*) points to the interface over which the next destination-based flow is sent.

    M2515-B# show ip route 1.0.0.0
    Routing entry for 1.0.0.0/8
    Known via "rip", distance 120, metric 1
    Redistributing via rip
    Advertised by rip (self originated)
    Last update from 192.168.75.7 on Serial1, 00:00:00 ago
    Routing Descriptor Blocks:
    * 192.168.57.7, from 192.168.57.7, 00:00:18 ago, via Serial0
    Route metric is 1, traffic share count is 1
    192.168.75.7, from 192.168.75.7, 00:00:00 ago, via Serial1
    Route metric is 1, traffic share count is 1
     
     

    BGP Multipath does not affect bestpath selection. For example, a router still designates one of the paths as the best path, according to the algorithm, and advertises this best path to its neighbors.
    These are the BGP Multipath features:

  • eBGP Multipath—maximum-paths n
  • iBGP Multipath—maximum-paths ibgp n
  • eiBGP Multipathmaximum-paths eibgp n

    R5#show ip bgp vpnv4 vrf VPN_A 0.0.0.0
    BGP routing table entry for 100:1:0.0.0.0/0, version 9
    Paths: (2 available, best #1, table VPN_A)
    Multipath: eiBGP
    Flag: 0x820
    Not advertised to any peer
    103 101
    10.3.3.3 (metric 30) from 10.3.3.3 (10.3.3.3)
    Origin incomplete, metric 0, localpref 100, valid, internal, multipath, best
    Extended Community: RT:100:1
    mpls labels in/out nolabel/20
    107 106
    10.7.7.7 (metric 30) from 10.7.7.7 (10.7.7.7)
    Origin incomplete, metric 0, localpref 100, valid, internal, multipath
    Extended Community: RT:100:1
    mpls labels in/out nolabel/29

Multicast Load balancing

Having said this, when you are running PIM multicast in the network and you have two (or more) equal cost paths back to the  network, PIM will choose the path with this highest ip address. It will NOT load balance the traffic. The main reason for this is to prevent traffic from looping around.

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