Best route selection - sequence

[deepa.muralidharan]

What is the sequence in which a route processor will decide a the best route to destination?

[massimiliano.serafino]

Hi, Look at Route Selection in Cisco Routers

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml

I hope this helps.

Best regards.

Massimiliano.

[deepa.muralidharan]

Hi,

Please check the scenario :

I have EIGRP & BGP enabled on a router.

Say, S1/0 connects to location A over EIGRP

& S 1/1 connects to location B over BGP.

Location A & Location B are both interconnected via MAN links.

Via S 1/0, only traffic to Location A needs to go & S1/1 should have only Location B traffic though it. In case of failure on one of the links, then there should be an automatic re-routing to reach the other location over MAN links.

Router eigrp as

network x.x.x.x

redistribute static

redistribute bgp AS metric 1000 100 255 1 1500

no auto-summary

!

router bgp AS

bgp log-neighbor-changes

redistribute static metric 1000

redistribute eigrp as metric 1000

redistribute eigrp as route-map traffic

neighbor x.x.x.x remote-as ASS

no auto-summary

!

route-map traffic permit 20

match ip address 35

set metric 500

!

access-list 35 permit x.x.0.0 0.0.255.255

access-list 35 permit y.y.0.0 0.0.255.255

access-list 35 permit z.z.0.0 0.0.255.255

!

Ip route u.u.0.0 255.255.0.0 Serial 1/0

Ip route v.v.0.0 255.255.0.0 Serial 1/0

!

With a configurtaion like this, what would happen? Can my objective be met?

[Alan Huseyin Kayahan]

Hello Deepa,

In Location B, let the router advertise Location A's network (Create a null route etc). In your router, the Location A route which is received from Location B via BGP, will stay in "r" (RIB Failure) mode and wont be installed in route table since EIGRP has a route already with a better Administrative Distance. Once EIGRP is down, the route to Location A that is waiting in r state will be installed into routing table. We made the redundancy for Location A.

Location B redundancy will be a little bit complex. The reason why is because this time we will require Location A to advertise Location B route via EIGRP externally! (should appear as D EX in your side, so use redistribution in Location A), but since EIGRP has a better AD than BGP, EIGRP will take over instantaneously, which we dont want. So you should apply the following to increase the AD for that D EX route specifically and let the BGP route stay primary.

access-list 10 permit LocationBsubnet LocationBmask

Yourrouter(config-router)#distance 201 x.x.x.x 255.255.255.255 10

x.x.x.x= Is the IP address of Location A EIGRP neighbor

Now Location B route that is advertised from Location A via EIGRP will have an Administrative Distance of 201. Since BGP route which should be the primary route for Location B has an AD of 200, it will be installed into routing table and EIGRP route will stay in topology table and wait BGP route to fail and disappear.

Regards