BGP with 4 routers in one AS running IBGP

[Danilo Dy]

Okay, I have 4 routers > R1, R2, R3, and R4.

R1 and R2 are located in Building-A. R3 and R4 are located in building-B. All four routers are in one AS.

This is their connections;

R1--Ethernet--R2

ISDN Leased Line

R4--Ethernet--R3

R1 to R2 is Ethernet, R2 to R3 is Leased Line, R3 to R4 is Ethernet, R4 to R1 is ISDN.

R1 is connected to ISP-1 and R3 is connected to ISP-2.

QUESTION:

1. Can I possibly run IBGP in four routers if all connections are active?

2. Can I configure ISDN as standby and will activate only if ATM fails.

3. If answer to question number one and two is yes. Can I still possibly run IBGP in four routers if ISDN is in standby mode?

4. If answer to question number two is yes. How to do it?

Thanks in advance:)

[thisisshanky · Accepted solution]

Hello,

You need to select an appropriate routing protocol for your network (IGP) such as OSPF or EIGRP.

1. Can I possibly run IBGP in four routers if all connections are active?

IBGP doesnt require, that the neighbors be physically connected. The only thing that is needed is neighbor reachability via IGP. Use loopback addresses for neighbor peering via IBGP inside your AS. Include the loopback network addresses, in your routing protocol, so that neighbor reachability is not a problem.

2. Can I configure ISDN as standby and will activate only if ATM fails.

Yes you can configure the ISDN to be standby. If you select OSPF to be routing protocol, you can configure the ISDN link as on-demand circuit. The link wont be up under normal circumstances, when a route change occurs, due to say, the leased line goes down, OSPF will automatically converge over the ISDN link. If you use EIGRP as routing protocol, you can use Dialer watch feature to keep the ISDN link as standby.

3. If answer to question number one and two is yes. Can I still possibly run IBGP in four routers if ISDN is in standby mode?

Yes, you can still configure IBGP in all 4 routers, with ISDN in standby mode. As I said before, you need to configure loopback addresses, and use them as neighbor addresses in BGP. IBGP only requires that the neighbor is reachable logically via routing protocol.

4. how to do it.

Some of these links might help!

a) For Dialer watch -

http://www.cisco.com/warp/public/129/bri-backup-map-watch.html

b) For OSPF demand circuit -

http://www.cisco.com/warp/public/129/config-bri-map.html

http://www.cisco.com/warp/public/104/dc.html

c) For configuring IBGP using loopback interfaces

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm

http://www.cisco.com/warp/public/459/23.html

Hope this helps! Let me know, if you need further assistance.

[Danilo Dy]

Thank you very much.

This is the second scenario.

The four routers (R1, R2, R3, R4) previously mentioned is in AS 1. I have two more routers (R5 and R6) in AS 2.

To repeat.

R1 and R2 is in Building-A (AS 1)

R3 and R4 is in Building-B (AS 1)

R5 and R6 is in Building-C (AS 2)

R1 and R2 connection is Ethernet

R2 and R3 connection is IP Tunnel Thru MPLS VPN (not actually Leased Line)

R3 and R4 connection is Ethernet

R4 and R1 connection is ISDN-30 (15 channels)

R5 and R6 connection is Ethernet

R5 and R1 connection is IP Tunnel Thru MPLS VPN

R6 and R4 connection is ISDN-30 (15 channels)

R6 and R1 connection is ISDN-30 (15 channels) >> Standby

AS 1 in R1 is peering with 2 upstream ISP (AS 3, AS 4), AS 1 in R3 is peering with one upstream ISP (AS5). R1 to R4 are using same IP block in their BGP configuration (/19).

1. Can R5 peer with R1 but to hear only AS 1, AS 3, and AS4 and beyond. Not AS5 and beyond? If yes, how it is done?

2. Can R6 peer with R4 but to hear only AS 1, AS 3, and AS4 and beyond. Not AS5 and beyond?

3. If R6 connection to R4 is lost, can R6 activate the ISDN-30 standby 15 channels to connect to R1? If yes, how it is done? Take note that this is already EBGP.

Thanks in advance:)