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Beginner

Re: Ask the Expert- EIGRP Unveiled

Good Day Peter/David,

 

I enjoyed the presentation and I have to admit I did pick up quite a bit of insight into the workings of EIGRP. With relation to the path selection or tuning of the EIGRP path, mention was made to the use of Delay over Bandwidth.

Are there any recommendations on how to select increments for the Delay manipulation, does it vary based on the link connection type or something else?   

Re: Ask the Expert- EIGRP Unveiled

Hello :)

 

First: Thanks for coming around and participating in the webcast! :D

 

Down to business: As the delay value is not used by any other element than EIGRP itself, you can adjust as you prefer. There are some things to consider, though:

 

  • There is no rule about how reasonable the delay must be.
  • The EIGRP formula  for the classic metrics would be the following: 256 * [ 10^7 / (bw) + (sum of delays) ] - based in fact that the magnitude of the BW value can be up to tens of millions, its easier for you to tweak the values of delay to make a path less preferred instead of making another path more preferred.
    • Approach this exercise in the following way: Set a delay value (say 1.000.000, for example) first, check the metrics you get and work your way up or down tweaking them until you achieve your goal (making one path less preferred over another).
  • Make sure you never increase it so high that it eventually hits the maximum possible value (16,777,215 in classic metric).
    Because this is the value used for Route Poisoning / Poisoned Reverse.
     
    Please, let us know if you have further questions! :D
     
    Thanks
Beginner

Re: Ask the Expert- EIGRP Unveiled

I came across one website one following eirgrp topology:

R1 connected to R2 with bandwidth=100Mbps & Delay=100, network 10.1.2.x

R2 connected to R3 with bandwidth=10Mbps & Delay=1000, network 10.2.3.x

R3 connected to R4 with bandwidth=100Mbps & Delay=100, network 10.3.4.x

R4 connected to network 10.4.5.x

 

Output:

R4# show ip eigrp topology 10.4.5.0/24
IP-EIGRP (AS 99): Topology entry for 10.4.5.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 28160
  Routing Descriptor Blocks:
  0.0.0.0 (FastEthernet0/0), from Connected, Send flag is 0x0
      Composite metric is (28160/0), Route is Internal
      Vector metric:
        Minimum bandwidth is 100000 Kbit
        Total delay is 100 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 0

R3# show ip eigrp topology 10.4.5.0/24
IP-EIGRP (AS 99): Topology entry for 10.4.5.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 30720
  Routing Descriptor Blocks:
  10.3.4.4 (FastEthernet0/1), from 10.3.4.4, Send flag is 0x0
      Composite metric is (30720/28160), Route is Internal
      Vector metric:
        Minimum bandwidth is 100000 Kbit
        Total delay is 200 microseconds        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 1

R2# show ip eigrp topology 10.4.5.0/24
IP-EIGRP (AS 99): Topology entry for 10.4.5.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 286720
  Routing Descriptor Blocks:
  10.2.3.3 (FastEthernet0/0), from 10.2.3.3, Send flag is 0x0
      Composite metric is (286720/30720), Route is Internal
      Vector metric:
        Minimum bandwidth is 10000 Kbit
        Total delay is 1200 microseconds        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
R1# show ip eigrp topology 10.4.5.0/24
IP-EIGRP (AS 99): Topology entry for 10.4.5.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 289280
  Routing Descriptor Blocks:
  10.1.2.2 (FastEthernet0/1), from 10.1.2.2, Send flag is 0x0
      Composite metric is (289280/286720), Route is Internal
      Vector metric:
        Minimum bandwidth is 10000 Kbit
        Total delay is 1300 microseconds        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 3

I'd like to know why R1 showing bandwidth 10MBps instead of 100MBps?






Hall of Fame Cisco Employee

Re: Ask the Expert- EIGRP Unveiled

Hello Kent,

Thank you for joining!

I'd like to know why R1 showing bandwidth 10MBps instead of 100MBps?

This is because R1's path to 10.4.5.0/24 goes through R2 and its 10Mbps link toward R3. The MTU component in EIGRP metric is the minimum path bandwidth - in other words, the lowest bandwidth, or the bottleneck, along the entire path to the destination. In this topology, the slowest link from R1 to 10.4.5.0/24 has a bandwidth of 10Mbps, and so that value is what R1 uses in its metric computations.

