BGP Next Hop Address Tracking

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(Mohammad Hasanuz Zaman) #2

Hi Rene,
So, Here BGP identified event by OSPF and update next hop after 5 sec but how BGP identified event between two eBGP Router that are not directly connected .Between eBGP Router there is a L2 segment . So My Question is in the scenario how BGP detect & update next hop. It will takes max 60 sec , right ? Thanks

br//zaman

(Lazaros Agapides) #3

Hello Zaman

Remember that a BGP neighbourship can be formed between two BGP routers even if they are not directly connected. The only prerequisite is that there is adequate routing between them (using IGPs) so that they can exchange BGP information. So, even in that case, if the routing table of a BGP router changes and the route to the other BGP router becomes unavailable, the change in the routing table will take place which will indeed trigger the BGP next hop tracking mechanism.

I hope this has been helpful!

Laz

(bahri a) #4

hi to everybody
i dont understand the topology .How did you connect them, use the switch ?

(Lazaros Agapides) #5

Hello Bahri

I assume it is this topology that you are talking about:
image
Essentially, the three routers are connected not as a point to point connection (obviously, since there are more than two interconnected devices), but as a multiple access network. This multiple access network can be interconnected by switches or any number of layer two devices. So in between the three routers there can indeed be a switch or any number of switches interconnecting these routers within a single subnet/network segment.

The switch was left out of the topology for simplicity’s sake…

I hope this has been helpful!

Laz

(Hoan N) #6

hi
I have same question as Zaman, please help answer the question.

Thank you

(Lazaros Agapides) #7

Hello Hoan

It seems that Zaman’s question above was responded to. If you have any more specific question, please share it with us.

Laz

(Alexander F) #8

Hi Rene,

I have Problems with that.
2 Inside Routers form iBGP over EIGRP(announcing Loopback). EIGRP is running between the Routers on their HSRP addresses:
Router 1 10.10.10.1
HSRP VIP 10.10.10.2
Router 2 10.10.10.3
–> forming EIGRP 10.10.10.1<->10.10.10.3 and Advertising Loopbacks for BGP
All good so far.
Every Router is connected to ISP(same AS but does not matter here) which give a default route only to each inside Router.
That means both inside Routers have default to ISP, but when 1 inside Router chrashes, BGP is not deleted between inside Routers. EIGRP is removed and route to Loopback of other router also, but then they try to reach iBGP neighbor via ISP of Course.

(Rene Molenaar) #9

Hi Alexander,

If you use private IP addresses on your network then in this case, traffic could match your default route and is forwarded to the ISP. They’ll drop it though since they don’t route private IP addresses.

If you want to prevent this, you could configure some null0 routes for your private ranges. When the router doesn’t have a more specific entry, it will drop the traffic instead of using the default route.

Rene

(Alexander F) #10

So I have to define a static route for bgp-used loopbacks with administrative distance worse than eigrp and point it to Null0?

(Lazaros Agapides) #11

Hello Alexander

Yes, I believe your description would would indeed provide the desired result.

Laz

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(Staut S) #12

Hello,

I don’t understand why R1 chooses R3 as next-hop in the first place. R2 has a lower router ID (2.2.2.2) and all BGP attributes are the same.

Also I saw that R3 has not been configured to neighbor with R2. Is this intended or perhaps there is no need for full IBGP adjacency for this lesson?

Many thanks,
Stefanita

(Rene Molenaar) #13

Hi Stefanita,

For this example, the iBGP adjacency between R2 and R3 isn’t required but to follow best practices, it should be there :slight_smile: It seems I added it for R2 but not for R3. It’s fixed now.

About the path selection, when everything is equal then it’s the oldest path that is selected. That could be R2 or R3, depending on which neighbor adjacency comes up first.

Rene

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(Staut S) #14

My bad. Indeed oldest path is selected before router ID. :slight_smile: Many thanks Rene!

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