Layer 3 Switching
Layer-3 (IP) SwitchesThe FS3-1600 switch can support Layer 3 (IP) switching and VLAN switching in hardware.
The Layer-3 switching capabilities of the FS3-1600 switch can be summarized as follows:
The FS3-1600 Layer-3 switches utilize completely separate hardware for Route Management and for Forwarding. The route management section identified sequences of packets as flows, which are then forwarded by the forwarding hardware.
Figure 10 - 2: IP Header Processing
Packet-by-Packet Routing Switch FunctionalityThe central tenet of the packet-by-packet routing switch is:
Switch if Possible, Route Only if Necessary.
This indicates that the routing switch:
- Employs its forwarding engine on all possible packets to maintain wire speed.
Employs its route management subsystem only for packets which are unknown by the routing switch, and therefore, cannot be handled by the forwarding engine. Figure 10-1 shows the decision process to determine whether an incoming packet is to be forwarded or routed.
You can see from Figure 10-1 that the routing switch reverts to layer-2 switching whenever a packet arrives with a MAC (Media Access Control) Address, which does not match the switchs MAC Address. This is because IP is a hop-by-hop protocol, which always replaces the packets MAC Destination Address with the MAC Address of the device at the end of this hop. Therefore, if the MAC Destination Address does not match, the packet must be a layer-2 packet with a final destination indicated by the MAC Destination Address.
However, if the MAC Destination Address of the incoming packet is a match for this switchs MAC Address, then the packet must be either a Management Frame destined for this switch or a hop-by-hop routed packet. The routing switch determines which type of packet by comparing the packets IP Destination Address with associated IP Addresses of this switch. Of course, the same comparison would be made for IPX or other protocol Addresses if the switch supports those protocols. If the IP Destination Address matches the IP Address of this switch, then the packet is determined to be a management packet destined for this switch.
Figure 10 - 1: IP Switching Decision Process
If the IP Destination Address does not match the IP address of the switch, the packet must be a layer-3 packet that must be forwarded to the next switch or router in the path. The FS3-1600 switch will begin searching its switching database to determine the appropriate destination. This search is performed using the Network ID portion of the IP address, and when found, it will insert the corresponding Next Hop address into the MAC address field of the packet.
Then resolve the logical port associated with the Next Hop address, and modify the FCS field of the Frame. The last step is to decrement the Time to Live value and recalculate the IP Header Checksum and put these into the new packet header. The packet is then transmitted to the new destination address. Figure 10-2 shows the fields of an IP header and identifies the fields that the FS3-1600 switch uses to make switching decisions and forward packets.
If the Network ID address is not found in the switching database, then the FS3-1600 switch will use RIP to request the needed destination address. Once this information is provided, the FS3-1600 switch will modify the packet headers and transmit the packet. If there is no response to the RIP request, the FS3-1600 switch will transmit the packet to a default destination address (typically a router) for resolution.
VLAN in a Layer-3 Routing Switch
The FS3-1600 Routing Switch utilizes both Logical Port Numbers and Physical Port Numbers in order to support VLANs and layer-3 switching. By using the abstraction of Logical Port Numbers, routing can occur from one VLAN to another transparently. Traditional routers used only Physical Port Numbers in their routing tables, which provided no support for VLANs.
The Layer-3 Switching DatabaseThe central element of any router or routing switch is the IP Switching Database. This data structure contains the key information required for the Layer-3 switch to forward incoming packets to the correct destination. The key design choices to be made involve the method of searching this database and how the entries are aged out.
As long as the IP Switching Database is small, the searching algorithm is non-critical. However, as the Database grows larger to accommodate larger networks, the searching algorithm is most critical to assure top performance.
The FS3-1600 switch utilizes a modified Hash List algorithm to optimize both complexity and performance for this application.
The Hash Buck algorithm works by creating an index into a hash list by performing an Exclusive-Or function with the destination MAC address. The result of this is a 2-byte index, of which the lower 13 bits are used to address a location in the hash list. The MAC address at this location is checked against the destination address looking for an exact match. If the addresses dont match then the switch will follow a pointer to the first hash bucket.
This hash bucket contains a number of IP addresses and MAC addresses that share the same index, and are limited to 64 bytes total. The addresses in this bucket are searched to find a match to the destination address. Once a match is found, the entry contains a pointer to the memory location that contains the destination information for that route, and the forwarding process begins
.Over time some of the entries in the switching database will become inactive and must be removed from the database. This aging process is handled in an efficient manner in the FS3-1600 Switch chipset.
When a forwarding address is found in the switching database, the Layer-3 switching processor in the FS3-1600 switch chipset takes that entry and places it at the end of a doubly linked list. This aging list contains every address entry in the database and organizes the addresses such that all the recently used addresses are at one end of the linked list, and the unused addresses are at the other end of the list. The FS3-1600 switch simply looks at the last entry on the list and removes it once it expires. This organization of the address entries simplifies the aging and removal process.
The result is a Layer-3 switching capability, which is state-of-the-art in performance, as well as having reasonable complexity, and reasonable sizes for the hash table, hash buckets, and total hash entries. The Layer-3 switching processor handles new entry creation automatically when new IP addresses are learned, as well as managing aging and replacement of older entries.
(QoS) is one of the most important new functions in the evolution of networks. It provides the capability to prioritize and forward each packet in order to meet the needs of a variety of services: data, voice, and video simultaneously. This requires multiple priorities to be defined and recognized by the Layer-3 switch.Quality of Service
FS3-1600 is providing four transmit queues on each port, together with Weighted Fair Queuing. These functions could be used, for example, to provide independent priorities for real-time video, real-time voice, guaranteed-delivery data, and best-effort data. Some major industry players have already announced plans to assure four levels of priority on future products. And the IEEE 802.1p standards group is refining a standard for QoS based on multiple priorities.
Priority assignment in the FS3-1600 switch is accomplished through explicit assignment by end stations, which have applications needing priority higher than best-effort data. This mechanism utilizes the IEEE 802.1p and 802.1Q tag structure. It requires only driver modification in end stations, which need to utilize QoS.
Additionally, implicit assignment of priorities is possible. This can be accomplished by intermediate devices, which could examine IP addresses and/or the TCP/UDP port numbers of packets in transit. However, this could be more difficult for network personnel to manage.
Figure 10-3 explains an example of how to apply FS3-1600 to your network. In Figure 10-3, the FS3-1600 contains three subnetworks: they are 100.100.100.0, 200.200.200.0, and 180.180.180.0. Both 100.100.100.0 and 200.200.200.0 subnetworks are deployed in Intranet by means of the Port-based VLAN function of the FS3-1600 switch. The FS3-1600 switch uses its configured default route address to forward Internet packets to the default gateway (180.180.180.254). The routing function between 100.100.100.0 and 200.200.200.0 subnetworks is performed by the Layer-3 switching function of the FS3-1600 switch. The gateway of 100.100.100.1 and 100.100.100.2 are suggested to set as 100.100.100.254, the gateway of 200.200.200.1 and 200.200.200.2 are suggested to set as 200.200.200.254.
Figure 10 - 3: A Deployment Example of FS3-1600 Routing Switch
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