Please feel welcome to ask further!

Best regards,
Peter

Highlighted
Community Manager

Re: Ask the Expert- EIGRP Unveiled

Hi Peter and David

Thanks for quite an extraordinary session. 

Please find help to solve the following questions from the session: 

 

Please help to clarify the following. When RTB is down and RTC has metric 25 and RTD has metric 21, which route is selected although RTB is FS?

We can influent a static route destination to be injected into EIGRP as an internal route? Can we do this on a route to a vrf or on vrf leak?

 

Beginner

Re: Ask the Expert- EIGRP Unveiled

Hello David and Peter,

This is a rather long question about EIGRP Unequal Cost Load Balancing - 'Variance' Command.

As I'm doing an EIGRP lab, I noticed a strange behavior with the 'variance' command. Its very possible that I'm missing something, which is why I wanted to ask here so if you guys can point out the cause of this behavior or if otherwise the documentation of this command is not accurate.

The issue is around the fact that for a route to be affected by the variance command it needs to pass the feasibility condition, i.e it is a feasible successor in the EIGRP topology table.

As for the topology (its based on INE EIGRP labs), Think of R6 as connected directly to two routers, R7 via Ethernet0/1.67 and R1 via Ethernet0/1.146. And frankly, the topology here does not matter that much, the end goal is that I'm trying to manipulate Delay on these two interfaces to unequally load balance to 155.1.9.0/24

Focusing on R6 and before changing anything:

R6(config-subif)#do sh ip eigrp top all | sec 155.1.9.0/24
P 155.1.9.0/24, 1 successors, FD is 76800, serno 662
via 155.1.67.7 (76800/51200), Ethernet0/1.67


R6(config-subif)#do sh ip eigrp top 155.1.9.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(150.1.6.6) for 155.1.9.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 76800
Descriptor Blocks:
155.1.67.7 (Ethernet0/1.67), from 155.1.67.7, Send flag is 0x0
Composite metric is (76800/51200), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 3000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 150.1.9.9

Now, I'm going to change the delay on VLAN67 to 300 and VLAN146 to 2100.

Here is where things get interesting:

R6(config-subif)#do sh ip eigrp top all | sec 155.1.9.0/24
P 155.1.9.0/24, 2 successors, FD is 76800, serno 701
via 155.1.67.7 (128000/51200), Ethernet0/1.67
via 155.1.146.1 (640000/102400), Ethernet0/1.146

R6(config-subif)#do sh ip eigrp top | sec 155.1.9.0/24
P 155.1.9.0/24, 2 successors, FD is 76800
via 155.1.67.7 (128000/51200), Ethernet0/1.67

R6(config-subif)#do sh ip eigrp top 155.1.9.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(150.1.6.6) for 155.1.9.0/24
State is Passive, Query origin flag is 1, 2 Successor(s), FD is 76800
Descriptor Blocks:
155.1.67.7 (Ethernet0/1.67), from 155.1.67.7, Send flag is 0x0
Composite metric is (128000/51200), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 5000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 150.1.9.9
155.1.146.1 (Ethernet0/1.146), from 155.1.146.1, Send flag is 0x0
Composite metric is (640000/102400), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 25000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 4
Originating router is 150.1.9.9

R6(config-router)#do sh ip rou 155.1.9.0
Routing entry for 155.1.9.0/24
Known via "eigrp 100", distance 90, metric 128000, type internal
Redistributing via eigrp 100
Last update from 155.1.67.7 on Ethernet0/1.67, 00:00:08 ago
Routing Descriptor Blocks:
* 155.1.67.7, from 155.1.67.7, 00:00:08 ago, via Ethernet0/1.67
Route metric is 128000, traffic share count is 1
Total delay is 5000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

 

Now according to the above, the route that is being learned from 155.1.146.1 has an RD of 102400 which is currently higher than the FD 76800 which means it did not satisfy the FC. All the show output above also confirms that, as it does not show in the topology table without using all-links keyword or with the exact prefix.

So, no matter how much the value of the variance command is, this route should NEVER be considered by EIGRP as it cannot guarantee that its loop free - correct? well, not in this case...

