07-11-2010 01:57 AM - edited 03-04-2019 09:01 AM
Hi All
Can any one help me understand why BGP require route to be learned via IGP first then only that route will be installed in BGP
table.I am not able to find any specific reason for that. Moreover does it make any diffrence for BGP if that route is also learned via
Static route
Thanks a Ton
Hemant Kumar
07-11-2010 03:21 AM
Hi Hemant,
Please understand this rule applies when bgp advertise subnets/prefixes using network commands.
for e.g if i use network 10.10.10.0 mask 255.255.255.0
the rule says i must have 10.10.10.0/24 must learned from IGP in order to advertise it further using BGP.
Now obvious question comes.WHY?
To answer this is BGP/IGP synchronization usually disabled.
If say BGP advertises the routes to some other AS which are not in the their own IGP means those routes doesnt even exists and tht spreads false information.
In order to avoid such situation this is required.
Again Static is an IGP so it doesnt make any difference. even you can use you directly connected subents.
Cheersss.......
Saurabh Joshi
07-11-2010 04:59 AM
Hi Saurabh
Got your point , but does that means any ISP router being used for Internet must be having all the routes of Internet in
BGP as well as IGP for BGP to reach any destination (We all know the internet routing table have now reached 13000 routes)
Another question I have got is that I am facing RIB Failure in my routing table , now BGP FAQ says that route showing rib failure
must have been learnt by some other IGP (most probably static) that is causing this problem. My question is that all BGP require
is an IGP route either static or dynamic for advertising that route further so why showing RIB failure
Please correct me if I am wrong
Thanks a Ton
Hemant Kumar
07-11-2010 07:16 AM
Hi Hemant,
Please note that Total Internet routes are more than 300000 now and definately they all are running with synchronization disabled and this rules applies to only routes learnt from IBGP .
A BGP router with synchronization enabled will not advertise iBGP-learned routes to other eBGP peers if it is not able to validate those routes in its IGP. Assuming that IGP has a route to iBGP-learned routes, the router will announce the iBGP routes to eBGP peers. Otherwise the router treats the route as not being synchronized with IGP and does not advertise it. Disabling synchronization using the no synchronization command under router BGP prevents BGP from validating iBGP routes in IGP.
To answer your another question
Yes if you have route in the RIB for the same destination with lower AD the route will give you RIB failure.
In your case Static give AD 1 so definately you will see RIB failure.
Let me know if this answers your query.
Thanks
Saurabh Joshi
07-11-2010 09:53 AM
Hi Saurabh
Got your point for IGP requirement
Going further for Rib failure : Why BGP is flashing RIB Failure , all BGP require is IGP route either Static or dynamic so why
giving error ?
An example or any link or any doc would be much appreciated
Thanks a Ton
Hemant Kumar
07-11-2010 11:47 AM
Hi Hemant
What does r RIB-Failure mean in the show ip bgp command output?
R1> show ip bgp BGP table version is 5, local router ID is 200.200.200.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path r> 6.6.6.0/24 10.10.13.3 0 130 0 30 i *> 7.7.7.0/24 10.10.13.3 0 125 0 30 iA. When BGP tries to install the bestpath prefix into Routing Information Base (RIB) (for example, the IP Routing table), RIB might reject the BGP route due to any of these reasons:
Route with better administrative distance already present in IGP. For example, if a static route already exists in IP Routing table.
Memory failure.
The number of routes in VPN routing/forwarding (VRF) exceeds the route-limit configured under the VRF instance.
In such cases, the prefixes that are rejected for these reasons are identified by r RIB Failure in the show ip bgp command output and are not advertised to the peers. This feature was first made available in Cisco IOS Software Release 12.2(08.05)T.
Regards
Chetan Kumar
07-11-2010 09:34 PM
Hi Saurabh
I am unable to understand that BGP considers update sent by it's own neighbor to be false , why so ?
Why is need of IGP for making a route to be correct ?
Can you provide some example or link
Thanks a lot
sameer
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