Redistribute IGP Information Into BGP

When using BGP to connect to the public Internet, you should advertise the minimum number of prefixes needed to make your site reachable from the Internet – the best approach is to configure the prefixes you want to advertise as you did in the Advertise IPv4 Prefixes to BGP Neighbors lab exercise. That makes little sense when using BGP as a generic routing protocol, for example, when connecting your sites to an MPLS/VPN service. In these scenarios, it’s better to redistribute local routing information (connected subnets, static routes, and IGP routes) into the BGP table; you’ll practice route redistribution approach in this exercise.

Your lab has two sites connected to an MPLS/VPN provider. Each site has a WAN edge router and a core router, running OSPF between them. Your task is to establish connectivity between your sites to enable the core routers to ping each other.

Lab topology

If you’re using netlab, you’ll start with a preconfigured lab – netlab will configure IP addressing, OSPF routing, and EBGP sessions. If you’re using any other lab environment, you’ll have to configure all of that manually. The following tables describe the OSPF and BGP setup; the Reference Information section contains IP addressing information.

BGP Routers and AS Numbers

Node/ASN	Router ID	Advertised prefixes
AS65000
c1	10.0.0.1
AS65001
c2	10.0.0.2
AS65100
x1	10.0.0.10
x2	10.0.0.11

BGP Neighbors

Node	Neighbor	Neighbor AS	Neighbor IPv4
c1	x1	65100	10.1.0.2
c2	x2	65100	10.1.0.10
x1	x2	65100	10.0.0.11
	c1	65000	10.1.0.1
x2	x1	65100	10.0.0.10
	c2	65001	10.1.0.9

OSPF Setup

Area 1 (AS 65000)

Router	Interface	IPv4 Address	Neighbor(s)
c1	Loopback	10.0.0.1/32
	Ethernet2	10.1.0.13/30	s1
s1	Loopback	10.0.0.3/32
	Ethernet1	10.1.0.14/30	c1
	Ethernet2	172.16.0.3/24	passive

Area 2 (AS 65001)

Router	Interface	IPv4 Address	Neighbor(s)
c2	Loopback	10.0.0.2/32
	Ethernet2	10.1.0.17/30	s2
s2	Loopback	10.0.0.4/32
	Ethernet1	10.1.0.18/30	c2
	Ethernet2	172.16.1.4/24	passive

Start the Lab

Assuming you already set up your lab infrastructure:

Change directory to basic/5-redistribute
Execute netlab up (other options)
Log into your devices with netlab connect and verify IP addresses, OSPF routing, and basic BGP configuration.

Note: netlab will configure IP addressing, OSPF routing, and EBGP sessions on your routers. If you’re not using netlab, you’ll have to configure them manually.

Configuration Tasks

You have to exchange OSPF routing information between the two sites using BGP. To get that done, you have to:

Configure OSPF-to-BGP redistribution on C1 and C2, usually using a command similar to redistribute ospf¹ within the BGP configuration.
Configure BGP-to-OSPF redistribution on C1 and C2, usually using a command similar to redistribute bgp² within the OSPF configuration.

Warning

Two-way redistribution between routing protocols could quickly become exceedingly complex. Advertising the default route into the edge routing protocol is often better than redistributing core routes. You might want to test this alternative as an optional part of the lab exercise.

Verification

The local OSPF prefixes you want to advertise to the remote site must be in your router’s BGP table first. A command similar to show ip bgp is thus a good starting point; it’s even better to use a command that displays only the locally-originated prefixes, such as show ip bgp regexp ^$. The following printout contains the information displayed on C1 running Arista EOS:

c1#show ip bgp regexp ^$
BGP routing table information for VRF default
Router identifier 10.0.0.1, local AS number 65000
Route status codes: s - suppressed contributor, * - valid, > - active, E - ECMP head, e - ECMP
                    S - Stale, c - Contributing to ECMP, b - backup, L - labeled-unicast
                    % - Pending BGP convergence
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI Origin Validation codes: V - valid, I - invalid, U - unknown
AS Path Attributes: Or-ID - Originator ID, C-LST - Cluster List, LL Nexthop - Link Local Nexthop

          Network                Next Hop              Metric  AIGP       LocPref Weight  Path
 * >      10.0.0.3/32            10.1.0.14             -       -          -       0       i
 * >      172.16.0.0/24          10.1.0.14             -       -          -       0       i

c1#show ip bgp
...

