Cisco 3550-24 - Catalyst SMI Switch Технічний паспорт - Сторінка 4

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provider 802.1Q tag is removed when the packets exit their network, leaving the original packets untouched. Other

Layer 2 protocols such as STP, VLAN Trunk Protocol (VTP), and Cisco Discovery Protocol will also be tunneled

through the service provider's network. The 802.1Q tunneling feature enables service providers to use a single VLAN

(up to 4096 VLAN IDs) to support customers who have multiple VLANs. In addition, the feature preserves

customers' original VLAN IDs and segregates trafﬁc from different customers within the service provider

infrastructure even when those customers use the same VLAN IDs. As a result, enterprise customers subscribing to

TLS can transparently send and receive trafﬁc from other metro sites through the service provider network as though

the sites were within the same corporate LAN.

In addition to Layer 2 VPN services, Multi-VRF CE (Virtual Routing Forwarding Customer Edge, also called

VRF-lite) and Border Gateway Protocol (BGP) on the Cisco Catalyst 3550 will enable the creation of a Layer 3 VPN

service by keeping separate routing tables for each VPN customer. Multi-VRF CE is a feature that allows a service

provider to support two or more VPNs on a single 3550 switch, where IP addresses can be overlapped among the

VPNs. Multi-VRF CE uses input interfaces to distinguish routes for different VPNs and forms virtual

packet-forwarding tables by associating one or more Layer 3 interfaces with each VRF. Interfaces in a VRF can be

either physical, such as Ethernet ports, or logical, such as VLAN SVIs, but an interface cannot belong to more than

one VRF at any time.

The Cisco Catalyst 3550 offers superior Layer 3 granular QoS features to enable service providers to provide high-

quality services with service-level agreements (SLAs). The QoS features ensure that network trafﬁc is classiﬁed,

prioritized, and congestion is avoided in the most efﬁcient manner possible. The Cisco Catalyst 3550 can classify,

reclassify, police, and mark the incoming packets before the packet is placed in the shared buffer. Packet classiﬁcation

allows the network elements to discriminate between various trafﬁc ﬂows and enforce policies based on Layer 2 and

Layer 3 QoS ﬁelds.

To implement QoS, ﬁrst, the Cisco Catalyst 3550 switches identify trafﬁc ﬂows, or packet groups, and classify or

reclassify these groups using either the Differentiated Services Code Point (DSCP) ﬁeld or the 802.1p class-of-service

(CoS) ﬁeld, or both. Classiﬁcation and reclassiﬁcation can be based on criteria as speciﬁc as the source/destination

IP address, source/destination Media Access Control (MAC) address, or the Layer 4 Transmission Control Protocol/

User Datagram Protocol (TCP/UDP) port. At the ingress, the Cisco Catalyst 3550 can also perform policing and

marking of the packet. To ensure proper policing and marking on a per packet basis, control plane and data plane

access control lists (ACLs) are supported on all ports.

After the packet goes through classiﬁcation, policing, and marking, it is assigned to the appropriate queue before

exiting the switch. The Cisco Catalyst 3550 Series supports four egress queues per port, which allows the service

provider to be more discriminating and speciﬁc in assigning priorities for the various applications. At egress, the

switch performs scheduling and congestion control. Scheduling is an algorithm that determines the order in which

the queues are processed. The switches support Weighted Round Robin (WRR) scheduling and strict priority

queuing. The WRR queuing algorithm ensures that the lower-priority packets are not entirely starved for bandwidth

and are serviced without compromising the priority settings administered by the service provider. Strict priority

queuing ensures that the highest-priority packets will always get serviced ﬁrst, ahead of all other trafﬁc, which allows

the other three queues to be serviced using WRR scheduling. In conjunction with scheduling, the Cisco Catalyst 3550

Series Gigabit Ethernet ports support congestion control via Weighted Random Early Detection (WRED). WRED

avoids congestion by setting thresholds at which packets are dropped before congestion occurs.

Cisco Systems, Inc.

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