BGP and EIGRP redistribution and route selection.

tmcquaig · ‎01-08-2008

I'm working on migrating from eigrp on ispA to bgp on ispB and migrating from ispA to ispB. Currently I have 3 sites with connections via ispA, ispB, and GRE/vpn.

From my main site I have a show ip eigrp topology for a remote site. I also have a show ip bgp and show ip route.

4506#show ip eigrp topology 10.12.100.0/24

IP-EIGRP (AS 4200): Topology entry for 10.12.100.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 434176

Routing Descriptor Blocks:

10.1.10.9 (Vlan10), from 10.1.10.9, Send flag is 0x0

Composite metric is (434176/433920), Route is Internal

Vector metric:

Minimum bandwidth is 6002 Kbit

Total delay is 300 microseconds

Reliability is 255/255

Load is 27/255

Minimum MTU is 1500

Hop count is 4

4506#show ip bgp 10.12.100.0/24

BGP routing table entry for 10.12.100.0/24, version 644

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

Advertised to update-groups:

1

Local

10.1.10.9 from 0.0.0.0 (10.51.58.2)

Origin incomplete, metric 100, localpref 100, weight 32768, valid, sourced, best

4506#show ip route 10.12.100.0

Routing entry for 10.12.100.0/24

Known via "eigrp 4200", distance 90, metric 434176, type internal

Redistributing via eigrp 4200, bgp 65351

Advertised by bgp 65351 metric 100

Last update from 10.1.10.9 on Vlan10, 00:11:20 ago

Routing Descriptor Blocks:

* 10.1.10.9, from 10.1.10.9, 00:11:20 ago, via Vlan10

Route metric is 434176, traffic share count is 1

Total delay is 300 microseconds, minimum bandwidth is 6002 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 27/255, Hops 4

4506#

The route should be to 10.1.10.5 not 10.1.10.9, or I should say I would rather it be.

Here is an example of another site that does not have ispA MPLS using the gateway for ispB 10.1.10.5.

4506#show ip bgp 10.13.1.0

BGP routing table entry for 10.13.1.0/24, version 6

Paths: (1 available, best #1, table Default-IP-Routing-Table)

Flag: 0x820

Not advertised to any peer

65301 7018 65001

10.1.10.5 from 10.1.10.5 (<ispA MPLS Router IP>)

Origin IGP, localpref 100, valid, external, best

4506#

10.1.10.9 is a 1841 I control for ispA

10.1.10.5 is a 3845 managed router from ispB.

mheusing · ‎01-09-2008

Hi,

If I understand correctly, you currently have two pathes between some sites, one with EIGRP, one with BGP.

From your output:

4506#show ip route 10.12.100.0

Routing entry for 10.12.100.0/24

Known via "eigrp 4200", distance 90, metric 434176, type internal

Redistributing via eigrp 4200, bgp 65351

The problem could be like this:

If a network is learned through EIGRP and redistributed in BGP the PE router in the MPLS SP will learn it from two BGP pathes:

1) through iBGP in the SP AS from the PE connected to the site, where the network resides

2) through eBGP from the other site your output is from, i.e. from a CE, which learned it through EIGRP

I would assume that the BGP selection process in the PE will prefer the eBGP path and thus the PE will not announce this prefix back to the CE.

Using EIGRP and BGP in your scenario might require to adjust administrative distances and surely requires proper filters for redistribution.

Any CE should only advertise in BGP networks local to that site. Your CE (4506) redistributes everything into BGP and thus the problem arises.

So I would suggest to setup filters for the redistribution from EIGRP into BGP on your 4506 and all other CE routers to only allow networks local to the respective CE site.

Hope this helps! Please rate all posts.

Regards, Martin