Name: EIGRP
Long Name: Enhanced Interior Gateway Routing Protocol
Supported Protocols: IP, IPX, Appletalk
Standard: Cisco Proprietary
Transport Protocol: IP
Routing Protocol Type: Hybrid
Algorithm: DUAL
Hello Timers: 5 seconds (high speed links) 60 seconds (wan links 1.5mb and lower)
Dead Timers: 15 seconds(high speed links) 180 seconds (wan links 1.5mb and lower)
EIGRP Active Timer: 3 minutes
Neighbor Discovery Type: Auto (multicast/unicast)
Multicast IP: 224.0.0.10
Internal AD: 90
External AD: 170
Route Summarization: Yes (auto by default)
RTP: Reliable Transport Protocol
EIGRP Protocol Messages:
HELLO
UPDATE (sent reliably via RTP)
QUERY (sent reliably via RTP)
REPLY (sent reliably via RTP)
ACK (acknowledgement)
EIGRP Update Message Content:
PREFIX
PREFIX LENGTH
METRIC: Bandwidth, Delay, Reliability and Load
OTHER ITEMS: MTU & Hop Count
Metric is calculated as follows:
Metric = 256 * ((10^7 / lowest bandwidth in path) + Cumulative Delay)
Metric with K value weights:
Metric = 256*((K1*Bw) + (K2*Bw)/(256-Load) + K3*Delay)*(K5/(Reliability + K4)))
EIGRP Load Balancing Default: Maximum Paths 4
Feasible Distance (FD): Metric based on local route to destination metric
Reported Distance (RD): Metric based on the neighbors metric to the destination metric
Successor Route: Lowest FD to the destination
Feasible Successor: Backup to destination if feasibility condition is met
Feasibility Condition: If a non-successor route’s RD is less than the FD, the route is a feasible successor
EIGRP Stub Options:
Connected: Advertise connected routes for interfaces matched with the network command
Summary: Advertise auto-summarized or manually configured summary routes
Redistributed: Advertise routes learned from configured redistribution
Receive-only: No routes are advertised
Static: Advertise static routes but must be used with the “redistribute static” command
2: Detailed Information
EIGRP Description:
EIGRP is a Cisco proprietary protocol primarily used on private networks. EIGRP is considered a hybrid distance vector protocol as it shares several attributes from distance vector and link state protocols. EIGRP used an automatic discovery process by sending EIGRP hello messages to the multicast address 224.0.0.10. EIGRP operates using three main tables: Topology Table, Routing Table and the Neighbor Table. Once neighbors have been discovered, EIGRP shares its entire topology table with its neighbor similar to that of a typical distance vector protocol. Local routers perform DUAL to place the best loop free routes into the routing table. Updates are sent using RTP (reliable transport protocol) for data exchange. EIGRP uses two messages as part of the update process (update & ACK).
EIGRP Update Process:
1. When two routers discover each other they exchange full topology table updates.
2. After the full topology (all prefixes) has been exchanged with a neighbor the updates are stopped, there is not a re-aging process and the topology table is not re-sent.
3. If a change occurs, a partial update is sent containing changed information
4. If neighbors fail and re-establish, the full topology table is re-sent thus repeating the cycle
Note: EIGRP Utilizes the split horizon rule, updates for prefixes will not be sent out via the same interface in which they were learned.
What triggers an EIGRP update?
Metric change
Link failure
Link Recovery
Other neighbors learn new prefixes
EIGRP Metric Information:
The EIGRP update process includes metrics for calculation for best routes. EIGRP uses bandwith, delay, reliability and load. By default, only bandwidth and delay are used. Both of these metrics are set via an interface command. Bandwidth is set to a default value of 1544 for T1 serial interfaces and auto learns bandwidth on higher speed links. Delay can be set to manipulate the routing metric for manual control of best path. By default EIGRP routing updates utilize up to 50% of the bandwidth of the circuit, because of this it is important the bandwidth on the interface is set correctly. EIGRP metrics weights can be added and are represented as “K” values.
Metric is calculated as follows:
Metric = 256 * ((10^7 / lowest bandwidth in path) + Cumulative Delay)
K Values Metric:
EIGRP metrics weights can be added and are represented as “K” values. K values can be changed per router but k values MUST match on each router in order to establish neighbor relationships.
