Vlan; Buffering And Flow Control; Figure 10-3 Buffering And Flow Control - Cisco ONS 15600 Reference Manual

10.4.3 VLAN

10.4.3 VLAN
The ASAP line card is transparent to VLAN tag information in the Ethernet frame.
VLANs are tunneled, and not terminated.
Note

10.5 Buffering and Flow Control

Because the STS circuits can often be oversubscribed (have a bandwidth lower than that needed to
support the traffic on the Ethernet port), a combination of buffering and local flow control is supported.
Initially, frames that arrive on an ingress interface that cannot be transmitted immediately on the egress
interface are placed in an ingress buffer. When this buffer starts filling up and is in danger of
overflowing, flow control is employed. Local flow control is flow control between the Ethernet interface
and the router or switch it is connected to, over the single Ethernet link. This is depicted in
Figure 10-3
Router
To prevent dropping of frames on an ingress Ethernet interface due to buffer congestion, an Ethernet
interface sends a PAUSE frame to its peer on its egress Ethernet interface. An Ethernet interface might
be capable of both sending and receiving PAUSE frames (symmetric flow control). On the other hand, it
might be capable of performing only one of the two (asymmetric flow control). Two factors determine
what an Ethernet interface ultimately ends up supporting:
•
•
The Ethernet interfaces are capable of using symmetric or asymmetric flow control to meter packet
receptions according to the requirements specified in IEEE 802.3x. This is done to avoid dropping
packets internally due to output or input queue congestion. When an Ethernet interface uses symmetric
flow control, it generates and obeys pause frames. When it uses asymmetric flow control, it only
generates pause frames. Through management control, the Ethernet interface allows an operator to turn
flow control ON or OFF (the default is ON).
When an Ethernet interface is set to be capable of generating PAUSE frames, it starts generating them
at a rate of X PAUSE frames per second when the ingress buffer passes above a certain level called the
High Water Mark. A PAUSE_TIME is specified in each PAUSE frame. The values of the PAUSE_TIME
parameter and the rate X parameter are determined by the system and are not user settable.
Cisco ONS 15600 Reference Manual, R8.0
10-6
Buffering and Flow Control
15xxx
Flow Control
- PAUSE
frames
Flow control capability of the interface as provisioned by the operator (fixed at OFF or ON)
Flow control capability of the peer as determined by AutoNegotiation
Ethernet links
15xxx
SONET network
Flow Control
Chapter 10 Ethernet Operation
Router
- PAUSE
frames
Figure 10-3.