Table of Contents
Advertisement
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-flow options in the
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
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-robin ECMP 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-flow
ECMP 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
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
ip equal-cost multi-path {round-robin | per-flow}
Figure
show ip route
5-10. On the Central XSR, next hops 1.1.1.2 and
command.
command which
Advertisement
Table of Contents
Troubleshooting
