CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies. Ben Piper
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СКАЧАТЬ just deals with Network layer addresses—usually IP addresses.

The figure shows the simple layer 2 (on the top) and layer 3 (at the bottom) topologies.

      IP creates an addressing scheme on top of the Data Link layer, giving each subnet a different CIDR—a combination of an IP subnet address and subnet mask:

       VLAN 700—192.168.70.0/26

       PPP—10.255.70.0/31

       VLAN 706—192.168.70.64/26

      

The OSI's dream of turning the network into a software abstraction begins to show cracks in the Network layer. Applications do indeed need to have some knowledge of the network, even if it's just IP addresses.

      Forwarding within a Subnet

      If the destination is in the same subnet, the node will simply communicate with the destination at the Data Link layer. For example, if Host A (192.168.70.3) attempts to ping Router A (192.168.70.62), the following will happen:

      1 Host A will note that Router A's IP is in the same subnet.

      2 Host A will send an Address Resolution Protocol (ARP) request to the broadcast destination address, asking who has 192.168.70.62.

      3 Switch A will flood the ARP request and Router A will receive it.

      4 Router A will send an ARP reply to Host A's MAC address. The reply will contain Router A's IP address (192.168.70.62) and MAC address.

      5 Switch A will forward the ARP reply to Host A.

      6 Host A will encapsulate the IP packet inside an Ethernet frame addressed to Router A's MAC address. Host A will set the Type field in the frame to 0x0800, indicating that the frame contains an IP packet.

      

A ping is an Internet Control Message Protocol (ICMP) echo request. ICMP is an integral part of IP.

      Forwarding between Subnets

      On the other hand, if Host A (192.168.70.3) attempts to ping Host B (192.168.70.67), the following happens:

      1 Host A compares its IP address with Router B's IP and determines that they are in different subnets.

      2 Host A consults its IP routing table for a closest-match route to 10.255.70.1. Not finding an exact match, the closest match is the default route (0.0.0.0/0). Host A's default gateway is 192.168.70.62, the IP belonging to Router A's Ethernet interface.

      3 Because Router A's and Host A's IP addresses are in the same subnet, Host A sends an ARP request asking for Router A's MAC address.

      4 Switch A floods the ARP request to Router A.

      5 Router A sends an ARP reply to Host A's MAC address. The reply contains Router A's IP address and MAC address.

      6 Host A encapsulates the IP packet inside an Ethernet frame addressed to Router A's MAC address. The Type field will contain the value 0x0800 to indicate that the Data field contains an IP packet.

      7 Router A receives the Ethernet frame and, based on the Type field in the Ethernet frame, knows it contains an IP packet.

      8 Router A looks at the destination IP address in the IP packet and checks its forwarding information base (FIB) for an exact match. Cisco Express Forwarding (CEF) uses the FIB to make forwarding decisions. The FIB is fed by the IP routing table (also known as the Routing Information Base, or RIB). Not finding an exact match for the destination IP address in the FIB, it will use the default route, which has Router B (10.255.70.1) as its next hop.

      9 Router A will encapsulate the IP packet in a PPP frame and send it to Router B.

      10 Router B will decapsulate the IP packet, look at the destination IP address, and check its FIB for a match. Because the destination IP (192.168.70.67) is in the same subnet as Router B's Ethernet interface, Router B will send an ARP request.

      11 Switch B will flood the ARP request to Host B, which will send an ARP reply to Router B's MAC address. The ARP reply will contain Host B's MAC address and IP address.

      12 Router B will encapsulate the IP packet in an Ethernet frame addressed to Host B's MAC address.

 The figure shows the layer view of the network.

      

It's crucial that a CIDR block belong to only one subnet—that is, one section of the network where all the connected nodes use the same Data Link layer protocol. A common mistake is to try to split a CIDR across different subnets that are usually in geographically separated areas, like different data centers. The rationale for subnet splitting is to achieve some sort of resiliency with minimal inconvenience, particularly by being able to migrate virtual machines from one site to another without changing any IP addresses. This requires using some network virtualization technology like Virtual Extensible LAN (VXLAN) to create the illusion of extending the subnet, when in fact it's stuffing Ethernet frames inside of IP packets and sending them across multiple subnets, in essence creating a virtual subnet! Remember that a subnet can't scale beyond a few hundred nodes—not even a virtual subnet.

      Address Resolution Protocol

      Most devices with an IP address—including СКАЧАТЬ