Routing

Routing and Performance

Routing requires a computer to be multihomed which means it has more than one network card. However some servers do not work very well when multihomed. These are:

• WINS
• Domain controllers
• Exchange

RIP

RIP routing tables provide at least:

• IP destination address
• Hop metric from 1 to 15.
• Next router IP address
• Timers
• Marker showing recent route changes

No more than 15 hops can be uses when using RIP on a network.

NT and RIP

NT 4.0 supports RIP. To use it you must:

• Have two network cards on the server with proper IP addresses and subnet masks set up.
• In the control panel, network applet, protocols tab, TCP/IP properties routing tab, check the "Enable IP Forwarding" checkbox.
• On the control panel, network applet, services tab, add the "RIP for Internet Protocol" service.

OSPF

Metric limit is 65535. Each node creates a network tree and considers itself to be at the root of the tree. Administrators can set costs for specific links or hops. OSPF can balance network load between various routes when multiple routes exist. Broadcasting is done less than RIP broadcasting. Information about multiple routers may be carried on routing update packets. Adjacent routers swap information rather than sending to all routers.

ARP

ARP is used to perform IP address resolution which is the linking of IP addresses to MAC (Media Access Control) hardware addresses. ARP uses a broadcast to do this by asking the host that has the given IP address to respond to the broadcast with its MAC address. This step is required to create the data packet at the data link layer of the OSI network model.

RFC 826 defines ARP. ICMP used for ARP reply, possibly Information Reply. Entries in ARP cache are kept for a maximum of 10 minutes according to Microsoft documentation. An entry that is not used within 2 minutes is deleted. On some systems, the 10 minute lifetime is renewed every time the entry is used. Windows NT deletes entries from its cache starting with the oldest, even if its lifetime is not expired, to make room for new entries as the ARP table capacity is reached.

ARP Commands

ARP command	Result
arp -a or arp -q	List the ARP cache
arp -s ipaddress macaddress	Add an arp entry
arp -d ipaddress	Remove an arp entry

Redundant IP addresses can cause broadcast storms.