Classful routing protocols are considered legacy and do not include subnet mask information with routing updates. Examples of classful routing protocols are RIPv1 and IGRP. Because subnet mask information is not included in updates, consistency of the mask is assumed throughout the network. Classful routing protocols also feature automatic summarization of routing updates when sent across a major classful network boundary. For example, the 10.16.0.0/16 network would be advertised as 10.0.0.0/8 when sent into a 172.16.0.0 domain.
BGP and EIGRP are not classful routing protocols, both engage in automatic summarization behavior by default, and in that sense they act classful. The no auto-summary command is used to disable this behavior. Classful routing protocols feature a fixed-length subnet mask (FLSM) as a result of their inherent limitations. The FLSM leads to inefficient use of addresses and limits the network’s overall routing efficiency. By default, classful routing protocols discard traffic bound for any unknown subnet of the major classful network. For example, if your classful routing protocol receives traffic destined for 10.16.0.0 and it knows of only the 10.8.0.0 and 10.4.0.0 subnets in its routing table, it discards the traffic—even if a default route is present! The ip classless command was introduced to change this behavior. The ip classless command allows the protocol to use the default route in this case. This command is on by default with Cisco IOS Release 12.0 and later routers.
As a classic example of a classless routing protocol, OSPF carries subnet mask information in updates. Wireless LAN Services Module (WLSM) is possible with such protocols.
Split horizon
Split horizon is a technique used by routing protocols to help prevent routing loops. The split-horizon rule states that an interface will not send routing information out an interface from which the routing information was originally received. Split horizon can cause problems in some topologies, such as hub-and-spoke Frame Relay configurations.
Administrative distance
If a router learns of a network from multiple sources (routing protocols or static configurations), it uses the administrative distance value to determine which route to install in the routing (forwarding) table. The default administrative distance values are listed here.
Source Administrative Distance
Connected interface - 0
Static route - 1
EIGRP summary route - 5
External BGP - 20
Internal EIGRP - 90
IGRP - 100
OSPF - 110
IS-IS - 115
RIP - 120
Exterior Gateway Protocol - 140
On-Demand Routing - 160
External EIGRP - 170
Internal BGP - 200
Unknown - 255
Administrators can create static routes that “float.” A floating static route means the administrator increases the administrative distance of the static route to be greater than the administrative distance of the dynamic routing protocol in use. This means the static route is relied on only when the dynamic route does not exist.
Source Administrative Distance
Connected interface - 0
Static route - 1
EIGRP summary route - 5
External BGP - 20
Internal EIGRP - 90
IGRP - 100
OSPF - 110
IS-IS - 115
RIP - 120
Exterior Gateway Protocol - 140
On-Demand Routing - 160
External EIGRP - 170
Internal BGP - 200
Unknown - 255
Administrators can create static routes that “float.” A floating static route means the administrator increases the administrative distance of the static route to be greater than the administrative distance of the dynamic routing protocol in use. This means the static route is relied on only when the dynamic route does not exist.
Routing decision criteria
Routers must determine the best route to send traffic on toward itsdestination. This is accomplished as follows (note that the order of operations is critical and fixed):
1. Valid next-hop IP address—when updates are received, the router first verifies that the next-hop IP address to reach the potential destination is valid.
2. Metric—the router then examines the metrics for the various routes that might exist from a particular protocol. For example, if OSPF has several routes to the destination, the router tries to install the route with the best metric (in this case, cost) into the routing table.
3. Administrative distance—if multiple routing protocols are running on the device, and multiple protocols are all presenting routes to the destination with valid next hops, the router examines administrative distance. The route sourced from the lowest administrative distance protocol or mechanism is installed in the routing table.
4. Prefix—the router examines the route’s prefix length. If no exact match exists in the routing table, the route is installed. Note that this might cause the routing table to be filled with the following entries: EIGRP 172.16.2.0/24 and RIP 172.16.2.0/19.
The subject of prefix length and the routing table, remember that when a router is looking for a match in the IP routing table for the destination address, it always looks for the longest possible prefix match. For example, if the routing table contains entries of 10.0.0.0/8, 10.2.0.0/16, and 10.2.1.0/24, and your traffic is destined for 10.2.1.0/24, the longest match prefix is selected.
1. Valid next-hop IP address—when updates are received, the router first verifies that the next-hop IP address to reach the potential destination is valid.
2. Metric—the router then examines the metrics for the various routes that might exist from a particular protocol. For example, if OSPF has several routes to the destination, the router tries to install the route with the best metric (in this case, cost) into the routing table.
3. Administrative distance—if multiple routing protocols are running on the device, and multiple protocols are all presenting routes to the destination with valid next hops, the router examines administrative distance. The route sourced from the lowest administrative distance protocol or mechanism is installed in the routing table.
4. Prefix—the router examines the route’s prefix length. If no exact match exists in the routing table, the route is installed. Note that this might cause the routing table to be filled with the following entries: EIGRP 172.16.2.0/24 and RIP 172.16.2.0/19.
The subject of prefix length and the routing table, remember that when a router is looking for a match in the IP routing table for the destination address, it always looks for the longest possible prefix match. For example, if the routing table contains entries of 10.0.0.0/8, 10.2.0.0/16, and 10.2.1.0/24, and your traffic is destined for 10.2.1.0/24, the longest match prefix is selected.
