Mustafa, I see what you mean, I think the 254 is confusing as well. Take a look here:
Example 1: R1----R2 Peer R1 and R2 between loopbacks without ebgp-multihop command. So even though TTL is 1, packets would survive BUT the problem is R1 and R2 won't send them in the first place because they they don't see the peer address as a connected route. If you configure disable-connected-check then R1 and R2 will send their TCP packets and connection will come up Example 2: R1----R2----R3 R1 and R3 are peered between loopbacks. Disable connected check makes no difference here because you need a multi-hop of 2. Once you configure multi-hop of 2, packets have a TTL of 2 so R1 and R3 will initiate the connection. So in summary it allows for directly connected EBGP peers who are peering with loopbacks to bring up the session without multi-hop configured. On Tue, Mar 16, 2010 at 1:41 PM, Mustafa Yadav <[email protected]>wrote: > hi bryan, > > > From the link you sent as below I undetstood that for ebgp session ttl > value is 254.Is that right? > > I have read /listened somewhere else it is for ibgp and for ebgp is > default one 1. > > And for the below example what exactly ebgp multihop 1 means?Is not it 1 > by default? > > > Defaults > > A BGP routing process will verify the connection of single-hop eBGP peering > session (*TTL=254)* to determine if the eBGP peer is directly connected to > the same network segment by default. If the peer is not directly connected > to same network segment, connection verification will prevent the peering > session from being established. > > > Examples > > In the following example, a single-hop eBGP peering session is configured > between two BGP peers that are reachable on the same network segment through > a local loopback interfaces on each router: > > BGP Peer 1 > > Router(config)# interface loopback 1 > > Router(config-if)# ip address 10.0.0.100 255.255.255 > > Router(config-if)# exit > > Router(config)# router bgp 64512 > > Router(config-router)# neighbor 192.168.0.200 remote-as 65534 > > Router(config-router)# neighbor 192.168.0.200 ebgp-multihop 1 > > Router(config-router)# neighbor 192.168.0.200 update-source loopback 2 > > Router(config-router)# neighbor 192.168.0.200 disable-connected-check > > Router(config-router)# end > > BGP Peer 2 > > Router(config)# interface loopback 2 > > Router(config-if)# ip address 192.168.0.200 255.255.255 > > Router(config-if)# exit > > Router(config)# router bgp 65534 > > Router(config-router)# neighbor 10.0.0.100 remote-as 64512 > > Router(config-router)# neighbor 10.0.0.100 ebgp-multihop 1 > > Router(config-router)# neighbor 10.0.0.100 update-source loopback 1 > > Router(config-router)# neighbor 10.0.0.100 disable-connected-check > > Router(config-router)# end > > > > On Tue, Mar 16, 2010 at 9:16 PM, Bryan Bartik <[email protected]>wrote: > >> Mustafa, >> >> It's in the command reference how it does this. It simply disables the >> check: >> >> >> http://www.cisco.com/en/US/docs/ios/12_3t/ip_route/command/reference/ip2_n1gt.html#wp1109875 >> >> On Tue, Mar 16, 2010 at 12:49 PM, Mustafa Yadav >> <[email protected]>wrote: >> >>> hi all, >>> >>> So far I know for e-bgp ttl is always 1 and if you wanna to establish >>> bgp between loopbacks you have to use ebgp multihop command.But today I have >>> learned disable-connected check is also helps to establish betwwen >>> looopbacks.How is it helping on this?Chaning ttl values or what? >>> >>> >>> >>> _______________________________________________ >>> For more information regarding industry leading CCIE Lab training, please >>> visit www.ipexpert.com >>> >>> >> >> >> -- >> Bryan Bartik >> CCIE #23707 (R&S, SP), CCNP >> Sr. Support Engineer - IPexpert, Inc. >> URL: http://www.IPexpert.com >> > > -- Bryan Bartik CCIE #23707 (R&S, SP), CCNP Sr. Support Engineer - IPexpert, Inc. URL: http://www.IPexpert.com
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