Improving LSP Scaling

RSVP maintains path and reserve state by periodically sending refresh messages.

Refresh messages allow each LSR along the path to properly maintain reservation state information and to recover from network failures. Because refresh messages are periodically sent for each path reservation, scaling the number of RSVP-TE LSPs is an issue. Additionally, network requirements for faster failure detection and improved LSP recovery times further exacerbate the scaling issue.

Several techniques are described in RFC 2961 RSVP Refresh Overhead Reduction to improve the scalability of RSVP. These techniques include the bundle message, message ID extension, and summary refresh extension. Support for these extensions is signaled between RSVP peers via the refresh-reduction-capable bit in the flags field of the common RSVP header. Additionally, the hello extension, described in RFC 3209, provides a fourth scaling mechanism for RSVP. The hello extension is designed so that either peer can use the mechanism regardless of how the other peer is configured. Therefore, support for the hello extension is not signaled between RSVP peers. The ExtremeXOS software supports and is compliant with the RSVP-TE scaling features described in RFC 2961.

These features include the following:

Bundle Message: RSVP bundle messages aggregate multiple RSVP messages within a single PDU. The messages are addressed directly to peer LSRs. Therefore, bundle messages are not sent with the IP Router Alert option. Bundling multiple RSVP messages into a single PDU reduces the per packet overhead associated with local delivery and local origination. Each bundle message must contain at least one RSVP message. Transmission of RSVP messages may be delayed up to the number of seconds configured for bundle time. The size of the bundle message is limited to the RSVP-TE interface MTU size. Bundle messaging is enabled using the enable mpls rsvp-te bundle-message command.

Summary Refresh Extension: A summary refresh message is used to refresh RSVP states along an LSP without having to explicitly send path and reserve refresh messages. This can substantially reduce the RSVP control bandwidth overhead. Summary refresh messages contain a list of message_ID objects. Each message_ID object identifies a path and reserve state to be refreshed. When summary refresh support is enabled, path and reserve refresh messages are suppressed. If the message identifier value indicates that the RSVP state has changed, the receiving LSR notifies the sender by transmitting a message_ID_NACK message. The summary refresh rate is enabled using the enable mpls rsvp-te summary-refresh command.

Message ID Extension: The message ID extension provides reliable delivery of RSVP messages. It also provides a simple mechanism for identifying refresh messages, which can greatly reduce refresh message processing on the receiving LSR. The message ID extension defines three new objects: message_ID, message_ID_ACK, and message_ID_NACK. The message_ID object contains a unique message identifier based on the sender's IP address. Only one message_ID object is inserted into an RSVP message. The receiving LSR can use the message_ID object to quickly refresh path and reserve states. If the message identifier value in the message_ID object is greater than the locally saved message identifier value, then the RSVP message represents a new or modified state. The receiving LSR must acknowledge an RSVP message using the message_ID_ACK object if the sender set the ACK_desired flag in the message_ID object, otherwise the message_ID acknowledgement is optional. The message_ID_ACK object may be included in any unrelated RSVP message or in an RSVP ACK message. Message ID extension is required for both bundle message and summary refresh, so this capability is automatically enabled if either of the other capabilities is enabled.

Hello Extension: The RSVP hello message provides a quick and simple mechanism for detecting the loss of a peer RSVP-TE LSR. The hello protocol is implemented using the RSVP soft-state model. RSVP hello messages may be enabled independently of each LSR peer. The hello protocol consists of two new objects: hello_request and hello_ACK. If configured, an LSR sends a hello_request every hello interval. If a hello_ACK is not received within a specified amount of time, the sending LSR assumes that its peer LSR is no longer active. Once a peer LSR is deemed inactive, all reservation states associated with LSPs established to or through the peer LSR must be freed and the LSPs torn down. The hello interval is configurable using the command configure mpls rsvp-te timers session hello-time.

You can improve LSP scaling by configuring the following RSVP-TE parameters:

Refresh Time: The refresh time specifies the interval for sending refresh path messages. RSVP refresh messages provide soft state link-level keep-alive information for previously established paths and enable the switch to detect when an LSP is no longer active. RSVP sessions are torn down if an RSVP refresh message is not received from a neighbor within [(keep-multiplier + 0.5) * 1.5 * refresh-time] seconds. The valid refresh time may be set to any value between 1 and 36000 seconds. The default setting is 30 seconds. Configuring a longer refresh time reduces both switch and network overhead.

Summary Refresh Time: The summary refresh time, specified in tenths of a second, indicates the time interval for sending summary refresh RSVP messages. The summary refresh time must be less than the configured refresh time. The default summary refresh time is zero, indicating that no summary refresh RSVP messages are sent. The summary refresh time value may be set to any value between zero to 100 (or 10 seconds). If configured, the bundled and summary refresh RSVP messages are only sent to RSVP-TE peers supporting RSVP refresh reduction.

Bundle Time: The bundle time, specified in tenths of a second, indicates the maximum amount of time a transmit buffer is held so that multiple RSVP messages can be bundled into a single PDU. The default bundle time is zero, indicating that RSVP message bundling is not enabled. The bundle time value can be set to any value between zero and 30 (or 3 seconds).