Summarization
Summarization is the process in which the administrator collapses many routes with a long mask to form another route with a shorter mask. Route summarization reduces the size of routing tables and makes routing function more efficiently. Route summarization also helps make networks more stable by reducing the number of updates that are sent when subnets change state. Route summarization makes classless interdomain routing (CIDR) possible. Variable-length subnet masking (VLSM) promotes the use of route summarization. Some dynamic routing protocols engage in route summarization automatically for changes in a major classful network, whereas others do not.
For any routing protocol within the scope of the CCIE written exam, an administrator can disable any automatic summarization that might be occurring and configure “manual” summarization. To engage in route summarization, find all the leftmost bits that are in common and create a mask that encompasses them. An example follows:-
The following routes exist in the routing table—all routes use a 24-bit mask:
10.108.48.0 = 00001010 01101100 00110000 00000000
10.108.49.0 = 00001010 01101100 00110001 00000000
10.108.50.0 = 00001010 01101100 00110010 00000000
10.108.51.0 = 00001010 01101100 00110011 00000000
10.108.52.0 = 00001010 01101100 00110100 00000000
10.108.53.0 = 00001010 01101100 00110101 00000000
10.108.54.0 = 00001010 01101100 00110110 00000000
10.108.55.0 = 00001010 01101100 00110111 00000000
Notice that the first 21 bits of the subnetwork IDs are all common. These can be masked off. You can use the single route entry for all these subnetworks as follows:
10.108.48.0/21
For any routing protocol within the scope of the CCIE written exam, an administrator can disable any automatic summarization that might be occurring and configure “manual” summarization. To engage in route summarization, find all the leftmost bits that are in common and create a mask that encompasses them. An example follows:-
The following routes exist in the routing table—all routes use a 24-bit mask:
10.108.48.0 = 00001010 01101100 00110000 00000000
10.108.49.0 = 00001010 01101100 00110001 00000000
10.108.50.0 = 00001010 01101100 00110010 00000000
10.108.51.0 = 00001010 01101100 00110011 00000000
10.108.52.0 = 00001010 01101100 00110100 00000000
10.108.53.0 = 00001010 01101100 00110101 00000000
10.108.54.0 = 00001010 01101100 00110110 00000000
10.108.55.0 = 00001010 01101100 00110111 00000000
Notice that the first 21 bits of the subnetwork IDs are all common. These can be masked off. You can use the single route entry for all these subnetworks as follows:
10.108.48.0/21
EIGRP Authentication
By default, no authentication is used for any routing protocol. Some protocols, such as RIPv2, IS-IS, and OSPF, can be configured to do simple password authentication between neighboring routers. In this type of authentication, a clear-text password is used. EIGRP does not support simple authentication. However, it can be configured to authenticate each packet exchanged, using an MD5 hash. This is more secure than clear text, as only the message digest is exchanged, not the password.
EIGRP authenticates each of its packets by including the hash in eachone. This helps verify the source of each routing update.
To configure EIGRP authentication, follow these steps:
Step 1. Configure a key chain to group the keys.
Step 2. Configure a key within that key chain.
Step 3. Configure the password or authentication string for thatkey. Repeat Steps 2 and 3 to add more keys if desired.
Step 4. Optionally configure a lifetime for the keys within that key chain. If you do this, be sure that the time is synchronized between the two routers.
Step 5. Enable authentication and assign a key chain to an interface.
Step 6. Designate MD5 as the type of authentication.
EIGRP Bandwidth Configuration
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary classless routing protocol that uses a complex metric based on bandwidth and delay.
By default, EIGRP limits itself to bursting to half the link bandwidth.
This limit is configurable per interface using the ip bandwidth-percent command. The following example assumes EIGRP AS 7 and limits
EIGRP to one quarter of the link bandwidth:
Router(config)#int s0/0/0
Router(config-if)#ip bandwidth-percent eigrp 7 25
The real issue with WAN links is that the router assumes that each link has 1544 kbps bandwidth. If interface Serial0/0/0 is attached to a 128k fractional T1, EIGRP assumes it can burst to 768k and could overwhelm the line. This is rectified by correctly identifying link bandwidth.
Router (config)#int serial 0/0/0
Router (config-if)#bandwidth 128
The following shows a situation in which these techniques can be combined.
In this example, R1 has a 256 kbps connection to the Frame Relay network and two permanent virtual circuits (PVCs) with committed information rates (CIR) of 128 Kpbs and 64 Kbps. EIGRP divides the interface bandwidth evenly between the number of neighbors on that interface. What value should be used for the interface bandwidth in this case? The usual suggestion is to use the CIR, but the two PVCs have different CIRs. You could use the bandwidth-percent command to allow SNMP reporting of the true bandwidth value, while adjusting the interface burst rate to 25 percent, or 64 kbps.
R1(config)#int serial 0/0/0
R1 (config-if)#bandwidth 256
R1 (config-if)#ip bandwidth-percent eigrp 7 25
A better solution is to use sub-interfaces and identify bandwidth separately.
In the following example, s0/0/0.1 bursts to 64 k, and s0/0/0.2 bursts to 32 k, using EIGRP’s default value of half the bandwidth.
R1(config)#int serial 0/0/0.1
R1 (config-if)#bandwidth 128
!
R1(config)#int serial 0/0/0.2
R1 (config-if)#bandwidth 64
In cases where the hub interface bandwidth is oversubscribed, it may be necessary to set bandwidth for each sub-interface arbitrarily low, and then specify an EIGRP bandwidth percent value over 100 in order to allow EIGRP to use half the PVC bandwidth.
