When I have two equal cost paths in RIP, which one is used? 2

[geo2500]

Hi,
I have a router A, which receives the same rip updates from 2 routers(B and C) with the same metric.
Which one, will A use for sending packets to these networks?

ps: B and C are for redundancy and when I install them, they will play HSRP. I just want to find out first, what will happen with the updates from B and C.
a simple diagram is as follows A------------switch-B,C
B,C are connected to a switch

 

[dloz]

When using RIP if they both have equal costs the first one learned will be used.

 

[KiscoKid]

Hmm disagree with previous poster.

When routes with same AD and metrics are installed into the routing table, load-balancing will occur over those routes. Tye type of load-balancing you get (i.e. per packet or per destination) depends on the switching mechanism deployed on the router. Fast/CEF switching etc by default uses per destination load balancing.

 

[leedsit]

Hi,

Totally Agree with KiscoKid

I believe upto 4 Equal cost paths ( Same Metric / Admin distance ) are entered into the routing table by default, upto 6 if configured.

If using Fast/CEF, then per destination is used by default, however you can on the interface configure IP LOAD-SHARING-PER-PACKET which does per packet load balancing.

Ave fun.......

LEEroy
MCNE6,CCNA2,CWNA, Project+

 

[jimbopalmer]

RIPv2 has no spec for load balancing, unlike OSPF where equal cost multipath routing is an optional part of the spec, or IGRP which allows non-equal cost multipath routing. A given router may do load balancing using vendor enhancments, but it is not a part of RIP, just a vendor add-on.

"In particular, RIP's small hop-count limit (16) restricted the size of internetworks; single metric (hop count) support of only equal-cost load balancing (in all-Cisco networks only!) did not allow for much routing flexibility in complex environments." -

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/igrp.htm

I tried to remain child-like, all I acheived was childish.

 

[dloz]

If we are talking about a cisco router than yes I was wrong by default it will store 4 routes and balance them. But here is what rfc 2453 says
"If the new metric is the same as the old one, it is simplest to do nothing further (beyond re-initializing the timeout, as specified above); but, there is a heuristic which could be applied. Normally, it is senseless to replace a route if the new route has the same metric as the existing route; this would cause the route to bounce back and forth, which would generate an intolerable number of triggered updates. However, if the existing route is showing signs of timing out, it may be better to switch to an equally-good alternative route immediately, rather than waiting for the timeout to happen. Therefore, if the new metric is the same as the old one, examine the timeout for the existing route. If it is at least halfway to the expiration point, switch to the new route. This heuristic is optional, but highly recommended."

http://www.faqs.org/rfcs/rfc2453.html