thanks sundeep, i was thinking from lab point of veiw where ibgp routers are multiple hops away ...and thier tcp session can be re-routed via multiple paths primary pos with MTU of 4470 and in case of POS failure via FE with 1500 bytes MTU.
the feature i got is Pmtud enabled by *defult*and in case when MSS is higher than any of underlying links..then update message is fragmented and sent. i am not sure of the affect of IP TCP PATH-mtu-discovery global config command on bgp sessions? On Thu, Nov 1, 2012 at 8:21 PM, sundeep sadhwani <[email protected] > wrote: > Hi Imran, > > 1) DF bit is not set for the BGP update message. > > From my experience, You can see BGP flapping in case you have device in > between like transmission devices or a switch working as a pure layer 2 > device which are dropping the packets if they dont support packet of 1500 > bytes). The MTU supported by the routers will be 1500 bytes so MSS would be > 1460(Excluding 20 bytes IP and 20 bytes TCP) > > I had faced this issue for one of my customer. BGP neighbor was flapping > every 3 minutes(Keepalives getting lost). After troubleshooting we found > out that the keepalive messages were getting queued behind the BGP update > messages(Considering BGP update message payload to be 1460 bytes.). We had > to ping and check the MTU supported by the underlying devices. Then we > manually adjusted the value of tcp mss using the global command on cisco > router(The interface level didnt help as it wont affect locally originated > traffic). > > 2) I am not sure about how path MTU discovery will work in this scenario > as it will only come into picture when you have devices that understand IP > in between the BGP neighbors. I am not sure what will happen if the > transmission devices or layer 2 devices in between are dropping the packets > of 1500 bytes silently(It means the IP router at the other is not receiving > icmp packets hence there will not be any ICMP unreachable message generated > from the IP router). > > > Regards. > > > > On Thu, Nov 1, 2012 at 8:24 PM, Tom Kacprzynski <[email protected]> wrote: > >> I think this a very good question to be tested in GNS3 with wireshark. >> Doing things you'll learn so much better than just reading about it. >> Please >> let us know what you find out. >> >> Regards, >> >> Tom >> >> On Thu, Nov 1, 2012 at 8:51 AM, Imran Ali <[email protected]> wrote: >> >> > Hi all, >> > >> > By default when bgp router sends update ...does it sets DF bit ? >> > question 1 >> > >> > in this case if the size of update packet let say is larger than >> > MTU of any of transit links ..then any transit router upon reception >> of >> > DF marked bgp update packet send an ICMP error , if icmp is filtered >> due >> > to any firewall then originating router will not reduce the MSS and >> this >> > results in oscilaitng bgp sessions >> > >> > >> > >> http://nagendrakumar-nagendra.blogspot.com/2010/03/bgp-path-mtu-discovery.html >> > >> > does any one have experinced the same or can comment on this ? >> > >> > >> > 2) if bgp path mtu is enabled by default ..then do the router >> undergo >> > path mtu discovery ..and figures out lowest mtu of transit links and >> then >> > adjusts the [mss = lowes mtu - (ip headers) - (tcp headers) ] ..? >> > >> > >> > Blogs and organic groups at http://www.ccie.net >> > >> > _______________________________________________________________________ >> > Subscription information may be found at: >> > http://www.groupstudy.com/list/CCIELab.html >> >> >> Blogs and organic groups at http://www.ccie.net >> >> _______________________________________________________________________ >> Subscription information may be found at: >> http://www.groupstudy.com/list/CCIELab.html >> >> >> >> >> >> >> >> > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
