Campus Network Design Fundamentals
| Now that you have an understanding of VLANs, the following sections introduce the two different ways that Layer 3 switching is implemented within Cisco switchesmultilayer switching and Cisco Express Forwarding. Multilayer Switching
Multilayer switching, as its name implies, allows switching to take place at different protocol layers. Switching can be performed only on Layers 2 and 3, or it can also include Layer 4. MLS is based on network flows. Key Point A network flow is a unidirectional sequence of packets between a source and a destination. Flows can be very specific. For example, a network flow can be identified by source and destination IP addresses, protocol numbers, and port numbers as well as the interface on which the packet enters the switch.
The three major components of MLS are as follows[3]:
MLS can be implemented in the following two ways:
Note Not all Catalyst switches and routers support MLS. Refer to specific product documentation on the Cisco website for device support information for switches[4] and routers.[5]
Key Point MLS allows communication between two devices that are in different VLANs (on different subnets) and that are connected to the same MLS-SE and that share a common MLS-RP. The communication bypasses the MLS-RP and instead uses the MLS-SE to relay the packets, thus improving overall performance.[6]
Figure 2-10 is an example network that illustrates MLS operation. Figure 2-10. The MLS-SE Offloads Work from the MLS-RP
In Figure 2-10, the MLS-RP and MLS-SE communicate using MLSP. The SE learns the MAC addresses of the RP (one for each VLAN that is running MLS). When device 1 (10.1.1.1/16) wants to send a packet to device 2 (10.2.2.2/16), device 1 creates a frame with the destination MAC address of its default gateway, the router, which in this case is the RP. The SE receives the frame, sees that it is for the RP, and therefore examines its MLS cache to see whether it has a match for this flow. In the case of the first packet in the flow, no match exists, so the SE forwards the frame to the RP. The SE also puts the frame in its MLS cache and marks the frame as a candidate entry. The MLS-RP receives the frame, decapsulates (unwraps) the frame, and examines the packet. The RP then examines its routing table to see whether it has a route to the destination of the packet; assuming that it does, the RP creates a new frame for the packet after decrementing the IP header Time to Live (TTL) field and recalculating the IP header checksum. The source MAC address of this frame is the MAC address of the RP; the destination MAC address of this frame is the MAC address of the destination device (or next-hop router). The RP then sends the frame through the SE. The MLS-SE receives the frame and compares it to its MLS cache; the SE recognizes that the frame is carrying the same packet as a candidate entry and is on its way back from the same RP. The SE therefore completes the MLS cache entry using information from the frame; this entry is now an enabler entry. The SE also forwards the frame out of the appropriate port toward its destination. When a subsequent packet in the same flow enters the switch, the SE examines its MLS cache to see whether it has a match. This time it does have a match, so it does not forward the frame to the RP. Instead, the SE rewrites the frame using the information in the MLS cache, including decrementing the TTL field, recalculating the IP header checksum, and using the MAC address of the RP as the source MAC address; the resulting frame looks as though it came from the RP. The SE then forwards the frame out of the appropriate port toward its destination. Note Network flows are unidirectional. Therefore, if device 1 and device 2 both send packets to each other, two flows would be recorded in the MLS cache, one for each direction. Note In Figure 2-10, the MLS cache is shown as having a "protocol" field. In the output of the display on the Catalyst switches this field is called a "port" field, even though it represents the protocol field in the IP header. The MLS-SE also keeps traffic statistics that can be exported to other utilities to be used, for example, for troubleshooting, accounting, or other functions. Cisco Express Forwarding
Cisco Express Forwarding (CEF), like MLS, aims to speed the data routing and forwarding process in a network. However, the two methods use different approaches. CEF uses two components to optimize the lookup of the information required to route packets: the Forwarding Information Base (FIB) for the Layer 3 information and the adjacency table for the Layer 2 information.[7] CEF creates an FIB by maintaining a copy of the forwarding information contained in the IP routing table. The information is indexed so that it can be quickly searched for matching entries as packets are processed. Whenever the routing table changes, the FIB is also changed so that it always contains up-to-date paths. A separate routing cache is not required. The adjacency table contains Layer 2 frame header information, including next-hop addresses, for all FIB entries. Each FIB entry can point to multiple adjacency table entries, for example, if two paths exist between devices for load balancing. After a packet is processed and the route is determined from the FIB, the Layer 2 next-hop and header information is retrieved from the adjacency table and a new frame is created to encapsulate the packet. Cisco Express Forwarding can be enabled on a router (for example, on a Cisco 7500 Series router) or on a switch with Layer 3 functionality (such as the Catalyst 8540 switch). Note Not all Catalyst switches support Cisco Express Forwarding. Refer to specific product documentation on the Cisco website[8] for device support information.
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