Routing Module 7

route summarization can happen in one of two ways:

Automatic-With automatic summarization, the router identifies adjacent networks and calculates the summarized route. o Auto-summarization is supported on classless and classful routing protocols. o Auto-summarization uses the default class boundary to summarize routes. o RIP (version 1 and version 2) and EIGRP support auto-summarization; OSPF does not. o For RIPv2 and EIGRP, you can disable automatic summarization. Manual-With manual summarization, an administrator identifies the summarized route to advertise. The specified route includes the summarized subnet address with the subnet mask that includes all summarized subnets.

Border Gateway Protocol (BGP)

BGP is an advanced distance vector protocol (also called a path vector protocol). BGP is an exterior gateway protocol (EGP) used for routing between autonomous systems. • BGP uses paths, rules, and policies instead of a metric for making routing decisions. • BGP is a classless protocol. • Internal BGP (iBGP) is used within an autonomous system; External BGP (eBGP) is used between autonomous systems. BGP is the protocol used on the Internet; ISPs use BGP to identify routes between autonomous systems. Very large networks can use BGP internally, but typically share routes on the Internet only if the AS has two (or more) connections to the Internet through different ISPs.

The following table shows the default administrative values for a Cisco router:

Connected interface 0 Static route 1 EIGRP summary route 5 EIGRP internal route 90 IGRP 100 OSPF 110 RIP 120 EIGRP external route 170

Metric

The metric is a value assigned to each route that identifies the distance or cost to the destination network. The metric is used by the routing protocol to identify and select the best route to the destination when multiple routes exist. A lower metric identifies a more preferred route

routing table

The routing table typically contains the following information: • The address of a known network. • The interface or next hop router used to reach the destination network. • A cost value (also called a metric) that identifies the desirability of the route to the destination network (using distance, delay, or cost). • A timeout value that identifies when the route expires.

FHRP includes the following:

• Hot Standby Router Protocol (HSRP) • Virtual Router Redundancy Protocol (VRRP) • Gateway Load Balancing Protocol (GLBP)

Dynamic NAT

Dynamic NAT automatically maps internal IP addresses with a dynamic port assignment. On the NAT device, the internal device is identified by the public IP address and the dynamic port number. Dynamic NAT allows internal (private) hosts to contact external (public) hosts, but not vice versa—external hosts cannot initiate communications with internal hosts. This implementation is also sometimes called Many-to-One NAT, because many internal private IP address are mapped to one public IP address on the NAT router.

Dynamic and Static NATT

Dynamic and Static NAT, where two IP addresses are given to the public NAT interface (one for dynamic NAT and one for static NAT), allows traffic to flow in both directions.

Enhanced Interior Gateway Routing Protocol (EIGRP)

EIGRP is a hybrid routing protocol developed by Cisco for routing within an AS. • EIGRP uses a composite number for the metric, which indicates bandwidth and delay for a link. The higher the bandwidth, the lower the metric. • EIGRP is a classless protocol. EIGRP is best suited for medium to large private networks.

scope

Each organization that has been assigned a network address from an ISP is considered an Autonomous System (AS). The organization is then free to create one large network or divide the network into subnets. Each autonomous system is identified by an AS number (ASN). This number can be locally administered (private ASN) or publicly registered (public ASN) if the AS is connected to the Internet. Routing protocols can be classified based on their scope, or whether traffic is routed within or between an autonomous system. • An Interior Gateway Protocol (IGP) routes traffic within an autonomous system. • An Exterior Gateway Protocol (EGP) routes traffic between autonomous systems.

Classful or classless

Early routing protocols were not capable of variable-length subnet masks (VLSM) and used only the default subnet masks to identify destination networks. Routing protocols can be identified based on their support for Classless Inter-Domain Routing (CIDR) features. • A classful protocol uses the IP address class and the default subnet mask to identify network addresses. Classful protocols do not support CIDR or VLSM. • A classless protocol ignores the IP address class and requires that a subnet mask value be included in all route advertisements. Classless protocols support CIDR and VLSM.

