What is Stuck In Active (EIGRP)?

Stuck In Active (EIGRP)

The EIGRP convergence is the process when going active on a route is sometimes also by the name of the underlying algorithm, diffusing update algorithm (DUAL).

When a successor path is lost and there is no feasible successor is identified, the EIGRP router sends out queries on all interfaces in an attempt to identify an alternate path. EIGRP routers don’t select the successor until the EIGRP router receives a reply to all queries. When a router has received a reply for all its query messages, that router can safely use the best of the routes confirmed to be loop-free. The process can and does work well in many cases, often converging to a new route in less than 10 seconds.  If a reply is missing for 3 minutes, the router becomes stuck in active (SIA). In that case, it reset the neighbor's relationship with the neighbors that did not reply to queries.

EIGRP sends every query and reply message using RTP, so every message is acknowledged using the EIGRP ACK message.

To limit the scope of queries, there are two tools: stub routers and route summarization this us to reduce the work performed by the DUAL and the scope of query messages.

EIGRP STUB

Stub routing is one way to limit queries. A stub router is one that is connected to more than two neighbors and should be a transit router. The EIGRP stub routing feature improves network stability, reduces resource utilization, and simplifies remote router (spoke) configuration. Stub routing is commonly used in a Hub-and-Spoke topology.

Here is the command:

R1(config-router)#EIGRP STUB?

  connected       Do advertise connected routes

  leak-map        Allow dynamic prefixes based on the leak-map

  receive-only    Set receive only neighbor

  redistributed   Do advertise redistributed routes

  static                 Do advertise static routes

  summary         Do advertise summary routes

·         Connected means the router advertises connected routes only for those interfaces that match with a network command.

·         Leak-map means the router advertises only which is specified by a leak-map

·         Receive-only means the router does not advertise any routes. This option can be used with any other option.

·         Redistribution means the router advertises only the redistributed routes.

·         Static means the router advertises only the static routes with redistributed static commands configured.

·         Summary means the router advertises only auto-summarized or statically configured summary routes

NOTE: By default, is connected and summary.

Before we start EIGRP configuration check out some important of EIGRP CCIE exam topics.

Here are the lists of all EIGRP labs and theories CCNA to CCIE

1.Enhanced Interior Gateway Routing Protocol (EIGRP)

2.Stuck In Active (EIGRP)

3.EIGRP Equal-Cost and Unequal-Cost load balancing

4.Route filtering passive interface

5.Route summarization

6.EIGRP STUB and Configuration

7.Routing Protocol Authentication (EIGRP ) and configuration

 

 

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What is EIGRP Equal-Cost and Unequal-Cost load balancing? What is variance? How to configure?

EIGRP Equal-Cost and Unequal-Cost load balancing

Routes with the lowest metric get installed in the routing table by default, if two or more routes have the same metric the router will install both the routes in the routing table this is called equal cost load balancing.

EIGRP does equal cost load balancing automatically, whereas unequal cost load balancing is not automatic, for unequal cost load balancing we need to enable Variance. EIGRP can load balancing up to six paths, and the default is four paths. 

The maximum number of paths are based on Cisco IOS versions and router platform. For the older Cisco IOS versions, the maximum-paths was 6. The latest version typically supports 16 or more.

 R1 (config)#router eigrp 100

R1 (config-router)#maximum-paths ?

  <1-32> Number of paths

Unequal cost load-balancing

EIGRP also supports unequal cost load balancing, it has to be done manually using variance. Variance is a multiplier value that ranges between ­(1 – 128) and the default is 1. The routes that can go for load balancing should satisfy the condition. The cost of the successor X variance > cost of the other routes to be used for load balancing

• ·         The variance is multiplied by the current Feasible Distance.
• ·         Any feasible successor whose calculated metric is less than or equal to the product of the variance and feasible distance is added to the IP routing table, assuming that the maximum-paths setting allows more routes
• ·         Routes that are neither successor nor feasible successor routes can never add to the IP routing table, regardless of the variance setting. 

Before we start  EIGRP configuration check out some important of EIGRP CCIE exam topics.

Here are the lists of all EIGRP labs and theories CCNA to CCIE

1.Enhanced Interior Gateway Routing Protocol (EIGRP)

2.Stuck In Active (EIGRP)

3.EIGRP Equal-Cost and Unequal-Cost load balancing

4.Route filtering passive interface

5.Route summarization

6.EIGRP STUB and Configuration

7.Routing Protocol Authentication (EIGRP ) and configuration

Configuration of EIGRP Equal cost load balancing-

Topology:

•      Configure the topology as per the diagram
•          Advertise the interfaces using EIGRP 100
•          Ensure the load balancing via traceroute.