Metric with K value weights:
EIGRP Metric = 256*((K1*Bw) + (K2*Bw)/(256-Load) + K3*Delay)*(K5/(Reliability + K4)))
EIGRP Load Balancing:
By default, EIGRP will load balance up for 4 equal cost paths. This is set with the “maximum-paths” EIGRP sub command. Up to 16 maximum paths can be set. EIGRP’s metric calculation is very detailed and it can be difficult to ensure all metrics are equal. The EIGRP variance can be used to tell the IOS that metrics within a certain value multiplier can be treated equal even if they are not an exact match. The variance multiplier can be set between 1 and 128.
An example of variance is as follows:
Best route through Serial0: 500300
Secondary route through Serial1: 600300
These two routes would not load balance, but with a variance of 2 as long as the secondary route is under the primary routes metric x 2 it would be treated equal. In this case, 500300 x 2 = 1000600 and since the secondary route of 600300 is smaller, the router would then load balance traffic for that route through S0 and S1.
EIGRP Route Types and Feasibility Condition:
As EIGRP builds its routing tables, a computation is performed called DUAL. Routes with the lowest metric are placed into the routing table. These routes are called successor routes. Due to CPU constraints and the query process routers constantly performing DUAL could introduce stability and convergence concerns. A built-in method of combatting this is by utilizing backup routes in that if the successor route is down, a backup route can be used in its place without the need for DUAL calculation. This backup route is called a feasible successor. How does a route become a feasible successor? It must meet feasibility condition and a router can have multiple feasible successors. When the successor fails, the next best feasible successor quickly become the successor and new feasible successors are calculated based on the new feasibility condition.
EIGRP Terms:
Feasible Distance (FD): Metric based on local route to destination metric
Reported Distance (RD): Metric based on the neighbors metric to the destination metric
Successor Route: Lowest FD to the destination
Feasible Successor: Backup to destination if feasibility condition is met
Feasibility Condition: If a non-successor route’s RD is less than the FD, the route is a feasible successor
EIGRP Convergence:
When EIGRP loses the successor route it looks for a feasible successor (FS). IF no FS is present meaning non-successor routes do not meet feasibility condition the route will perform DUAL. This process will attempt to discover a loop-free alternative route to reach the prefix for which the successor route was lost. This process is called going active.
Going Active:
The state is changed from passive (working) to active (non-working). A query message is sent to every neighbor except the neighbor with the failed route. The query message asks the neighbor if a loop-free route exists and if it does it will reply with an EIGRP reply message saying it does and not forward the query any further. If loop free route does NOT exist the query is forwarded and it will wait for a response before sending an EIGRP reply to the original requestor. When the original router that sent the query receives all the responses back, a new route is confirmed to be loop-free.
Stuck in Active:
When a router does not receive an EIGRP reply to a query within 3 minutes the route will become stuck in active (SIA). When a route is SIA the router is assumed to also be failed thus the neighbor relationship is reset and the learning process will start over again. In IOS version 12.2 or higher, a second SIA message is sent half-way through the active timer (90 seconds) sending a second request (SIA Query). This helps prevent the route from going SIA in the event the neighbor relationship is still established but a response was not received.
Note: Stuck in Active routes can be very harmful to the health of an EIGRP infrastructure, it is important to limit the SIA query range and design the network with the intention of limiting SIA routes.
Limiting the Query Range:
There are two main ways to limit the EIGRP query range:
Network Summarization
EIGRP Stub Networks
When a hub router is advertising networks via a summary, it will automatically respond on behalf of the neighbors for the routes in the summary advertisement. Therefore EIGRP query messages will never be forwarded past routers that are summarizing prefixes that would normally receive a query.
This same logic is performed for hub routers connected to EIGRP stubs, the hub routers answer on behalf of the stubs.
Note: EIGRP queries can be extended beyond the autonomous system, it is a common misconception that query ranges can be limited by splitting up EIGRP infrastructure through multiple AS and redistribution.
EIGRP Stub Networks:
A branch router should be configured as an EIGRP stub. There is no need for a branch router with a single EIGRP neighbor to advertise EIGRP learned routes from one neighbor to others. By default stub routers will only advertise connected and manually configured summary routers to their neighbor (hub).
