Unclear on Variance (can you believe it!?!)

mikearama

Hey techies.

Okay, not really on Var... that's clear enough. I'm unclear on what the Var multiplier does to the routing table.

I have a route from Network A to Network M... say, AD of 10, total FD of 20.

Other routes inc:

AD of 10, FD of 30
AD of 15, FD of 35
AD of 25, FD of 40

With the Variance set to 2, everything with FD less than / equal to 40 could load balance. Does my last route get included in the routing table or not? It's not a feasible successor, but it does qualify under the Var.

Thanks for the help.
Mike

kpjungle

mikearama wrote:

Hey techies.

Okay, not really on Var... that's clear enough. I'm unclear on what the Var multiplier does to the routing table.

I have a route from Network A to Network M... say, AD of 10, total FD of 20.

Other routes inc:

AD of 10, FD of 30
AD of 15, FD of 35
AD of 25, FD of 40

With the Variance set to 2, everything with FD less than / equal to 40 could load balance. Does my last route get included in the routing table or not? It's not a feasible successor, but it does qualify under the Var.

Thanks for the help.
Mike

Hi,

Was unsure about this myself. It has to qualify as a feasible successor to get considered a candidate for variance. If it is a feassible successor, it will enter the routing table if its FD is < variance*current_successor_FD.

Plazma

The hardest thing for me in EIGRP was all feasibility stuff so don't feel bad

tech-airman

mikearama wrote:

Hey techies.

Okay, not really on Var... that's clear enough. I'm unclear on what the Var multiplier does to the routing table.

I have a route from Network A to Network M... say, AD of 10, total FD of 20.

Other routes inc:

AD of 10, FD of 30
AD of 15, FD of 35
AD of 25, FD of 40

With the Variance set to 2, everything with FD less than / equal to 40 could load balance. Does my last route get included in the routing table or not? It's not a feasible successor, but it does qualify under the Var.

Thanks for the help.
Mike

Mikearama,

It depends. The thing that it depends on is if you're talking about the router's "routing table" or the IP-EIGRP Routing Table.

For a basic Cisco router without any dynamic routing protocols configured on it, by simple use of the command "Router#ip routing" the router is capable of forwarding packets. Configure logical addresses on the interfaces, configure static routes, and the router is ready to forward packets. At this stage, there are three components operating:

1. Routing Process
2. Routing Table
3. Forwarding Process

For static routing, the routing process receives the routing information from: 1) interface networks (connected) and 2) configured static routes (static) routes. Even at this simple stage, the routing process looks at the routes are offered to it, compares administrative distances, then installs or doesn't install routes into the router's Routing Table.

Now, with the configuration EIGRP and therefore the addition of a dynamic routing protocol, the router components now look like this:

1. IP-EIGRP Neighbor Table
2. IP-EIGRP Topology Table
3. DUAL
4. IP-EIGRP Routing Table
5. Routing Process
6. Routing Table
7. Forwarding Process

This is why I said "it depends" earlier because there are the IP-EIGRP Routing Table and the Routing Table for the router.

So as you may or may not know, after initial EIGRP neighbor adjacencies have been formed, EIGRP routers send EIGRP Update packets to each other. These EIGRP Update packets contains ALL of the route information that it knows which is used by the local EIGRP router to fill up it's IP-EIGRP Topology Table.

Then DUAL looks at the IP-EIGRP Topology Table then starts to seek out Sucessor routes. By default, variance is 1. What that means is that only the best routes from the IP-EIGRP Topology Table are used as Successor Routes to be installed into the IP-EIGRP Routing Table. The best route to a specific network is determined by the route with the lowest Feasible Distance to that specific network. Once the Sucessor routes are decided by DUAL, the Successor routes gets installed into the IP-EIGRP Routing Table. This is how EIGRP equal cost load balancing works. Then, DUAL has to decide which routes will be Feasible Successor routes. That is done by applying the feasibility condition of "non-sucessor route's Advertised Distance must be LESS THAN the Successor route's Feasible Distance." If the route in the IP-EIGRP Topology Table meet that criteria, then the Feasible Successor routes are installed into the IP-EIGRP Routing Table. Note, that's as far as the Feasible Successor routes get to the Routing Table for the router, kinda like sitting in inactive reserve just in case the Sucessor route goes down. But by configuring a different variance number, if a Feasible Successor's route's Feasible Distance is less than the variance multiple of the Successor route's Feasible Distance, then that Feasible Successor route gets promoted from Feasible Successor to Successor route. Then this newly promoted Successor route is offered from the IP-EIGRP Routing Table to the Routing Process for the router. Then this newly promoted Successor route is compared with routes offered by other routing protocols to the Routing Process, then Administrative Distances are compared, since EIGRP is the only routing protocol configured on this local router, that newly promoted Successor route SHOULD be installed into the Routing Table for the router. This is how EIGRP UNequal cost load balancing works.

So to directly answer your question of "Does my last route get included in the routing table or not?" it depends:

1. ONLY EIGRP knows of the route to the network and ONLY EIGRP is configured on the local router = Yes.
2. EIGRP and other routing protocols know of the network = It depends on how the route to the network was redistributed into each other.
3. EIGRP and other routing protocols are configured on the local router = It depends on which routing protocol's route to the network being offered to the Routing Process wins the Administrative Distance comparison.
   [list=1:3014b08891]
4. If EIGRP's Administrative Distance is better than the other routing protocol's Administrative Distance, then yes, that newly promoted Successor route will be installed into the Routing Table.
5. If EIGRP's Administrative Distance is worse than the other routing protocol's Administrative Distance, then no, that newly promoted Successor route will NOT be installed into the Routing Table.

[/list:o:3014b08891]

I hope this helps.

Source:

1. Route Selection in Cisco Routers - Cisco Systems - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml
2. How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP? - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml
3. variance (EIGRP) - Cisco IOS Release 12.0 Network Protocols Command Reference, Part 1 - EIGRP Commands - Cisco Systems - http://www.cisco.com/en/US/docs/ios/12_0/np1/command/reference/1reigrp.html#wp1018926