CCNP Enterprise Certification Study Guide: Implementing and Operating Cisco Enterprise Network Core Technologies. Ben Piper

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servers, routers, and switches—maintain an ARP cache to store ARP replies. The purpose of the ARP cache is to avoid having to send an ARP request every time the node needs to resolve an IP address to a MAC address.

      When a node needs to resolve the MAC address of an IP address not in its ARP cache, it sends an ARP request to the broadcast address (FFFF.FFFF.FFFF). Upon receiving a reply, it stores the mapping in its ARP cache. The following example illustrates the process using two switches:

       SW3 has a switched virtual interface (SVI) in VLAN 20 with an IP address of 10.10.20.3.

       SW4 has an SVI also in VLAN 20 with an IP address of 10.10.20.4.

      SW3:

      SW4:

      ! ARP Snooping debugging has been enabled on SW4. Note the destination broadcast ! address. SW4# ARP Packet (Gi1/0/20) Src: 0c3c.8aab.8014, Dst: ffff.ffff.ffff, SM: 0c3c.8aab.8014, SI: 10.10.20.3, TM: ffff.ffff.ffff, TI: 10.10.20.3 Packet bridged by platform. ARP Packet (Gi1/1/20) Src: 0c3c.8aab.8014, Dst: ffff.ffff.ffff, SM: 0c3c.8aab.8014, SI: 10.10.20.3, TM: ffff.ffff.ffff, TI: 10.10.20.3 Packet bridged by platform. ! Although not shown in the output, SW4's ARP reply is addressed to SW3's ! SVI MAC address.

      The default timeout for an ARP entry is 4 hours. You can modify this on a per-interface basis, as shown on SW3:

      SW3#show interfaces vlan 20 | i ARP Encapsulation ARPA, loopback not set ARP type: ARPA, ARP Timeout 04:00:00 SW3#configure terminal Enter configuration commands, one per line. End with CNTL/Z. SW3(config)#interface vlan 20 SW3(config-if)#arp timeout ? <0-2147483> Seconds

      You'll hear disagreement as to whether ARP is a layer 2 or layer 3 protocol, some even going so far as to call it a layer 2.5 protocol! ARP packets fit the definition of what the OSI model calls protocol control information. In addition to just providing a mapping between MAC and IP addresses, the fact that a node sends ARP packets indicates its willingness to use IP. In that respect, ARP is decidedly a layer 2 protocol.

      Fragmentation

      IPv6 differs from IPv4 when it comes to fragmentation. IPv4 packets can be fragmented by any router along the path unless the DF bit is set. IPv6 can be fragmented only by the sender. If an IPv6 packet will exceed an intermediate router's interface MTU, the router will respond to the sender with an ICMPv6 “packet too big” message and discard the packet.

      Routing vs. Forwarding

      What's the difference between routing and forwarding? Not much, really. Forwarding is about sending the data one step closer to its destination. Routing is about figuring out what that next step is.

      The routing versus forwarding distinction has nothing to do with layers. Recall that switches perform a crude version of routing by snooping the data plane to find out which port a MAC address is connected to. They compile this into a MAC address table, which they use to make forwarding decisions.

      When it comes to IP, route calculation and route advertisements are performed by interior gateway routing protocols such as Enhanced Interior Gateway Routing Protocol (EIGRP) and Open Shortest Path First (OSPF). Although we don't normally think of them in this way, routing protocols are actually applications that run on routers. They just populate the IP routing table that feeds into the FIB, but CEF does the forwarding.

      Layer 4: The Transport Layer

Application protocol Transport protocol Source IP Source port Destination IP Destination port
HTTP TCP 192.168.88.10 5230 18.213.128.4 80
HTTP TCP 192.168.88.10 5231 18.213.128.4 81
DNS UDP 192.168.88.10 56801 192.168.88.1 53

      

The protocol data unit for TCP is called a segment, and for UDP it's called a datagram.

      When a host receives an IP packet, the host's networking stack looks at the Protocol field to determine to which upper-layer protocol to send the data. If the Protocol field in the IP header is 6, the data contains a TCP segment. If it's 17, then it contains a UDP datagram. Consequently, a single host can use the same UDP and TCP port numbers simultaneously.

      

Transport layer protocols aren't always necessary. The interior gateway protocols EIGRP and OSPF ride directly over IP, using the IP protocol numbers 88 and 89, respectively.

      Transmission Control Protocol

      RFC

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