EIGRP Stub Options:
Connected: Advertise connected routes for interfaces matched with the network command
Summary: Advertise auto-summarized or manually configured summary routes
Redistributed: Advertise routes learned from configured redistribution
Receive-only: No routes are advertised
Static: Advertise static routes but must be used with the “redistribute static” command
3: EIGRP Command Reference
EIGRP Commands/sub commands:
Enable EIGRP:
router eigrp “asn”
Example: router eigrp 10
Configure networks to advertise:
network “ip address” “wildcard mask
Example: network 192.168.1.0 0.0.0.255
Clear EIGRP neighbors:
clear ip eigrp neighbors
Static Neighbor Configuration:
neighbor “ip address” “interface”
Example: neighbor 10.1.1.2 S0
Auto Summarization:
enable (default): auto-summary
disable: no auto-summary
Example: no auto-summary
Enable EIGRP Variance:
variance “1-128” (default=1)
Example: variance 2
Configure EIGRP Active Timer:
timers active-time “minutes” (default: 3)
Disable: no timers active-time
Example: timers active-time 150
Configure K Values:
metric weights “tos” “k1” “k2” “k3” “k4” “k5”
Example: metric weights 0 2 0 1 0 0
Configure EIGRP Stub:
eigrp stub [receive only | connected | static | summary]
Example: eigrp stub connected
Enable EIGRP Logging:
eigrp log-neighbor-changes
EIGRP Interface Commands:
Configure link bandwidth used:
ip bandwidth-percent eigrp “%”
Example: ip bandwidth-percent eigrp 20
Configure EIGRP Summary Address:
ip summary-address eigrp “as” “ip address” “mask”
Example: ip summary-address eigrp 10 10.10.1.0 255.255.252.0
Changing the Hello Interval:
ip hello-interval eigrp “as” “seconds”
Example: ip hello-interval eigrp 10 15
Changing the Hold Time:
ip hold-time eigrp “as” “seconds”
Example: ip hold-time eigrp 10 45
Disable Split Horizon:
no ip split-horizon eigrp “as”
Example: no ip split-horizon eigrp 10
Show/Debug commands:
Display neighbors discovered by eigrp: show ip eigrp neigbors
Display neighbor detail (verify stub routing): show ip eigrp neighbor detail
Display the EIGRP topology table: show ip eigrp topology
Display the configured EIGRP interfaces: show ip eigrp interfaces
Display EIGRP traffic: show ip eigrp traffic
Display EIGRP routes from the IP routing table: show ip route eigrp
EIGRP Packet Debug: debug ip eigrp packet
EIGRP Neighbor Debug: debug ip eigrp neighbor
Long Name: Enhanced Interior Gateway Routing Protocol
Supported Protocols: IP, IPX, Appletalk
Standard: Cisco Proprietary
Transport Protocol: IP
Routing Protocol Type: Hybrid
Algorithm: DUAL
Hello Timers: 5 seconds (high speed links) 60 seconds (wan links 1.5mb and lower)
Dead Timers: 15 seconds(high speed links) 180 seconds (wan links 1.5mb and lower)
EIGRP Active Timer: 3 minutes
Neighbor Discovery Type: Auto (multicast/unicast)
Multicast IP: 224.0.0.10
Internal AD: 90
External AD: 170
Route Summarization: Yes (auto by default)
RTP: Reliable Transport Protocol
EIGRP Protocol Messages:
HELLO
UPDATE (sent reliably via RTP)
QUERY (sent reliably via RTP)
REPLY (sent reliably via RTP)
ACK (acknowledgement)
EIGRP Update Message Content:
PREFIX
PREFIX LENGTH
METRIC: Bandwidth, Delay, Reliability and Load
OTHER ITEMS: MTU & Hop Count
Metric is calculated as follows:
Metric = 256 * ((10^7 / lowest bandwidth in path) + Cumulative Delay)
Metric with K value weights:
Metric = 256*((K1*Bw) + (K2*Bw)/(256-Load) + K3*Delay)*(K5/(Reliability + K4)))
EIGRP Load Balancing Default: Maximum Paths 4
Feasible Distance (FD): Metric based on local route to destination metric
Reported Distance (RD): Metric based on the neighbors metric to the destination metric
Successor Route: Lowest FD to the destination
Feasible Successor: Backup to destination if feasibility condition is met
Feasibility Condition: If a non-successor route’s RD is less than the FD, the route is a feasible successor
EIGRP Stub Options:
Connected: Advertise connected routes for interfaces matched with the network command
Summary: Advertise auto-summarized or manually configured summary routes
Redistributed: Advertise routes learned from configured redistribution
Receive-only: No routes are advertised
Static: Advertise static routes but must be used with the “redistribute static” command
2: Detailed Information
EIGRP Description:
EIGRP is a Cisco proprietary protocol primarily used on private networks. EIGRP is considered a hybrid distance vector protocol as it shares several attributes from distance vector and link state protocols. EIGRP used an automatic discovery process by sending EIGRP hello messages to the multicast address 224.0.0.10. EIGRP operates using three main tables: Topology Table, Routing Table and the Neighbor Table. Once neighbors have been discovered, EIGRP shares its entire topology table with its neighbor similar to that of a typical distance vector protocol. Local routers perform DUAL to place the best loop free routes into the routing table. Updates are sent using RTP (reliable transport protocol) for data exchange. EIGRP uses two messages as part of the update process (update & ACK).
