BGP redundancy... and dual ISP links on each router

[jquintard]

Hello,

I have small topology like this :

                    /--(Ge0/0.10)-- ISP 1
(Ge0/1) Router A ---
                    \--(Ge0/0.20)-- ISP 2

I want to redundant router A with another router. A current pratice is to use HSRP vith an iBGP session like this :

(Ge0/1) Router A (Ge0/0) -- ISP 1
           |
        HSRP/IBGP
           |
(Ge0/1) Router B (Ge0/0) -- ISP 2

But I want to redundant each ISP on each router (two eBGP sessions with each ISP) :

                   /--(Ge0/0.10)-- ISP 1
(Ge0/1) Router A --
           |       \--(Ge0/0.20)-- ISP 2
           |
        HSRP/IBGP
           |
           |       /--(Ge0/0.10)-- ISP 1
(Ge0/1) Router B --
                   \--(Ge0/0.20)-- ISP 2 

Two questions about that :

1. What is the best solution for that new topology ? In my case ISP 2 is not a backup alternative to ISP 1, it's a best path for specific AS with a link cost more than the ISP 1 link. So in the case where a router is down I must have this two links up, so I need to eBGP session on each router.

2. What is the gain to use GLBP instead HSRP in this case ?!

Thanks Jerome

[Marwan ALshawi]

Hi

First of all you need to be careful when use HSRP as you stated that the two ISPs being used at the same time based some bgp policies while hsrp will send the traffic to the active router only and based on the bgp config that active router will will send some traffic back to the second router over the iBgp if any config or design error you might get asymmetrical routing

Glbp is not the best option in your case will make the possibility of asymmetrical routing very high

I would say go with hsrp and ibgp between routers and align the bgp policies in noth sides ibgp and ebgp to make sure no asymmetrical routing

Or if you can use igp in the LAN and redistribution whither bgp you can do get the end to end routing automated too

Hope this help

If helpful rate

Sent from Cisco Technical Support iPhone App

[jquintard]

First of all you need to be careful when use HSRP as you stated that the two ISPs being used at the same time based some bgp policies while hsrp will send the traffic to the active router only and based on the bgp config that active router will will send some traffic back to the second router over the iBgp if any config or design error you might get asymmetrical routing

So 2 routers with eBGP and HSRP only. iBGP announces between routers must be disabled...

Glbp is not the best option in your case will make the possibility of asymmetrical routing very high

Ok

Or if you can use igp in the LAN and redistribution whither bgp you can do get the end to end routing automated too 

Why ? I dont understand where the use of an IGP is a solution in this case...

[Marwan ALshawi]

about HSRP and iBGP

you still need iBGP as some route might be prefered over ISP2 over R2 while HSRP will send the traffic to R1 and R1 will send it back to R2 based on BGP table

about the IGP, good question and to answer your question:

if you have a L3 device such as router or L3 siwtch and this L3 device lets say will exching routing using OSPF with other WAN edge routers,

now in each edge router you have BGP policies to make some route prefered

if use redistribution with some route maps that aligned with BGP policies you will get the route path as desried

for example

router one will be prefered for all routes coming from AS 100 while router 2 will be prefered for all routes has AS 200 in its path

in this way you can make a BGP policy in the inbound direction toward the ISP with the required AS patch list and changing local preference for example also tag the route lets say in R1 you tag this route with 100

when you redistribute from BGP to OSPF make the route with tag 100 less metric

in this case R1 wil be prefered while R2 will redisribute same route with higher metric will be back up for that route

hope this help

if helpful rate

[jquintard]

router one will be prefered for all routes coming from AS 100 while router 2 will be prefered for all routes has AS 200 in its path  

I think it's not possible. If :

- HSRP track link between each router (Gi0/0 WAN, Gi0/1 LAN, Gi0/2 subnet between this 2 routers)

- R1 and R2 are not neighbor (same AS but never declared as neighbor so no iBGP session)

R2 can be used only if R1 is down,

R1 can be used only if R2 is down

It's correct ?

The only problem I see is with data incoming from each ISP. What is the metric to uze to force traffic on R1, if R1 is UP and R2 is R1 is down.

[smitesh kharecha]

Hi Jerome,

They are many solution for the config your want.

It depends which one your want you use.

Secondly, are you importing whole / partial bgp table from both ISP or just default route.

If your are having default route from both ISP, you can design solution something like static route have of the subnet going via ISP 1 and and another half going via ISP 2 and having full subnet and whole subnet as well.

Internally between your router, use IGP so that you can have sync of routing table on both of your router.

HTH,

Smitesh

[Mahesh Gohil]

Hi,

                   /--(Ge0/0.10)-- ISP 1
(Ge0/1) Router A --
           |       \--(Ge0/0.20)-- ISP 2
           |
        HSRP/IBGP
           |
           |       /--(Ge0/0.10)-- ISP 1
(Ge0/1) Router B --
                   \--(Ge0/0.20)-- ISP 2 
 
 I am confused with this approach. Say you have asked for one physical link from ISP1 (ge0/0) then how come
 the subinterface (ge0/0.20) can be terminated at ISP2. I mean a single physical link can not be connected to two differet
 ISP. Please let me know if I misunderstood.
 
 To support the above solution you might need four physical link. isn't it?

Regards

Mahesh

[jquintard]

I am confused with this approach. Say you have asked for one physical link from ISP1 (ge0/0) then how come
the subinterface (ge0/0.20) can be terminated at ISP2. I mean a single physical link can not be connected to two differet
ISP. Please let me know if I misunderstood. 

There are two sub interfaces, each with one ISP. Where is the problem ???

[jquintard]

Secondly, are you importing whole / partial bgp table from both ISP or just default route.

Partial table.

In fact I have only routes originaly from ISP 2 AS.

If the final desination is on this table, I use ISP 2 otherwise I use the ISP 1 default route

[Mahesh Gohil]

Don't mind but think practically.

Regards/Mahesh