Dual BGP links to same ISP using redundant routers

[DANIEL MCKEEHAN]

What is the recommended setup for the following situation where there is redundant border routers connecting to the ISP with full tables?   Needs to advertise a single /24 in a redundant configuration.

[chrihussey]

Generally, if you want to prefer one router over the other you can IBGP peer between the two routers, give the preferred router higher local preference for the outbound and AS path prepend the /24 advertisement out the least preferred router so the primary is used on the inbound from the ISP.

If you prefer both to be used equally, just accept the ISP routes in both, IBGP for redundancy and advertise the /24 equally.

There are always caveats or intricacies to each, tt all depends on what you wish to achieve.

Hope this helps

[DANIEL MCKEEHAN]

I understand preferencing the outbound routes but the question is what is the best setup given the above design.  Imagine the Nexus as a L2/L3 with a vlan 999 that contains a network 1.1.1.0/24 that is connected through to the UCS.  Someone needs to announce this ideally more then one and someone needs to be the gateway for that network ideally more then one.  This is a non existing network and can be setup however I want. 

The ASR's can't do the announcement using traditional eBGP because they could black hole the traffic if it gets isolated. 

If its setup as a L2 to get to the UCS then STP will isolate all but one path.  Not ideal for load balancing. 

If its a L3 then I'm not sure how to do the setup except for putting a bunch of /30's on all the interfaces and then I guess use OSPF to setup all the routing?

Then there is the issue is that someone needs to be the gateway for that network maybe using GLBP but this needs to be able to handle device isolation.  

[chrihussey · Accepted solution]

OK, there are multiple ways to do this, but consider:

1- /30s between each of the Nexus and their neighboring ASR and run OSPF

2- It can be L2 between the Nexus but with OSPF peering across one or two VLANs, or just have a dedicated L3 /30 link.

3- The ASRs OSPF and IBGP peer across the link between the two.

4- The ASRs also EBGP peer with the ISP

5- The ASRs advertise the default route to their neighboring Nexus (default-information originate), in addition to Internet routes it may be a good idea to have the ISP also advertise the 0.0.0.0 to you to support this.

6- The ASRs use the OSPF learned routes (probably have to summarize in OSPF) to support their advertisements to the ISP. (BGP needs to have a supporting route in its IGP before it will advertise out)

7- The Nexus used HSRP, VRRP or GLBP for the default gateways of all the local LANs.

So in this scenario:

1- If an ASR gets isolated it won't have any supporting IGP routes and would withdraw its advertisements to its EBGP neighbor.

2- Both Nexus would choose it's nearest ASR neighbor to route to the internet

3- Based on the earlier response, depending on how you have the BGP set up, either one or both routers would be forwarding and receiving traffic to the Internet.

Again, there is always a tweak needed, but hope it gives you an idea and is of some help.

Regards 

[Bekzod Fakhriddinov]

Also you will need to change cost on the ospf interface and make HSRP/VRRP active switch most preferred by ospf and if it fails then ospf will select HSRP/VRRP standby switch.