EIGRP Update Process:
1. When two routers discover each other they exchange full topology table updates.
2. After the full topology (all prefixes) has been exchanged with a neighbor the updates are stopped, there is not a re-aging process and the topology table is not re-sent.
3. If a change occurs, a partial update is sent containing changed information
4. If neighbors fail and re-establish, the full topology table is re-sent thus repeating the cycle
Note: EIGRP Utilizes the split horizon rule, updates for prefixes will not be sent out via the same interface in which they were learned.
What triggers an EIGRP update?
Metric change
Link failure
Link Recovery
Other neighbors learn new prefixes
EIGRP Metric Information:
The EIGRP update process includes metrics for calculation for best routes. EIGRP uses bandwith, delay, reliability and load. By default, only bandwidth and delay are used. Both of these metrics are set via an interface command. Bandwidth is set to a default value of 1544 for T1 serial interfaces and auto learns bandwidth on higher speed links. Delay can be set to manipulate the routing metric for manual control of best path. By default EIGRP routing updates utilize up to 50% of the bandwidth of the circuit, because of this it is important the bandwidth on the interface is set correctly. EIGRP metrics weights can be added and are represented as “K” values.
Metric is calculated as follows:
Metric = 256 * ((10^7 / lowest bandwidth in path) + Cumulative Delay)
K Values Metric:
EIGRP metrics weights can be added and are represented as “K” values. K values can be changed per router but k values MUST match on each router in order to establish neighbor relationships.
Metric with K value weights:
EIGRP Metric = 256*((K1*Bw) + (K2*Bw)/(256-Load) + K3*Delay)*(K5/(Reliability + K4)))
EIGRP Load Balancing:
By default, EIGRP will load balance up for 4 equal cost paths. This is set with the “maximum-paths” EIGRP sub command. Up to 16 maximum paths can be set. EIGRP’s metric calculation is very detailed and it can be difficult to ensure all metrics are equal. The EIGRP variance can be used to tell the IOS that metrics within a certain value multiplier can be treated equal even if they are not an exact match. The variance multiplier can be set between 1 and 128.
An example of variance is as follows:
Best route through Serial0: 500300
Secondary route through Serial1: 600300
These two routes would not load balance, but with a variance of 2 as long as the secondary route is under the primary routes metric x 2 it would be treated equal. In this case, 500300 x 2 = 1000600 and since the secondary route of 600300 is smaller, the router would then load balance traffic for that route through S0 and S1.
EIGRP Route Types and Feasibility Condition:
As EIGRP builds its routing tables, a computation is performed called DUAL. Routes with the lowest metric are placed into the routing table. These routes are called successor routes. Due to CPU constraints and the query process routers constantly performing DUAL could introduce stability and convergence concerns. A built-in method of combatting this is by utilizing backup routes in that if the successor route is down, a backup route can be used in its place without the need for DUAL calculation. This backup route is called a feasible successor. How does a route become a feasible successor? It must meet feasibility condition and a router can have multiple feasible successors. When the successor fails, the next best feasible successor quickly become the successor and new feasible successors are calculated based on the new feasibility condition.
EIGRP Terms:
Feasible Distance (FD): Metric based on local route to destination metric
Reported Distance (RD): Metric based on the neighbors metric to the destination metric
Successor Route: Lowest FD to the destination
Feasible Successor: Backup to destination if feasibility condition is met
Feasibility Condition: If a non-successor route’s RD is less than the FD, the route is a feasible successor
EIGRP Convergence:
When EIGRP loses the successor route it looks for a feasible successor (FS). IF no FS is present meaning non-successor routes do not meet feasibility condition the route will perform DUAL. This process will attempt to discover a loop-free alternative route to reach the prefix for which the successor route was lost. This process is called going active.