R1#show ip interface  brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        10.1.1.1        YES manual up                    up

Serial3/0              1.1.1.1         YES manual up                    up

Serial3/3              4.4.4.2         YES manual up                    up

R2#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        20.1.1.1        YES manual up                    up

FastEthernet1/0        unassigned      YES unset  administratively down down

FastEthernet1/1        unassigned      YES unset  administratively down down

GigabitEthernet2/0     unassigned      YES unset  administratively down down

Serial3/0              1.1.1.2         YES manual up                    up

Serial3/1              2.2.2.1         YES manual up                    up

R3#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        30.1.1.1        YES manual up                    up

FastEthernet1/0        unassigned      YES unset  administratively down down

FastEthernet1/1        unassigned      YES unset  administratively down down

GigabitEthernet2/0     unassigned      YES unset  administratively down down

Serial3/0              unassigned      YES unset  administratively down down

Serial3/1              2.2.2.2         YES manual up                    up

Serial3/2              3.3.3.1         YES manual up                    up

R4#show ip interface brief

Interface              IP-Address      OK? Method Status                Protocol

FastEthernet0/0        40.1.1.1        YES manual up                    up

Serial3/2              3.3.3.2         YES manual up                    up

Serial3/3              4.4.4.1         YES manual up                    up

R1(config)#router eigrp 100

R1(config-router)#network 1.0.0.0

R1(config-router)#network 4.0.0.0

R1(config-router)#network 10.0.0.0

R1(config-router)#no auto-summary

R2(config)#router eigrp 100

R2(config-router)#network 20.0.0.0

R2(config-router)#network 1.0.0.0

R2(config-router)#network 2.0.0.0

R2(config-router)#no auto-summary

R3(config)#router eigrp 100

R3(config-router)#network 30.0.0.0

R3(config-router)#network 2.0.0.0

R3(config-router)#network 3.0.0.0

R3(config-router)#no auto-summary

R4(config)#router eigrp 100

R4(config-router)#network 4.0.0.0

R4(config-router)#network 3.0.0.0

R4(config-router)#network 40.0.0.0

R4(config-router)#no auto-summary

R1#show ip eigrp 100 neighbors

EIGRP-IPv4 Neighbors for AS(100)

H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq

                                                   (sec)         (ms)       Cnt Num

1   4.4.4.1                 Se3/3                    12 00:15:50   37   222  0  8

0   1.1.1.2                 Se3/0                    13 00:17:52   67   402  0  20

R3#show  ip eigrp neighbors

EIGRP-IPv4 Neighbors for AS(100)

H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq

                                                   (sec)         (ms)       Cnt Num

1   3.3.3.2                 Se3/2                    12 00:17:17   41   246  0  9

0   2.2.2.1                 Se3/1                    11 00:17:45   48   288  0  19

R1#show ip route eigrp

Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP

       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

       E1 - OSPF external type 1, E2 - OSPF external type 2

       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

       ia - IS-IS inter area, * - candidate default, U - per-user static route

       o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP

       + - replicated route, % - next hop override

Gateway of last resort is not set

D     2.0.0.0/8 [90/2681856] via 1.1.1.2, 00:17:19, Serial3/0

D     3.0.0.0/8 [90/2681856] via 4.4.4.1, 00:17:19, Serial3/3

D     20.0.0.0/8 [90/2172416] via 1.1.1.2, 00:17:19, Serial3/0

D     30.0.0.0/8 [90/2684416] via 4.4.4.1, 00:17:19, Serial3/3

                 [90/2684416] via 1.1.1.2, 00:17:19, Serial3/0

D     40.0.0.0/8 [90/2172416] via 4.4.4.1, 00:17:19, Serial3/3

The above output you can see that the router R1 installed both routes in the routing table as it is using both the routes to reach the network 30.1.1.1.

R1#show  ip eigrp topology

EIGRP-IPv4 Topology Table for AS(100)/ID(10.1.1.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

       r - reply Status, s - sia Status

P 2.0.0.0/8, 1 successors, FD is 2681856

        via 1.1.1.2 (2681856/2169856), Serial3/0

P 4.0.0.0/8, 1 successors, FD is 2169856

        via Connected, Serial3/3

P 10.0.0.0/8, 1 successors, FD is 28160

        via Connected, FastEthernet0/0

P 30.0.0.0/8, 2 successors, FD is 2684416

        via 1.1.1.2 (2684416/2172416), Serial3/0

        via 4.4.4.1 (2684416/2172416), Serial3/3

P 3.0.0.0/8, 1 successors, FD is 2681856

        via 4.4.4.1 (2681856/2169856), Serial3/3

P 40.0.0.0/8, 1 successors, FD is 2172416

        via 4.4.4.1 (2172416/28160), Serial3/3

P 1.0.0.0/8, 1 successors, FD is 2169856

        via Connected, Serial3/0

P 20.0.0.0/8, 1 successors, FD is 2172416

        via 1.1.1.2 (2172416/28160), Serial3/0

R1#traceroute 30.1.1.1

Type escape sequence to abort.

