Bài giảng Mạng máy tính nâng cao - Chapter 07: Variable length subnet masking

VARIABLE LENGTH SUBNET MASKINGMẠNG MÁY TÍNH NÂNG CAOChapter 07Objectives Define VLSM and briefly describe the reasons for its use Divide a major network into subnets of different sizes using VLSM Define route aggregation and summarization as they relate to VLSM Configure a router using VLSM Identify the key features of RIP v1 and RIP v2 Identify the important differences between RIP v1 and RIP v2 Configure RIP v2 Verify and troubleshoot RIP v2 operation Issues with IP Addressing Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the addressIn 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classesAs early as 1992, the IETF identified two specific concerns:IP address exhaustionRouting table growth IP Addressing Solutions Short term to extension to IPv4Subneting masking: RFCs 950, 1812Variable-Length Subnet Masks & Classless InterDomain Routing notation (1993, RFC 1519)Network Address Translation (1994, RFC 1631)Address allocation for private Internets (1996, RFC 1918)Route summarization: RFC 1518Ultimate solution: IPv6 128-bit address spaceClassful IP Addressing Classful Routing Protocol classful routing protocols(i.e. RIPv1) do not send subnet masks in their routing updates The reason is that the Subnet mask is directly related to the network addressClassless IP Addressing Classless Inter-domain Routing (CIDR –RFC 1517)Allows for: More efficient use of IPv4 address spaceRoute summarizationRequires subnet mask to be included in routing update because address class is meaninglessRecall purpose of a subnet mask:To determine the network and host portion of an IP addressCIDR and Route Summarization CIDR & Route SummarizationVariable Length Subnet Masking (VLSM): Allows a subnet to be further sub-netted according to individual needsCIDR allows for routes to be summarized as a single routeClassless Routing Protocol Characteristics of classless routing protocols:Routing updates include the subnet maskSupports VLSMSupports Route Summarization What is VLSM ? VLSM is Action Classful routing Only allows for one subnet mask for all networksVLSM & classless routingThis is the process of subnetting a subnetMore than one subnet mask can be used VLSM is supported Open Shortest Path First (OSPF).Integrated Intermediate System to Intermediate System (Integrated IS-IS).Enhanced Interior Gateway Routing Protocol (EIGRP).RIP v2.Static routing. A waste of space When to use VLSM? VLSM and the Routing Table Sub - Subnets Calculating subnets with VLSM VLSM Example Calculating subnets with VLSM Calculating subnets with VLSM All Zeros and All Ones Subnets Subnet Zero If a network address is subnetted, the first subnet obtained after subnetting the network address is called subnet zeroThe All-Ones Subnet When a network address is subnetted, the last subnet obtained is called the all-ones subnet.Using Subnet Zero and the All-Ones Subnet It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0 by entering the ip subnet-zero global configuration command.Route Summarization Route summarization done by CIDR Routes are summarized with masks that are less than that of the default classful maskRoute summarization example CIDR (Classless Inter-Domain Routing) By using a prefix address to summarizes routes, administrators can keep routing table entries manageable, which means the followingMore efficient routingA reduced number of CPU cycles when recalculating a routing table, or when sorting through the routing table entries to find a matchReduced router memory requirements Route summarization is also known as:Route aggregationSupernettingSupernetting is essentially the inverse of subnetting. Supernetting Example Company XYZ needs to address 400 hosts. Its ISP gives them two contiguous Class C addresses: 207.21.54.0/24 207.21.55.0/24Company XYZ can use a prefix of 207.21.54.0 /23 to supernet these two contiguous networks. (Yielding 510 hosts)CIDR and the Provider CIDR and the Provider MẠNG MÁY TÍNH NÂNG CAORIP Version 2 RIP v1: Characteristics IGP protocol, is classful routing.It is a distance vector protocol that uses a hop count metric. The maximum number of hops is 15. By default, routing updates are broadcast every 30 seconds.The router applies the one subnet mask that is configured on the receiving interface to receive network information.Load balancing over as many as six equal-cost paths, with four paths as the default.RIP v1: Limitations It does not send subnet mask information in its updates. It sends updates as broadcasts on 255.255.255.255. It does not support authentication. It is not able to support VLSM or classless interdomain routing (CIDR). RIP v1: Configuration RIPv1: Discontiguous Networks Because the subnet mask is not included in the update, RIPv1 and other classful routing protocols must summarize networks at major network boundariesRIPv1: Discontiguous Networks RIP v2 features Send out subnet mask information with the route update.Support VLSM or classless interdomain routing (CIDR). Provides for authentication in its updates.Provides Multicast routing updates, using the Class D address 224.0.0.9.Routing update is includes a next-hop route ip address.Comparing RIP v1 and v2 RIP v2: Configuration Verifying RIP v2 : SHOW IP PROTOCOLS Compatibility with RIP v1 Interface FastEthernet0/0 is configured to send and receive RIP v1 updates. FastEthernet0/1 is configured to send both version 1 and 2 updates. FastEthernet0/2 has no special configuration and therefore sends and receives version 2 by default.Compatibility with RIP v1 NewYorkinterface fastethernet0/0ip address 192.168.50.129 255.255.255.192ip rip send version 1ip rip receive version 1interface fastethernet0/1ip address 172.25.150.193 255.255.255.240ip rip send version 1 2interface fastethernet0/2ip address 172.25.150.225 225.255.255.240router ripversion 2network 172.25.0.0network 192.168.50.0RIPv2 and VLSM Authentication Whatever the reason, it is good practice to authenticate routing information transmitted between routers. RIPv2, EIGRP, OSPF, IS-IS, and BGP can be configured to authenticate routing information. This practice ensures routers will only accept routing information from other routers that have been configured with the same password or authentication information. Note: Authentication does not encrypt the routing table. Authentication Configuring authentication Router(config)#key chain RomeoRouter(config-keychain)#key 1Router(config-keychain-key)#key-string JulietRouter(config)#interface fastethernet 0/0Router(config-if)#ip rip authentication key-chain RomeoRouter(config-if)#ip rip authentication mode md5Note: The password must be the same on both routers (Juliet), but the name of the key (Romeo) can be different.If the command ip rip authentication mode md5 is not added, the interface will use the default clear text authentication.