The Link Bundling feature allows you to group multiple point-to-point links together into one logical link and provide higher bidirectional bandwidth, redundancy, and load balancing between two routers. A virtual interface is assigned to the bundled link. The component links can be dynamically added and deleted from the virtual interface.
The virtual interface is treated as a single interface on which one can configure an IP address and other software features used by the link bundle. Packets sent to the link bundle are forwarded to one of the links in the bundle.
A link bundle is simply a group of ports that are bundled together and act as a single link. The advantages of link bundles are as follows:
Multiple links can span several line cards to form a single interface. Thus, the failure of a single link does not cause a loss of connectivity.
Bundled interfaces increase bandwidth availability, because traffic is forwarded over all available members of the bundle. Therefore, traffic can flow on the available links if one of the links within a bundle fails. Bandwidth can be added without interrupting packet flow.
All the individual links within a single bundle must be of the same type and the same speed.
Cisco IOS XR software supports these methods of forming bundles of Ethernet interfaces:
IEEE 802.3ad—Standard technology that employs a Link Aggregation Control Protocol (LACP) to ensure that all the member links in a bundle are compatible. Links that are incompatible or have failed are automatically removed from a bundle.
EtherChannel —Cisco proprietary technology that allows the user to configure links to join a bundle, but has no mechanisms to check whether the links in a bundle are compatible.
Load balancing is supported on all links in the bundle. Load balancing function is a forwarding mechanism to distribute traffic over multiple links based on Layer 3 routing information in the router. There are two types of load balancing schemes:
Per-Destination Load Balancing
Per-Packet Load Balancing
When a traffic stream arrives at the router, per-packet load balancing allows the traffic to be evenly distributed among multiple equal cost links. Per-packet schemes make routing decision based on round-robin techniques, regardless of the individual source-destination hosts.
Only Per-Destination Load Balancing is supported.
Per-destination load balancing allows the router to distribute packets over one of the links in the bundle to achieve load sharing. The scheme is realized through a hash calculating based on the source-destination address and user sessions.
When the per-destination load balancing is enabled, all packets for a certain source-destination pair will go through the same link, though there are multiple links available. In other words, per-destination load balancing can ensure that packets for a certain source-destination pair could arrive in order.
By default, load balancing on Layer 2 link bundles is done based on the MAC SA/DA fields in the packet header. Layer 3 load balancing for link bundles is done on Ethernet Flow Points (EFPs) and is based on the IPv4 source and destination addresses in the packet.When Layer 3 service-specific load balancing is configured, all egressing bundles are load balanced based on the IPv4 source and destination addresses. When packets do not have IPv4 addresses, default load-balancing is used.
Layer 3 load balancing for link bundles is enabled globally, using the following command:
All Quality of Service (QoS) features, currently supported on physical interfaces and subinterfaces, are also supported on all Link Bundle interfaces and subinterfaces. QoS is configured on Link Bundles in the same way that it is configured on individual interfaces. However, the following points should be noted:
When a QoS policy is applied on a bundle (ingress or egress directions), the policy is applied at each member interface. Any queues and policers in the policy map (ingress or egress directions) will be replicated on each bundle member.
If a QoS policy is not applied to a bundle interface or bundle VLAN, both the ingress and egress traffic will use the per link members port default queue.
Link bundle members may appear across multiple Network Processing Units and linecards. The shape rate specified in the bundle policymap is not an aggregate for all bundle members. The shape rate applied to the bundle will depend on the load balancing of the links. For example, if a policy map with a shape rate of 10 Mbps is applied to a bundle with two member links, and if the traffic is always load-balanced to the same member link, then an overall rate of 10 Mbps will apply to the bundle. However, if the traffic is load-balanced evenly between the two links, the overall shape rate for the bundle will be 20 Mbps.
Example 1 shows how a traffic policy is applied on an Ethernet link bundle, in the ingress direction. The policy is applied to all interfaces that are members of the Ethernet link bundle.
Example 1 Applying a Traffic Policy to an Ethernet Link Bundle
interface Bundle-Ether bundle-id
service-policy input policy-1
QoS for POS link bundling
For POS link bundles, percentage-based bandwidth is supported for policers and output queues. Time-based queue limit is supported for output queues.
For input QoS, queuing is not supported and thus bandwidth is used for policer only. As a member link is added or removed from a bundle with input QoS configured, the aggregate bundle bandwidth for that affected line card will change. One input QoS policy instance is assigned for each SIP 700 line card that is part of the POS link bundle.
Output QoS Policy setup
Example 2 : Output QoS policy supported on POS link bundles
When a member link is added to a bundle with output QoS configured, the policy-map of the bundle is applied to the member link.
Example 2 shows the output QoS policy supported on POS link bundles.
priority level 1
police rate percent 10
bandwidth percent 30
queue-limit 100 ms
queue-limit 100 ms
These sections provide references related to implementing QoS on Link Bundles.
No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.
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