Tracing the route to 30.1.1.1

VRF info: (vrf in name/id, vrf out name/id)

  1 1.1.1.2 76 msec

    4.4.4.1 112 msec

    1.1.1.2 52 msec

  2 3.3.3.1 100 msec

    2.2.2.2 68 msec

    3.3.3.1 48 msec

now R1 is doing equal cost load balancing vea R2 and R4 because both the routes are having equal cost to reach network 30.1.1.1

EIGRP (basic)

EIGRP is a classless, distance vector routing protocol that uses the concept of an autonomous system to describe a set of contiguous routers that run the same routing protocol and share routing information, which also includes subnet mask in its route updates.

EIGRP is sometimes referred to as a hybrid routing protocol or an advanced distance vector protocol because is has characteristics of both distance-vector some link-state protocols. For example, EIGRP doesn't send link-state packets like OSPF does. instead, it traditional distance-vector updates that include information about networks plus the cost o reaching them from the perspective of the advertising router.

EIGRP has link-state characteristics as well it synchronize network topology information between neighbors at startup and then sends specific updates only when topology change occur.

EIGRP has a default hop count of 100, with maximum of 255. In EIGRP speak, hop count refers to how many router an EIGRP update packet can go through before it will be discarded, which limits the size of the autonomous system (AS).

Here's a list of some powerful features:

• Support for IP and IPv6 
• Considered classless 
• Support for VLSM/CIDR
• Support for summaries and discontinuous networks
• Efficient neighbor discovery
• Communication via Reliable Transport Protocol (RTP)
• Best path selection via diffusing Update Algorithm (DUAL)
• Reduced bandwidth usage with bounded updates
• No broadcast  

   

Neighbour Discovery- before EIGRP routers can exchange routes with each other, they must become neighbors, and there are three conditions that must be met before this can happen.

• Hello and Acknowledge received
• Autonomous System (AS) numbers match 
• Identical metrics (K values)

EIGRP routers that belong to different AS don't become neighbor and will not share routing information unless redistribution.

Hello messages are used to identify neighbors and serve a keepalive mechanism between neighbor. EIGRP hello messages are send to multicast address 224.0.0.10 and FF20::A in IPv6. Hello messages send every 5 seconds.

Reported/advertisement distance (AD)  this is the metric of a remote network, as reported by a neighbor. In other words Reported Distance is the neighbor's distance to the destination as reported in an EIGRP packet received from neighbor. It is also the routing table metric of the neighbor router.

In short: AD is the neighbor router distance to the destination.

Feasible distance (FD) this is the best metric among all paths to a remote network, the route with the lowest FD that you'll find in the routing table because its considered the best path. The metric of a feasible distance is calculated using the metric reported by the neighbor that's referred to as the reported or advertised distance plus the metric to the neighbor reporting the route.

In short : FD is the total cost from local router to destination.

Feasible successor (FC): FC is basically an entry in the topology table that represent a path that's inferior to the successor route. FC is define as a path whose advertised distance is less than the feasible distance of the current successor and considered a backup route.

In short: Feasible Successor is the backup route and stored in the topology table.

Neighbor table: Each router keeps state information about adjacent neighbors. when a newly discovered neighbor is found, its address and interface are recorded in the routing table, stored in RAM.

In short: neighbor table contain the list of directly connected router.

Topology table: is populated by the neighbor table and the Diffusing Update Algorithm (DUAL) calculates the best loop-free path to each remote network. it contains all the destinations advertisement by neighboring routers.

In short: topology table contain list all the best route learned from neighbor.

Successor: a successor route is the best route to a remote network. a successor will be copied from the topology table to routing table.

Reliable Transport Protocol (RTP): EIGRP uses RTP and its function is to deliver EIGRP packets between neighbors in a reliable and ordered way. It can use multicast or unicast and to keep things efficient not all packets are sent reliable. Reliable means that when we send a packet we want to get an acknowledgment from the other side to make sure that they received it.

Here's description of the five different types of packets used by EIGRP:

Hello   A hello packets are used to discover EIGRP neighbors and sent via unreliable multicast, meaning  its doesn't require an acknowledgement. 

Update packets have routing information and are sent reliable to whatever router that require this information. Update packets can be sent to a single neighbor using unicast or to a group of neighbors using multicast.

Query packets are used when your EIGRP router hast information about a certain network and doesn't have any backup paths. router will send query packets to its neighbors asking them if they have information about this particular network.   

Reply packets are used in response to the query packets.

ACK packets are used to acknowledge the receipt of update, query and reply packets. ACK packets are sent by using unicast.

EIGRP metric:

 EIGRP uses diffusing update algorithm (DUAL) for selecting and maintain the best path to each remote network. EIGRP uses bandwidth, delay, reliability, load, MTU, and hop count, out of these six component the first four are combined together using well known formula to produce a single number that we call composite metric. by default EIGRP uses bandwidth and delay in the metric calculation 

Formula with default K values (K1=1, K2=0, K3=1, K4=0, K5=0)

Metric=[K1*BW + ((K2*BW) / (256 - LOAD)) + K3 * delay]

EIGRP metric 

BW= (107/lowest bandwidth in kbps)*256

Delay=(sum of total delay/10)*256

By default, EIGRP metric = bandwidth (slowest link) + delay (sum of delays)