Enterasys Networks X-PeditionTM manual Equal-Cost Multi-Path Ecmp, Configuration Considerations

IP Routing Protocols

Equal-Cost Multi-Path (ECMP)

Equal-Cost Multi-Path (ECMP) is a technique to forward packets along multiple paths of equal cost, aggregating multiple physical links into one virtual link to effectively increase the total bandwidth of a connection. Internally, the XSR decides which next hop to use in the event that more than one choice is available in the forwarding table and by searching this table, the forwarding engine identifies paths by the next hop.

The XSR offers two methods to calculate the next hop, described as round robin (per packet) and per-flowoptions in the ip equal-costmulti-path{round-robin per-flow}command. They are characterized as follows:

•Per packet round robin - This selection method has the advantage of bearing very little impact on performance and not requiring much time for computation as well as offering perfect load balancing. Disadvantages include disruptions caused by flow-based traffic; a variable path MTU where the overall path MTU might change on a packet-by-packet basis, thus negating the usefulness of path MTU discovery; variable latencies with negative implications on TCP traffic, where packets might be received out of order; and less reliable debugging utilities (ping, traceroute) which may produce wrong results.

•Per flow round robin - this selection method is nondisruptive and lends itself to session-based traffic. It configures all packets belonging to a certain flow (as determined by source/ destination IP address and protocol number) to take the same path and directs each new flow toward the next available next hop. Disadvantages include a possible detrimental impact on performance depending on the algorithm selected and unbalanced traffic flows owing to the nature of the method.

ECMP is enabled globally so it applies to static, BGP and OSPF routes as well as all interfaces. If enabled, a maximum of three paths is allowed to a particular destination. If ECMP is disabled, one path to a particular destination is permitted.

You can display configured equal cost routes by using the show ip route command which describes routing descriptor blocks, with one for each route. An asterisk placed (*) next to a block entry corresponds to the active route used for new traffic.

Configuration Considerations

When configuring ECMP on the XSR, keep in mind the following considerations:

•We highly recommend you employ ECMP over similar physical interfaces. If you enable round-robinECMP over dissimilar physical links and those link speeds differ, results might be unpredictable because the order of incoming packets may be reversed. If you enable per-flowECMP in this instance, packet order will not be a problem but balancing will be inefficient.

•The XSR supports ECMP over VPN but without tunnel balancing in circumstances where two or more tunnels share the same peer at equal cost. For example, ECMP is not supported if two tunnels are created from a remote XSR to a central XSR, each through different ISPs, with both tunnels sharing the same peer.

•The XSR does support load balancing in a VPN topology where an ECMP route is configured on an XSR to different peers, as shown in Figure 5-10. On the Central XSR, next hops 1.1.1.2 and 1.1.1.3 are selected as the ECMP route through the VPN1 interface and static routes are configured over FastEthernet interfaces to Peer1 and Peer2. In this scenario, dual levels of routing are performed: one at the virtual interface level where ECMP applies and two, at the physical interface level where static routes are used.

5-34 Configuring IP