IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 2 subnets
C 10.10.1.0 is directly connected, FastEthernet0/0.10
C 10.20.1.0 is directly connected, FastEthernet0/0.20

Content 4.1 Describing Routing Between VLANs 4.1.4 Explaining Multilayer Switching Traditionally, a switch makes forwarding decisions by looking at the Layer 2 header, whereas a router makes forwarding decisions by looking at the Layer 3 header. A multilayer switch combines the functionality of a switch and a router into one device, therefore enabling the device to switch traffic when the source and destination are in the same VLAN and to route traffic when the source and destination are in different VLANs (that is, different subnets). In Figure , traffic between PC A and PC B are switched at Layer 2, whereas traffic between PC B and PC C are switched at Layer 3. Multilayer switches forward frames and packets at wire speed by using application-specific integrated circuit (ASIC) hardware. Specific Layer 2 and Layer 3 components, such as routing tables or access control lists (ACLs), are cached into hardware. These tables are stored in content-addressable memory (CAM) and ternary content-addressable memory (TCAM). Layer 2 forwarding in hardware is based on the destination MAC address. The Layer 2 switch learns and records the source MAC addresses from all frames that it receives. The MAC address table lists MAC addresses paired with the associated VLANs and interfaces. When a frame is received on an interface, the switch determines which VLAN the frame originated from, searches all interfaces that belong to that VLAN for the destination MAC, and forwards the frame out the appropriate interface. Figure describes how a Layer 2 switch forwards packets. Layer 3 forwarding is based on the destination IP address. Layer 3 forwarding occurs when a packet is routed from a source in one subnet to a destination in another subnet. When a multilayer switch (MLS) sees its own MAC address in the Layer 2 header, it recognizes that the packet is either destined for itself or is to be routed. If the packet is not destined for the MLS, the destination IP address is compared against the Layer 3 forwarding table for the longest match. In addition, router ACL checks are performed. In this case, the frame header needs to be rewritten with new source and destination MAC addresses. Figures and describe how a Layer 3 switch forwards packets.

Content 4.1 Describing Routing Between VLANs 4.1.5 Frame Rewrite Figure shows how the frame and packet header would be altered if CEF is used to forward frames. When frames are received on an interface, the trailer checksum is first calculated to verify accurate delivery of the frame. The frame is discarded if the calculation is not accurate. Next the payload is extracted. The IP header checksum is tested to verify that it is an accurate IP header. Once the packet is processed, IP unicast packets are rewritten on the output interface as follows:

The source MAC address changes from the sender MAC address to the router MAC address.
The destination MAC address changes from the router MAC to the next-hop MAC address.
The TTL is decremented by one and, as a result, the IP header checksum is recalculated.
The frame checksum is recalculated.

Routing, switching, ACL, and QoS tables are stored in a high-speed table memory so that forwarding decisions and restrictions can be made in high-speed hardware. Cisco Catalyst switches create and use two primary table architectures:

CAM table: Primary table used to make Layer 2 forwarding decisions. The table is built by recording the source address and inbound port of all frames. When a frame arrives at the switch with a destination MAC address of an entry in the CAM table, the frame is forwarded out only through the port associated with that specific MAC address.
TCAM table: Stores ACL, QoS, and other information generally associated with upper-layer processing.

Table lookups are done with efficient search algorithms. A “key” is created to compare the frame to the table content. For example, the destination MAC address and VLAN ID (VID) of a frame constitute the key for a Layer 2 table lookup. This key is fed into a hashing algorithm, which produces a pointer into the table. The system uses the pointer to access a smaller specific area of the table without requiring searching the entire table. In a Layer 2 table, all bits of all information are significant for frame forwarding (for example, VLANs, destination MAC addresses, and destination protocol types). However, in more complicated tables associated with upper-layer forwarding criteria, some bits of information may be too inconsequential to analyze. For example, an ACL may require a match on the first 24 bits of an IP address, but the last 8 bits may be insignificant information. In specific high-end switch platforms, the TCAM is a portion of memory designed for rapid, hardware-based table lookups of Layer 3 and Layer 4 information. In the TCAM, a single lookup provides all Layer 2 and Layer 3 forwarding information for frames, including CAM and ACL information. Figure displays the ACL information stored in the TCAM table that would result in a packet being permitted or denied. TCAM matching is based on three values: 0, 1, or X (where X is either number), hence the term “ternary.” The memory structure is broken into a series of patterns and masks. Masks are shared among a specific number of patterns and are used as wildcards in some content fields. The following two ACL entries are referenced in Figure , which shows how their values are stored in the TCAM: access-list 101 permit ip host 10.1.1.1 any
access-list 101 deny ip 10.1.1.0 0.0.0.255 any The TCAM table entries in Figure consist of the following types of regions:

Longest match region: Each longest match region consists of groups of Layer 3 address entries (“buckets”) organized in decreasing order by mask length. All entries within a bucket share the same mask value and key size. The buckets can change their size dynamically by borrowing address entries from neighboring buckets. Although the size of the whole protocol region is fixed, you can reconfigure it. The reconfigured size of the protocol region takes effect only after the next system reboot.
First-match region: The first-match region consists of ACL entries. Lookup stops after the first match of the entry.

Content 4.2 Enabling Routing Between VLANs 4.2.1 Describing Layer 3 SV An SVI is a virtual Layer 3 interface that can be configured for any VLAN that exists on a Layer 3 switch. It is virtual in that there is no physical interface for the VLAN, and yet it can accept configuration parameters applied to Layer 3 router interfaces. The SVI for the VLAN provides Layer 3 processing for packets from all switch ports associated with that VLAN. Only one SVI can be associated with a VLAN. You configure an SVI for a VLAN for the following reasons:

To provide a default gateway for a VLAN so that traffic can be routed between VLANs
To provide fallback bridging if it is required for non-routable protocols
To provide Layer 3 IP connectivity to the switch
To support routing protocol and bridging configurations

By default, an SVI is created for the default VLAN (VLAN1) to permit remote switch administration. Additional SVIs must be explicitly created. SVIs are created the first time a VLAN interface configuration mode is entered for a particular VLAN SVI. The VLAN corresponds to the VLAN tag associated with data frames on an Ethernet trunk or to the VLAN ID (VID) configured for an access port. An IP address is assigned in interface configuration mode to each VLAN SVI that is to route traffic off of and on to the local VLAN.

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