Intermediate System to Intermediate System (IS-IS)

IS-IS is a link-state routing protocol used for routing within an AS. • IS-IS uses relative link cost for the metric. • IS-IS is a classless protocol. • The original IS-IS protocol was not used for routing IP packets; use Integrated IS-IS to include IP routing support. • IS-IS divides a large network into areas. There is no area 0 requirement, and IS-IS provides greater flexibility than OSPF for creating and connecting areas. • L1 routers share routes within an area; L2 routers share routes between areas; an L1/L2 router can share routes with both L1 and L2 routers. • A network link is the boundary between one area and another area. IS-IS is best suited for large private networks; it supports larger networks than OSPF. IS-IS is typically used within an ISP and easily supports IPv6 routing.

split horizon

Most distant vector routing protocols use a technique called split horizon to prevent routing loops. Split horizon does this by making sure that a router cannot send network information backwards.

Open Shortest Path First (OSPF)

OSPF is a link-state routing protocol used for routing within an AS. • OSPF uses relative link cost for the metric. • OSPF is a classless protocol. • OSPF divides a large network into areas. o Each autonomous system requires an area 0 that identifies the network backbone. o All areas are connected to area 0, either directly or indirectly through another area. o Routes between areas must pass through area 0. • Internal routers share routes within an area; area border routers share routes between areas; autonomous system boundary routers share routes outside of the AS. • A router is the boundary between one area and another area. OSPF is best suited for large private networks.

Destination network address translation (DNAT)

One commonly used implementation of static NAT is called port forwarding. Port forwarding allows incoming traffic addressed to a specific port to move through the firewall and be transparently forwarded to a specific host on the private network. Inbound requests are addressed to the port used by the internal service on the router's public IP address (such as port 80 for a web server). This is often called the public port. Port forwarding associates the inbound port number with the IP address and port of a host on the private network. This port is often called the private port. Based on the public port number, incoming traffic is redirected to the private IP address and port of the destination host on the internal network.

Routing Information Protocol (RIP)

RIP is a distance vector routing protocol used for routing within an autonomous system (e.g., an IGP). • RIP uses hop count as the metric. • RIP networks are limited in size to a maximum of 15 hops between any two networks. A network with a hop count of 16 indicates an unreachable network. • RIP v1 is a classful protocol; RIP v2 is a classless protocol. RIP is best suited for small private networks.

Dynamic

Routers can dynamically learn about networks by sharing routing information with other routers. The routing protocol defines how routers communicate with each other in order to share and learn about other networks. The routing protocol determines: • The information that is contained in the routing table. • How messages are routed from one network to another. • How topology changes (i.e., updates to the routing table) are communicated between routers. Use a routing protocol to allow a router to learn about other networks automatically. The routing protocol generates some network traffic for the process of sharing routes, but it has the advantage of being dynamic and automatic (i.e., changes in the network are propagated automatically to other routers).

Static NAT (SNAT)

Static NAT maps a single private IP address to a single public IP address on the NAT router. Static NAT is used to take a server on the private network (such as a web server) and make it available on the Internet. Using a static mapping allows external hosts to contact internal hosts—external hosts contact the internal server using the public IP address and the static port. This implementation is called One-to-One NAT, because one private IP address is mapped to one public IP address. In addition to static NAT, the term SNAT also means source NAT, stateful NAT, and secure NAT. Although the terms vary, the function is the same.

Static

Static routing requires that entries in the routing table be configured manually. • Network entries remain in the routing table until manually removed. • When changes to the network occur, static entries must be modified, added, or removed.

Router

a device that sends packets from one network to another. Routers receive packets, read their headers to find addressing information, and forward them to the correct destination on the network or Internet

First Hop Redundancy Protocol (FHRP)

a fault-tolerant approach that ensures hosts can communicate outside their local subnet. FHRP allows hosts to dynamically switch between the main router and one or more redundant routers should an outage occur. By doing this, FHRP protects against a single point of failure. Using FHRP, a group of two or more routers actively manage a single virtual router MAC address and IP address as their default router address. This configuration ensures that if a router fails, a backup router takes responsibility as the default gateway. With FHRP, LAN clients send traffic to the virtual router, and the physical router handles the forwarding of that traffic. The difference between the virtual and physical routers is transparent to clients.

distance vector method

a router shares its entire routing table with its immediate neighbors. Routes learned from neighboring routers are added to the routing table and are shared with its neighbors.

