Information Technology Reference
In-Depth Information
Unequal weight load sharing is also possible. The weight is assigned by the routing
protocols with different traffic share counts. IOS currently supports a maximum of eight
paths per prefix.
Per-session load sharing has the potential problem of traffic polarization. In other words,
traffic would always use the same link if the same hash function is used on all the routers.
A new algorithm is integrated into Cisco IOS software Releases 12.0(11)S2 and later to
allow a unique ID for each router. The ID is automatically generated or can be fixed with
the optional keyword id. The ultimate goal is that the hash functions in each router are
completely different and independent.
ip cef load-sharing algorithm universal [id]
To see the ID, use
show ip cef detail
. Example 2-8 shows a snapshot of the output.
CEF Detail
Example 2-8
router#show ip cef detail
IP CEF with switching (Table Version 5), flags=0x0
5 routes, 0 reresolve, 0 unresolved (0 old, 0 new), peak 0
8 leaves, 11 nodes, 12400 bytes, 8 inserts, 0 invalidations
0 load sharing elements, 0 bytes, 0 references
universal per-destination load sharing algorithm, id 24D5ED01
2(0) CEF resets, 0 revisions of existing leaves
Resolution Timer: Exponential (currently 1s, peak 1s)
0 in-place/0 aborted modifications
refcounts: 796 leaf, 795 node
id 24D5ED01
Per-Packet Load Sharing
Per-packet load sharing allows the router to send successive data packets over paths without
regard to individual hosts or user sessions. It uses the round-robin method to determine
which path each packet takes to the destination. Per-packet load sharing ensures more even
balancing over multiple links.
Per-packet load sharing is most effective when the bulk of the data passing through parallel
links is for a single session. Per-session load sharing in this case overloads a single link
while other links have very little traffic. Enabling per-packet load sharing allows you to use
alternative paths for the same busy session.
Although path utilization with per-packet load sharing is better, packets for a given source-
destination host pair may take different paths. This can introduce reordering of packets,
which might be inappropriate for certain types of data traffic that depend on packets
arriving at the destination in sequence, such as voice over IP traffic.
