Information About IPv4
You can configure IP on the switch to assign IP addresses to network interfaces. When you assign IP addresses, you enable the interfaces and allow communication with the hosts on those interfaces.
You can configure an IP address as primary or secondary on a switch. An interface can have one primary IP address and multiple secondary addresses. All networking switches on an interface should share the same primary IP address because the packets that are generated by the switch always use the primary IPv4 address. Each IPv4 packet is based on the information from a source or destination IP address. See the Multiple IPv4 Addresses section.
You can use a subnet to mask the IP addresses. A mask is used to determine what subnet an IP address belongs to. An IP address contains the network address and the host address. A mask identifies the bits that denote the network number in an IP address. When you use the mask to subnet a network, the mask is then referred to as a subnet mask. Subnet masks are 32-bit values that allow the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address.
The IP feature in the Cisco NX-OS system is responsible for handling IPv4 packets, as well as the forwarding of IPv4 packets, which includes IPv4 unicast and multicast route lookup, reverse path forwarding (RPF) checks, and software access control list (ACL) forwarding. The IP feature also manages the network interface IP address configuration, duplicate address checks, static routes, and packet send and receive interface for IP clients.
Multiple IPv4 Addresses
The Cisco NX-OS system supports multiple IP addresses per interface. You can specify an unlimited number of secondary addresses for a variety of situations. The most common situations are as follows:
When there are not enough host IP addresses for a particular network interface. For example, if your subnet allows up to 254 hosts per logical subnet, but on one physical subnet you must have 300 host addresses, then you can use secondary IP addresses on the routers or access servers to allow you to have two logical subnets using one physical subnet.
Two subnets of a single network might otherwise be separated by another network. You can create a single network from subnets that are physically separated by another network by using a secondary address. In these instances, the first network is extended, or layered on top of the second network. A subnet cannot appear on more than one active interface of the router at a time.
Note If any switch on a network segment uses a secondary IPv4 address, all other switches on that same network interface must also use a secondary address from the same network or subnet. The inconsistent use of secondary addresses on a network segment can quickly cause routing loops.
Address Resolution Protocol
Networking switches and Layer 3 switches use Address Resolution Protocol (ARP) to map IP (network layer) addresses to (Media Access Control [MAC]-layer) addresses to enable IP packets to be sent across networks. Before a switch sends a packet to another switch, it looks in its own ARP cache to see if there is a MAC address and corresponding IP address for the destination switch. If there is no entry, the source switch sends a broadcast message to every switch on the network.
Each switch compares the IP address to its own. Only the switch with the matching IP address replies to the switch that sends the data with a packet that contains the MAC address for the switch. The source switch adds the destination switch MAC address to its ARP table for future reference, creates a data-link header and trailer that encapsulates the packet, and proceeds to transfer the data. The following figure shows the ARP broadcast and response process.
When the destination switch lies on a remote network which is beyond another switch, the process is the same except that the switch that sends the data sends an ARP request for the MAC address of the default gateway. After the address is resolved and the default gateway receives the packet, the default gateway broadcasts the destination IP address over the networks connected to it. The switch on the destination switch network uses ARP to obtain the MAC address of the destination switch and delivers the packet. ARP is enabled by default.
The default system-defined CoPP policy rate-limits ARP broadcast packets. The default system-defined CoPP policy prevents an ARP broadcast storm from affecting the control plane traffic but does not affect bridged packets.
ARP caching minimizes broadcasts and limits wasteful use of network resources. The mapping of IP addresses to MAC addresses occurs at each hop (switch) on the network for every packet sent over an internetwork, which may affect network performance.
ARP caching stores network addresses and the associated data-link addresses in memory for a period of time, which minimizes the use of valuable network resources to broadcast for the same address each time a packet is sent. You must maintain the cache entries since the cache entries are set to expire periodically because the information might become outdated. Every switch on a network updates its tables as addresses are broadcast.
Static and Dynamic Entries in the ARP Cache
You must manually configure the IP addresses, subnet masks, gateways, and corresponding MAC addresses for each interface of each switch when using static routes. Static routing enables more control but requires more work to maintain the route table. You must update the table each time you add or change routes.
Dynamic routing uses protocols that enable the switches in a network to exchange routing table information with each other. Dynamic routing is more efficient than static routing because the route table is automatically updated unless you add a time limit to the cache. The default time limit is 25 minutes but you can modify the time limit if the network has many routes that are added and deleted from the cache.
Devices That Do Not Use ARP
When a network is divided into two segments, a bridge joins the segments and filters traffic to each segment based on MAC addresses. The bridge builds its own address table that uses MAC addresses only, as opposed to a switch, which has an ARP cache that contains both IP addresses and the corresponding MAC addresses.
Passive hubs are central-connection switches that physically connect other switches in a network. They send messages out on all their ports to the switches and operate at Layer 1 but do not maintain an address table.