          Network                Next Hop              Metric  AIGP       LocPref Weight  Path
 * >      10.0.0.3/32            10.1.0.14             -       -          -       0       i
 * >      10.0.0.4/32            10.1.0.2              0       -          100     0       65100 65001 i
 * >      172.16.0.0/24          10.1.0.14             -       -          -       0       i
 * >      172.16.1.0/24          10.1.0.2              0       -          100     0       65100 65001 i

After validating the BGP table on C1 and C2, check that the BGP routes are redistributed into intra-site OSPF – inspect the routing table and OSPF topology database on S1 and S2. This is what Arista EOS displays on S1:

s1>show ip route

VRF: default
Codes: C - connected, S - static, K - kernel,
       O - OSPF, IA - OSPF inter area, E1 - OSPF external type 1,
       E2 - OSPF external type 2, N1 - OSPF NSSA external type 1,
       N2 - OSPF NSSA external type2, B - Other BGP Routes,
       B I - iBGP, B E - eBGP, R - RIP, I L1 - IS-IS level 1,
       I L2 - IS-IS level 2, O3 - OSPFv3, A B - BGP Aggregate,
       A O - OSPF Summary, NG - Nexthop Group Static Route,
       V - VXLAN Control Service, M - Martian,
       DH - DHCP client installed default route,
       DP - Dynamic Policy Route, L - VRF Leaked,
       G  - gRIBI, RC - Route Cache Route

Gateway of last resort:
 S        0.0.0.0/0 [1/0] via 192.168.121.1, Management0

 O        10.0.0.1/32 [110/20] via 10.1.0.13, Ethernet1
 C        10.0.0.3/32 is directly connected, Loopback0
 O E2     10.0.0.4/32 [110/1] via 10.1.0.13, Ethernet1
 C        10.1.0.12/30 is directly connected, Ethernet1
 C        172.16.0.0/24 is directly connected, Ethernet2
 O E2     172.16.1.0/24 [110/1] via 10.1.0.13, Ethernet1
 C        192.168.121.0/24 is directly connected, Management0

Reference Information

You might find the following information useful if you’re not using netlab to build the lab:

Lab Wiring

This lab uses a superset of the 4-router lab topology. You can still use that topology without the S1/S2 routers, and redistribute connected prefixes into BGP.

Point-to-Point Links

Origin Device	Origin Port	Destination Device	Destination Port
c1	Ethernet1	x1	swp1
x1	swp2	x2	swp2
c2	Ethernet1	x2	swp3
c1	Ethernet2	s1	Ethernet1
c2	Ethernet2	s2	Ethernet1

Stub Links

Origin Device	Origin Port	Description
s1	Ethernet2	s1 -> stub
s2	Ethernet2	s2 -> stub

Lab Addressing

Node/Interface	IPv4 Address	Description
c1	10.0.0.1/32	Loopback
Ethernet1	10.1.0.1/30	c1 -> x1
Ethernet2	10.1.0.13/30	c1 -> s1
c2	10.0.0.2/32	Loopback
Ethernet1	10.1.0.9/30	c2 -> x2
Ethernet2	10.1.0.17/30	c2 -> s2
x1	10.0.0.10/32	Loopback
swp1	10.1.0.2/30	x1 -> c1
swp2	10.1.0.5/30	x1 -> x2
x2	10.0.0.11/32	Loopback
swp2	10.1.0.6/30	x2 -> x1
swp3	10.1.0.10/30	x2 -> c2
s1	10.0.0.3/32	Loopback
Ethernet1	10.1.0.14/30	s1 -> c1
Ethernet2	172.16.0.3/24	s1 -> stub
s2	10.0.0.4/32	Loopback
Ethernet1	10.1.0.18/30	s2 -> c2
Ethernet2	172.16.1.4/24	s2 -> stub

You might have to add OSPF process number to the command. ↩
You might have to add BGP AS number to the command. Some older platforms have to be told to redistribute subnets into OSPF. ↩

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