Route redistribution

a way of exchanging routing information between two different routing protocols. Route redistribution involves placing the routes learned from one routing domain, such as RIP, into another routing domain, such as EIGRP

Network Address Translation (NAT)

allows you to connect a private network to the Internet without obtaining registered addresses for every host

hybrid method

combines characteristics of the distance vector and link-state methods. A router shares its full routing table at startup, followed by partial updates when changes occur.

route summarization

groups contiguous networks that use the same routing path, advertising a single route as the destination for the grouped subnets. Keep in mind that summarization: • Reduces the size of the routing table. A single route to the summarized network takes the place of multiple routes to individual subnets. • Speeds convergence. The accessibility of each subnet address is indicated by the accessibility of the summarized address. • Retains all necessary routing information, so all networks are still reachable after summarization.

administrative distance

is a number assigned to a source of routing information (such as a static route or a specific routing protocol). The router uses this value to select the source of information to use when multiple routes to a destination exist. A smaller number indicates a more trusted route

convergence

is used to describe the condition when all routers have the same (or correct) routing information.

When route redistribution occurs, you have to address several issues.

o Metrics. Each routing protocol has its own way of determining the best path to a network. RIP uses hops, and EIGRP and IGRP both use a composite metric of bandwidth, delay, reliability, load, and MTU size. Because of the differences in metric calculations, when redistributing routes, you lose all metrics and must manually specify the cost metric for each routing domain. This is because RIP has no way of translating bandwidth, delay, reliability, load, and MTU size into hops, and vice versa. o Classful vs. classless. Some routing protocols are classful and do not send subnet mask information in the routing updates (e.g., RIP and IGRP), and some protocols are classless and do send subnet mask information in the routing updates (e.g., EIGRP). This causes problems when VLSM and CIDR routes need to be redistributed from a classless routing protocol into a classful routing protocol.

link-state method

routers share only their directly connected routes using special packets called link-state advertisements (LSAs) and link-state packets (LSPs). These route advertisements are flooded (forwarded) throughout the network. Routers use this information to build a topology database of the network.

Routing update method

the different routing protocol methods have the following characteristics: • The distance vector method is simpler and requires less processing power for routers. It is best suited for small networks. • The link-state method uses less network traffic for sending routing information, converges faster, and is less prone to errors. It is the best choice for large networks or for sharing routes over WAN links. • A hybrid method reduces the negative effects of the distance vector method while gaining many of the benefits of the link-state method.

The metric can be calculated based on the following criteria:

• Hop count is the number of routers between the current router and the destination network. • Bandwidth, or time, is an actual measure of how long it takes to reach the destination network (delay). For example, high-speed links might be associated with a lower metric cost. • Link cost is a relative number that represents the cost for using the route. For example, it could relate to the actual cost of using a link, such as an expensive WAN link, or it might identify the desirability of using a specific link. Be aware that comparing route metrics used by different routing protocols is not useful. For example, a metric of 10 for a routing protocol that uses bandwidth might indicate a better route than a metric of 4 for a protocol that uses hop count.

Private addresses are translated to the public address of the NAT router:

• Hosts on the private network share the IP address of the NAT router or a pool of addresses assigned for the network. • The NAT router maps port numbers to private IP addresses. Responses to Internet requests include the port number appended by the NAT router. This allows the NAT router to forward responses back to the correct private host. • Technically speaking, NAT translates one address to another. Port address translation (PAT) associates a port number with the translated address. o With only NAT, you would need a public address for each private host. NAT associates a single public address with a single private address. o PAT allows multiple private hosts to share a single public address. Each private host is associated with a unique port number on the NAT router. o Because virtually all NAT routers perform PAT, you are normally using PAT and not just NAT when you use a NAT router. (NAT is usually synonymous with PAT.) • NAT supports a limit of 5,000 concurrent connections. • NAT provides some security for the private network, because it translates or hides private addresses. • A NAT router can act as a limited-function DHCP server, assigning addresses to private hosts. • A NAT router can forward DNS requests to the Internet.