Layer 2 switches determine which port is connected to a device to which the message is addressed and send only to that port, unlike a hub, which sends the message out all of its ports. However, Layer 3 switches are switches that build an ARP cache (table).
Reverse ARP (RARP) as defined by RFC 903 works the same way as ARP, except that the RARP request packet requests an IP address instead of a MAC address. RARP often is used by diskless workstations because this type of device has no way to store IP addresses to use when they boot. The only address that is known is the MAC address because it is burned into the hardware.
Use of RARP requires an RARP server on the same network segment as the router interface. Following figure illustrates how RARP works.
There are several limitations of RARP. Because of these limitations, most businesses use DHCP to assign IP addresses dynamically. DHCP is cost effective and requires less maintenance than RARP. The following are the most important limitations:
Because RARP uses hardware addresses, if the internetwork is large with many physical networks, a RARP server must be on every segment with an additional server for redundancy. Maintaining two servers for every segment is costly.
Each server must be configured with a table of static mappings between the hardware addresses and IP addresses. Maintenance of the IP addresses is difficult.
RARP only provides IP addresses of the hosts and not subnet masks or default gateways.
Proxy ARP enables a switch that is physically located on one network appear to be logically part of a different physical network connected to the same switch or firewall. Proxy ARP allows you to hide a switch with a public IP address on a private network behind a router and still have the switch appear to be on the public network in front of the router. By hiding its identity, the router accepts responsibility for routing packets to the real destination. Proxy ARP can help switches on a subnet reach remote subnets without configuring routing or a default gateway.
When switches are not in the same data link layer network but in the same IP network, they try to transmit data to each other as if they are on the local network. However, the router that separates the switches does not send a broadcast message because routers do not pass hardware-layer broadcasts and the addresses cannot be resolved.
When you enable Proxy ARP on the switch and it receives an ARP request, it identifies the request as a request for a system that is not on the local LAN. The switch responds as if it is the remote destination for which the broadcast is addressed, with an ARP response that associates the MAC address of the switch with the IP address of the remote destination. The local switch believes that it is directly connected to the destination, while in reality its packets are being forwarded from the local subnetwork toward the destination subnetwork by their local switch. By default, Proxy ARP is disabled.
Local Proxy ARP
You can use local Proxy ARP to enable a switch to respond to ARP requests for IP addresses within a subnet where normally no routing is required. When you enable local Proxy ARP, ARP responds to all ARP requests for IP addresses within the subnet and forwards all traffic between hosts in the subnet. Use this feature only on subnets where hosts are intentionally prevented from communicating directly by the configuration on the switch to which they are connected.
Gratuitous ARP sends a request with identical source IP address and destination IP address to detect duplicate IP addresses. Cisco NX-OS supports enabling or disabling gratuitous ARP requests or ARP cache updates.
When forwarding an incoming IP packet, if the Address Resolution Protocol (ARP) request for the next-hop is not resolved, packets are punted to the central processing unit (CPU) for ARP resolution. The CPU resolves the MAC address for the next-hop and programs the hardware.
The Cisco Nexus 3000 Series device hardware has glean rate limiters to protect the supervisor from the glean traffic. If the maximum number of entries is exceeded, the packets for which the ARP request is not resolved continues to be processed in the software instead of getting dropped in the hardware.
When an ARP request is sent, the software adds a /32 drop adjacency in the hardware to prevent the packets to the same next-hop IP address to be forwarded to the supervisor. When the ARP is resolved, the hardware entry is updated with the correct MAC address. If the ARP entry is not resolved before a timeout period, the entry is removed from the hardware.
You can use ICMP to provide message packets that report errors and other information that is relevant to IP processing. ICMP generates error messages, such as ICMP destination unreachable messages, ICMP Echo Requests (which send a packet on a round trip between two hosts) and Echo Reply messages. ICMP also provides many diagnostic functions and can send and redirect error packets to the host. By default, ICMP is enabled.
Some of the ICMP message types are as follows:
Network error messages
Network congestion messages
ICMP redirects are disabled on interfaces where the local proxy ARP feature is enabled.
ICMP Unreachable Support to Set Source Interface
You can configure an interface IP address for the ICMP source IP field to handle ICMP error messages. When ICMP packets are constructed in a network stack, the packets use the configured interface IP address. You can select Ethernet, loopback, or port channel interfaces to configure the IP address. If you do not select these options, the switch will behave as in earlier releases.
IPv4 supports Virtual Routing and Forwarding instances (VRFs). By default, Cisco NX-OS places you in the default VRF unless you specifically configure another VRF. For more information, see Configuring Layer 3 Virtualization.
IPv4 Routes with ECMP
If all next-hops for a route are glean, drop, or punt, all next-hops are programmed as-is in the Multipath hardware table.
If some next-hops for a route are glean, drop, or punt, and the remaining next-hops are not, then only non glean, drop, or punt next-hops are programmed in the Multipath hardware table.
When a specific next-hop for ECMP route is resolved (ARP ND resolved), then the Multipath hardware table is updated accordingly.