I'm going now to configure variance of 5.

 

R6(config-router)#do sh ip rou 155.1.9.0
Routing entry for 155.1.9.0/24
Known via "eigrp 100", distance 90, metric 128000, type internal
Redistributing via eigrp 100
Last update from 155.1.146.1 on Ethernet0/1.146, 00:00:07 ago
Routing Descriptor Blocks:
155.1.146.1, from 155.1.146.1, 00:00:07 ago, via Ethernet0/1.146
Route metric is 640000, traffic share count is 1
Total delay is 25000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 4
155.1.67.7, from 155.1.67.7, 00:00:07 ago, via Ethernet0/1.67
Route metric is 128000, traffic share count is 5
Total delay is 5000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

R6(config-router)#do sh ip rou | sec 155.1.9.0
D 155.1.9.0/24 [90/640000] via 155.1.146.1, 00:00:17, Ethernet0/1.146
[90/128000] via 155.1.67.7, 00:00:17, Ethernet0/1.67

R6(config-router)#do sh ip eigrp top 155.1.9.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(150.1.6.6) for 155.1.9.0/24
State is Passive, Query origin flag is 1, 2 Successor(s), FD is 76800
Descriptor Blocks:
155.1.67.7 (Ethernet0/1.67), from 155.1.67.7, Send flag is 0x0
Composite metric is (128000/51200), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 5000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 150.1.9.9
155.1.146.1 (Ethernet0/1.146), from 155.1.146.1, Send flag is 0x0
Composite metric is (640000/102400), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 25000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 4

So as you can see, even though that route does not satisfy the FC, EIGRP still decided to use it and install it in the routing table, Even unequally load balancing (ratio is 5 to 1)

 

If you guys can shed some light on why this is happening, that would be really great.

 

Thanks in advance.

Hall of Fame Cisco Employee

Re: Ask the Expert- EIGRP Unveiled

Hi Hilda,

On both David's and my behalf, thank you!

Regarding the questions:

Please help to clarify the following. When RTB is down and RTC has metric 25 and RTD has metric 21, which route is selected although RTB is FS?

I assume the question revolves around the scenario:

A-B-E = 20, RD(B) = 10, B = successor
A-C-E = 25, RD(C) = 10, C = feasible successor
A-D-E = 21, RD(D) = 20, D = only a neighbor, not a feasible successor
FD = 20

If B fails, A will find out that the neighbor providing the least distance is D but it does not satisfy the Feasible Condition. Therefore, while A still points to B (that has died in the meantime), it starts a Diffusing Computation, sends out Queries to C and D, waits for all Replies to come back, and then is allowed to simply pick whatever neighbor providing the smallest computed distance. In this case, it will be neighbor D, and the new Feasible Distance will be 21. Therefore, A will move the path from using B to using D as the next hop. Neighbor C will never be used in this scenario.

We can influent a static route destination to be injected into EIGRP as an internal route? Can we do this on a route to a vrf or on vrf leak?

The only way for a static route to appear in EIGRP as an internal route is when the route is

  1. Configured using the outgoing interface only (such as ip route 192.0.2.0 255.255.255.0 GigabitEthernet0/0/0)
  2. Included in EIGRP using a network command (such as network 192.0.2.0 0.0.0.255)

This is because a static route pointing out its outgoing interface is considered to be directly connected, and so falls under a network command in EIGRP. However, this is not how we should configure static routes through multi-access interfaces, and so this option is mostly of academical value only. However, every route injected into EIGRP through a redistribute command will be treated as external. This is intentional - being able to tell internal and external routes apart is a necessary step in avoiding routing loops caused through redistribution which cannot be prevented even with Feasible Condition (because their metrics can be manipulated arbitrarily).

Maybe you can explain why you need a static route to appear as internal in EIGRP. This Ask The Expert session will be closed soon but I encourage you to post your question in the Routing area on this forum, and we will be sure to continue the discussion there.

Thank you!

Best regards,
Peter

Beginner

Re: Ask the Expert- EIGRP Unveiled

Hello @Peter Paluch,

Can you please review my post that is just above your previous one, when you get a chance?

Thank you!

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