Thank you Marta. I saw that but this makes it very clear. The confusion was because of this not being explicit in the document. It makes sense because redistribution needs the route to exist in the routing table.
Because of my lack of experience with BGP i started to question this well known concept. Thank you for taking the time to explain this very clearly. -Srikant On Thu, Sep 27, 2012 at 11:22 AM, Marta Sokolowska < [email protected]> wrote: > Having a route to discard address on the trigger router is necessary - > otherwise this router will not advertise routes to its peers. But it can be > *any* route (not necessarily pointing to Null0), including default route > :-) > > As an example (considering scenario from Lab 16): > > if I want to redistribute the following four routes from R2 (which is my > trigger router): > > R2# show run | incl route > [...] > ip route 199.100.222.0 255.255.255.0 Null0 tag 199 > ip route 199.100.224.0 255.255.255.0 Null0 tag 199 > ip route 199.100.226.0 255.255.255.0 Null0 tag 199 > ip route 199.100.228.0 255.255.255.0 Null0 tag 199 > > with the next-hop of 192.0.2.1: > > R2# sh run | sec route-map > route-map r-RTBH permit 10 > match tag 199 > set local-preference 100 > set origin igp > set community no-export > set ip next-hop 192.0.2.1 > > I have to have a static route for 192.0.2.1 on R2 (pointing to Null0) to > be able to redistribute those four networks to my BGP peers: > > R2(config)# ip route 192.0.2.1 255.255.255.255 Null0 > > Now, if I remove the route above from R2, those four redistributed > networks will be gone from routing/BGP table of my BGP peers (R5 and R6): > > R2#sh ip cef 192.0.2.1 > 0.0.0.0/0 > no route > > R5# sh logg > [...] > *Sep 27 14:33:35.351: BGP(0): 2.2.2.2 rcv UPDATE about 199.100.228.0/24-- > withdrawn > *Sep 27 14:33:35.351: BGP(0): no valid path for 199.100.228.0/24 > *Sep 27 14:33:35.351: BGP(0): 2.2.2.2 rcv UPDATE about 199.100.226.0/24-- > withdrawn > *Sep 27 14:33:35.351: BGP(0): no valid path for 199.100.226.0/24 > *Sep 27 14:33:35.351: BGP(0): 2.2.2.2 rcv UPDATE about 199.100.224.0/24-- > withdrawn > *Sep 27 14:33:35.351: BGP(0): no valid path for 199.100.224.0/24 > *Sep 27 14:33:35.351: BGP(0): 2.2.2.2 rcv UPDATE about 199.100.222.0/24-- > withdrawn > *Sep 27 14:33:35.351: BGP(0): no valid path for 199.100.222.0/24 > > and it is because R2 stopped redistributing them: > > R2#sh ip bgp 199.100.222.0 > BGP routing table entry for 199.100.222.0/24, version 18 > Paths: (1 available, no best path) > Flag: 0x820 > Not advertised to any peer > Local > *192.0.2.1 (inaccessible) from 0.0.0.0 (2.2.2.2)* > Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced > Community: no-export > > But having default route on R2 is enough to start advertising those four > routes to R5 and R6 again: > > R2(config)#ip route 0.0.0.0 0.0.0.0 200.13.25.5 > > R2#sh ip cef 192.0.2.1 > 0.0.0.0/0 > nexthop 200.13.25.5 FastEthernet0/1.5 > > R2#sh ip bgp 199.100.222.0 > BGP routing table entry for 199.100.222.0/24, version 38 > Paths: (1 available, best #1, table Default-IP-Routing-Table, not > advertised to EBGP peer) > Flag: 0x820 > Advertised to update-groups: > 2 > Local > 192.0.2.1 from 0.0.0.0 (2.2.2.2) > Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, > best > Community: no-export > > But of course it would be safer to have specific route to 192.0.2.1 > (discard network) than relying on default route. > > > Marta Sokolowska. > > 2012/9/26 GuardGrid <[email protected]> > > Hi, >> Is there any info on RTBH on the DOC CD? >> >> I could only find this >> http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6642/prod_white_paper0900aecd80313fac.pdf >> >> http://packetlife.net/blog/2009/jul/6/remotely-triggered-black-hole-rtbh-routing/ >> >> and obviously this will not be available during the exam. >> >> I thought I understood the basic concept here but I am unable to get this >> to work without adding a route for the discard address on the trigger >> router. >> Both the links above only suggest that we add the route to null0 on the >> edge routers and only the route for the destination (victim) to be added on >> the trigger router. >> >> Destination RTBH >> ================= >> 1. Add route to null0 for the discard address on the edge routers >> 2. On the Trigger router Create a route map and match for a specific tag >> and set the next hop to that discard address. >> 3. As a best practice also set the origin to IGP (make it more preferred) >> and community to no-export (don't advertise outside the AS) >> 4. Add this route map for static redistribution under the BGP process on >> the trigger router. >> >> 5. Now when the victim in question is under attack then add a static >> route pointing to null0 with that tag on the trigger router so that this >> will be redistributed >> via BGP to the edge routers . >> >> >> Can you tell me what I am missing here? >> >> I am having to add a route for the discard addr on the trigger router as >> well which I do not see mentioned in any doc. >> >> >> -Srikant >> >> >> _______________________________________________ >> 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 >> > > > > -- > -- > > Marta SokoĊowska. > >
_______________________________________________ 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