Going Active:
The state is changed from passive (working) to active (non-working). A query message is sent to every neighbor except the neighbor with the failed route. The query message asks the neighbor if a loop-free route exists and if it does it will reply with an EIGRP reply message saying it does and not forward the query any further. If loop free route does NOT exist the query is forwarded and it will wait for a response before sending an EIGRP reply to the original requestor. When the original router that sent the query receives all the responses back, a new route is confirmed to be loop-free.
Stuck in Active:
When a router does not receive an EIGRP reply to a query within 3 minutes the route will become stuck in active (SIA). When a route is SIA the router is assumed to also be failed thus the neighbor relationship is reset and the learning process will start over again. In IOS version 12.2 or higher, a second SIA message is sent half-way through the active timer (90 seconds) sending a second request (SIA Query). This helps prevent the route from going SIA in the event the neighbor relationship is still established but a response was not received.
Note: Stuck in Active routes can be very harmful to the health of an EIGRP infrastructure, it is important to limit the SIA query range and design the network with the intention of limiting SIA routes.
Limiting the Query Range:
There are two main ways to limit the EIGRP query range:
Network Summarization
EIGRP Stub Networks
When a hub router is advertising networks via a summary, it will automatically respond on behalf of the neighbors for the routes in the summary advertisement. Therefore EIGRP query messages will never be forwarded past routers that are summarizing prefixes that would normally receive a query.
This same logic is performed for hub routers connected to EIGRP stubs, the hub routers answer on behalf of the stubs.
Note: EIGRP queries can be extended beyond the autonomous system, it is a common misconception that query ranges can be limited by splitting up EIGRP infrastructure through multiple AS and redistribution.
EIGRP Stub Networks:
A branch router should be configured as an EIGRP stub. There is no need for a branch router with a single EIGRP neighbor to advertise EIGRP learned routes from one neighbor to others. By default stub routers will only advertise connected and manually configured summary routers to their neighbor (hub).
EIGRP Stub Options:
Connected: Advertise connected routes for interfaces matched with the network command
Summary: Advertise auto-summarized or manually configured summary routes
Redistributed: Advertise routes learned from configured redistribution
Receive-only: No routes are advertised
Static: Advertise static routes but must be used with the “redistribute static” command
3: EIGRP Command Reference
EIGRP Commands/sub commands:
Enable EIGRP:
router eigrp “asn”
Example: router eigrp 10
Configure networks to advertise:
network “ip address” “wildcard mask
Example: network 192.168.1.0 0.0.0.255
Clear EIGRP neighbors:
clear ip eigrp neighbors
Static Neighbor Configuration:
neighbor “ip address” “interface”
Example: neighbor 10.1.1.2 S0
Auto Summarization:
enable (default): auto-summary
disable: no auto-summary
Example: no auto-summary
Enable EIGRP Variance:
variance “1-128” (default=1)
Example: variance 2
Configure EIGRP Active Timer:
timers active-time “minutes” (default: 3)
Disable: no timers active-time
Example: timers active-time 150
Configure K Values:
metric weights “tos” “k1” “k2” “k3” “k4” “k5”
Example: metric weights 0 2 0 1 0 0
Configure EIGRP Stub:
eigrp stub [receive only | connected | static | summary]
Example: eigrp stub connected
Enable EIGRP Logging:
eigrp log-neighbor-changes
EIGRP Interface Commands:
Configure link bandwidth used:
ip bandwidth-percent eigrp “%”
Example: ip bandwidth-percent eigrp 20
Configure EIGRP Summary Address:
ip summary-address eigrp “as” “ip address” “mask”
Example: ip summary-address eigrp 10 10.10.1.0 255.255.252.0
Changing the Hello Interval:
ip hello-interval eigrp “as” “seconds”
Example: ip hello-interval eigrp 10 15
Changing the Hold Time:
ip hold-time eigrp “as” “seconds”
Example: ip hold-time eigrp 10 45
Disable Split Horizon:
no ip split-horizon eigrp “as”
Example: no ip split-horizon eigrp 10
Show/Debug commands:
Display neighbors discovered by eigrp: show ip eigrp neigbors
Display neighbor detail (verify stub routing): show ip eigrp neighbor detail
Display the EIGRP topology table: show ip eigrp topology
Display the configured EIGRP interfaces: show ip eigrp interfaces
Display EIGRP traffic: show ip eigrp traffic
Display EIGRP routes from the IP routing table: show ip route eigrp
EIGRP Packet Debug: debug ip eigrp packet
EIGRP Neighbor Debug: debug ip eigrp neighbor
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very useful
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